The Copenhagen Diagnosis, 2009: “Updating the World on the Latest Climate Science”
I. Allison, N.L. Bindoff, R.A. Bindschadler, P.M. Cox, N. de Noblet, M.H. England, J.E. Francis, N.
Gruber, A.M. Haywood, D.J. Karoly, G. Kaser, C. Le Quéré, T.M. Lenton, M.E. Mann, B.I. McNeil,
A.J. Pitman, S. Rahmstorf, E. Rignot, H.J. Schellnhuber, S.H. Schneider, S.C. Sherwood, R.C.J.
Somerville, K. Steffen, E.J. Steig, M. Visbeck, A.J. Weaver. The University of New South Wales
Climate Change Research Centre (CCRC), Sydney, Australia, 60pp.
Stephen Gray from the UNSW Climate Change Research Centre (CCRC) provided support in the
compilation of this report and contributed as Technical Editor. Robert Beale (UNSW Faculty of
Science) and Michael Molitor (UNSW CCRC) provided editorial advice on sections of the report.
Alex Sen Gupta (UNSW CCRC) provided Figure 15 and Darrell Kaufman (Northern Arizona
University) provided Figure 20.
Updating the World on the Latest Climate Science
Ian Allison, Nathan Bindoff, Robert Bindschadler, Peter Cox, Nathalie de Noblet-Ducoudré, Matthew England, Jane Francis, Nicolas Gruber, Alan Haywood, David Karoly, Georg Kaser, Corinne Le Quéré, Tim Lenton, Michael Mann, Ben McNeil, Andy Pitman, Stefan Rahmstorf, Eric Rignot, Hans Joachim Schellnhuber, Stephen Schneider, Steven Sherwood, Richard Somerville, Konrad Steffen, Eric Steig, Martin Visbeck, Andrew Weaver
Preface, Executive Summary, Greenhouse Gases and the Carbon Cycle, The Atmosphere,
Extreme Events, Land Surface, Permafrost and Hydrates, Glaciers and Ice-Caps, Ice-Sheets of Greenland and Antarctica, Ice Shelves, Sea-Ice, The Oceans, Global Sea Level, Abrupt Change and Tipping Points, Lessons from the Past, The Future, References, Biographies.
It is over three years since the drafting of text was completed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In the meantime, many hundreds of papers have been published on a suite of topics related to human-induced climate change. The purpose of this report is to synthesize the most policy-relevant climate science published since the close-off of material for the last IPCC report. The rationale is two-fold. Firstly, this report serves as an interim evaluation of the evolving science midway through an IPCC cycle ? IPCC AR5 is not due for completion until 2013.
Secondly, and most importantly, the report serves as a handbook of science updates that supplements the IPCC AR4 in time for Copenhagen in December, 2009, and any national or international climate change policy negotiations that follow.
This report covers the range of topics evaluated by Working Group I of the IPCC, namely the Physical Science Basis.
A- an analysis of greenhouse gas emissions and their atmospheric concentrations, as well as the global carbon cycle;
B-coverage of the atmosphere, the land-surface, the oceans, and all of the major components of the cryosphere (land-ice, glaciers, ice shelves, sea-ice and permafrost);
C- paleoclimate, extreme events, sea level, future projections, abrupt change and tipping points;
D- separate boxes devoted to explaining some of the common misconceptions surrounding climate change science.
The report has been purposefully written with a target readership of policy-makers, stakeholders, the media and the broader public. Each section begins with a set of key points that summarises the main findings. The science contained in the report is based on the most credible and significant peer-reviewed literature available at the time of publication. The authors primarily comprise previous IPCC lead authors familiar with the rigor and completeness required for a scientific assessment of this nature.
The most significant recent climate change findings are:
Surging greenhouse gas emissions: Global carbon dioxide emissions from fossil fuels in 2008 were nearly 40% higher than those in 1990. Even if global emission rates are stabilized at present-day levels, just 20 more years of emissions would give a 25% probability that warming exceeds 2degrees C, even with zero emissions after 2030. Every year of delayed action increases the chances of exceeding 2 degrees C warming.
Recent global temperatures demonstrate human-induced warming: Over the past 25 years temperatures have increased at a rate of 0.19degreesC per decade, in very good agreement with predictions based on greenhouse gas increases. Even over the past ten years, despite a decrease in solar forcing, the trend continues to be one of warming. Natural, short-term fluctuations are occurring as usual, but there have been no significant changes in the underlying warming trend.
Acceleration of melting of ice-sheets, glaciers and ice-caps: A wide array of satellite and ice measurements now demonstrate beyond doubt that both the Greenland and Antarctic ice-sheets are losing mass at an increasing rate. Melting of glaciers and ice-caps in other parts of the world has also accelerated since 1990.
Rapid Arctic sea-ice decline: Summer-time melting of Arctic sea-ice has accelerated far beyond the expectations of climate models. The area of sea-ice melt during 2007-2009 was about 40% greater than the average prediction from IPCC AR4 climate models.
Current sea-level rise underestimated: Satellites show recent global average sea-level rise (3.4 mm/yr over the past 15 years) to be ~80% above past IPCC predictions. This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps, and the Greenland and West-Antarctic ice-sheets. Sea-level predictions revised: By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4; for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as ~ 2 meters sea level rise by 2100. Sea level will continue to rise for centuries after global temperatures have been stabilized, and several meters of sea level rise must be expected over the next few centuries.
Delay in action risks irreversible damage: Several vulnerable elements in the climate system (e.g. continental ice-sheets, Amazon rainforest, West African monsoon and others) could be pushed towards abrupt or irreversible change if warming continues in a business-as-usual way throughout this century. The risk of transgressing critical thresholds (“tipping points”) increases strongly with ongoing climate change. Thus waiting for higher levels of scientific certainty could mean that some tipping points will be crossed before they are recognized.
The turning point must come soon: If global warming is to be limited to a maximum of 2 degrees C above pre-industrial values, global emissions need to peak between 2015 and 2020 and then decline rapidly. To stabilize climate, a decarbonized global society ? with near-zero emissions of CO2 and other long-lived greenhouse gases ? needs to be reached well within this century. More specifically, the average annual per-capita emissions will have to shrink to well under 1 metric ton CO2 by 2050. This is 80-95% below the per-capita emissions in developed nations in 2000.
Greenhouse gases and the carbon cycle
– Global carbon dioxide (CO2) emissions from fossil fuel burning in 2008 were 40% higher than those in 1990, with a three-fold acceleration over the past 18 years.
– Global CO2 emissions from fossil fuel burning are tracking near the highest scenarios considered so far by the IPCC.
– The fraction of CO2 emissions absorbed by the land and ocean CO2 reservoirs has likely decreased by ~5% (from 60 to 55%) in the past 50 years, though interannual variability is large.
Global Carbon Dioxide Emissions
In 2008, combined global emissions of carbon dioxide (CO2) from fossil fuel burning, cement production and land use change (mainly deforestation) were 27% higher than in the year 1990 (Le Qu?r? et al. 2009). Of this combined total, the CO2 emissions from fossil fuel burning and cement production were 40%
higher in 2008 compared to 1990. The global rate of increase of fossil fuel CO2 emissions has accelerated three-fold over the last 18 years, increasing from 1.0% per year in the 1990s to 3.4% per year between 2000-2008 (Figure 1). The accelerated growth in fossil fuel CO2 emissions since 2000 was primarily
caused by fast growth rates in developing countries (particularly China) in part due to increased international trade of goods (Peters and Hertwich 2008), and by the slowdown of previous improvements in the CO2 intensity of the global economy (Raupach et al. 2007). The observed acceleration in fossil fuel CO2 emissions is tracking high-end emissions scenarios used by IPCC AR4 (Nakicenovic et al. 2000). In contrast, CO2 emissions from land use change were relatively constant in the past few decades. Preliminary figures suggest total CO2 emissions have dropped in 2009, but this is a temporary effect resulting from the global recession and no sign of the transformation required for stabilizing greenhouse gases in the atmosphere.
The concentration of CO2 in the atmosphere reached 385 parts per million (ppm) in 2008 (Figure 2). The atmospheric CO2 concentration is more than 105 ppm above its natural preindustrial level. The present concentration is higher than at any time in the last 800,000 years, and potentially the last 3 to 20
million years (Luthi et al. 2008; Tripati et al. 2009; Raymo et al. 1996). CO2 levels increased at a rate of 1.9 ppm/year between 2000 and 2008, compared to 1.5 ppm/yr in the 1990s. This rate of increase of atmospheric CO2 is more than ten times faster than the highest rate that has been detected in ice core data; such high
rates would be discernable in ice cores if they had occurred at any time in the last 22,000 years (Joos and Spahni 2008).
The concentration of methane (CH4) in the atmosphere increased since 2007 to 1800 parts per billion (ppb) after almost a decade of little change (Figure 2). The causes of the recent increase in CH4 have not yet been determined. The spatial distribution of the CH4 increase shows that an increase in Northern Hemisphere
CH4 emissions has played a role and could dominate the signal
~Figure 1. Observed global CO2 emissions from fossil fuel burning and cement production compared with IPCC emissions scenarios (Le Qu?r? et al. 2009). Observations are from the US Department of Energy Carbon Dioxide Information Center (CDIAC) up to 2006. 2007 and 2008 are based on BP economic data. The emission scenarios are averaged over families of scenarios presented in Nakicenovic et al (2000). The shaded
area covers all scenarios used to project climate change by the IPCC. Emissions in 2009 are projected to be ~3% below 2008 levels, close to the level of emissions in 2007. This reduction is equivalent to a temporary halt in global emissions for a period of only 2-4 weeks. But the source of the increase is unknown.
CH4 is emitted by many industrial processes (ruminant farming, rice agriculture, biomass burning, coal mining, and gas & oil industry) and by natural reservoirs (wetlands, permafrost and peatlands). Annual industrial emissions of CH4 are not available as they are difficult to quantify. CH4 emissions from natural reservoirs can increase under warming conditions. This has been observed from permafrost thawing in Sweden (see
Permafrost section), but no large-scale evidence is available to clearly connect this process to the recent CH4 increase. If the CH4 increase is caused by the response of natural reservoirs to warming, it could continue for decades to centuries and enhance the greenhouse gas burden of the atmosphere.
Figure 2. Concentration of CO2 (top) and CH4 (bottom) in the atmosphere. The trends with seasonal cycle removed are shown in red. CO2 and CH4 are the two most important anthropogenic greenhouse gases. Data are from the Earth System Laboratory of the US National Oceanic and Atmospheric Administration. CO2 is
averaged globally. CH4 is shown for the Mauna Loa station only.
Carbon Sinks and Future Vulnerabilities
The oceanic and terrestrial CO2 reservoirs the CO2 sinks have continued to absorb more than half of the total emissions of CO2. However the fraction of emissions absorbed by the reservoirs has likely decreased by ~5% (from 60 to 55%) in the past 50 years (Canadell et al. 2007). The uncertainty in this estimate is large because of the significant background interannual variability and because of uncertainty in CO2 emissions from land use change. The response of the land and ocean CO2 sinks to climate variability and recent climate change can account for the decrease in uptake efficiency of the sinks suggested by the observations (Le Quéré et al. 2009). A long-term decrease in the efficiency of the land and ocean CO2 sinks would enhance
climate change via an increase in the amount of CO2 remaining in the atmosphere. Many new studies have shown a recent decrease in the efficiency of the oceanic carbon sink at removing anthropogenic CO2 from the atmosphere. In the Southern Ocean, the CO2 sink has not increased since 1981 in spite of the large
increase in atmospheric CO2 (Le Quéré et al. 2007; Metzl 2009; Takahashi et al. 2009).
The Southern Ocean trends have been attributed to an increase in winds, itself a likely consequence of
ozone depletion (Lovenduski et al. 2008). Similarly, in the North Atlantic, the CO2 sink decreased by ~50% since 1990 (Schuster et al. 2009), though part of the decrease has been associated with natural variability (Thomas et al. 2008). Future vulnerabilities of the global CO2 sinks (ocean and land) have not been revised since the IPCC AR4. Our current understanding indicates that the natural CO2 sinks will decrease in efficiency during this century, and the terrestrial sink could even start to emit CO2 (Friedlingstein et al. 2006). The response of the sinks to elevated CO2 and climate change is shown in models to amplify global warming by 5-30%. The observations available so far are insufficient to provide greater certainty, but they do not exclude the largest global warming amplification projected by the models (Le Quéré et al. 2009).
Is the greenhouse effect already saturated, so that adding more CO2 makes no difference?
No, not even remotely. It isn?t even saturated on the runaway greenhouse planet Venus, with its atmosphere made up of 96% CO2 and a surface temperature of 467 degrees C, hotter even than Mercury (Weart and Pierrehumbert 2007). The reason is simple: the air gets ever thinner when we go up higher in the atmosphere. Heat radiation escaping into space mostly occurs higher up in the atmosphere, not at the surface on average from an altitude of about 5.5 km. It is here that adding more CO2 does make a difference. When we add more CO2, the layer near the surface where the CO2 effect is largely saturated gets thicker # one can visualize this as a layer of fog, visible only in the infrared. When this fog layer gets thicker, radiation can only escape to space from higher up in the atmosphere, and the radiative equilibrium temperature of -18 degrees C therefore also occurs higher up. That upward shift heats the surface, because temperature increases by 6.5 ?C per kilometer as one goes down through the atmosphere due to the pressure increase. Thus, adding 1 km to the “CO2 fog layer” that envelopes our Earth will heat the surface climate by about 6.5 degrees C.
– Global air temperature, humidity and rainfall trend patterns exhibit a distinct fingerprint that cannot be explained by phenomena apart from increased atmospheric greenhouse gas concentrations.
– Every year this century (2001-2008) has been among the top 10 warmest years since instrumental records began, despite solar irradiance being relatively weak over the past few years.
– Global atmospheric temperatures maintain a strong warming trend since the 1970s (~0.6?C), consistent with expectations of greenhouse induced warming.
Global Temperature Trends
IPCC AR4 presented ?an unambiguous picture of the ongoing warming of the climate system.? The atmospheric warming trend continues to climb despite 2008 being cooler than 2007 (Figure 3)
Figure 3. (top) Mean surface temperature change (?C) for 2001-2007 relative to the baseline period of 1951-1980 and (bottom) global average temperature 1850-2009 relative to the baseline period 1880-1920 estimated from the (top) NASA/GISS data set and (bottom) NASA/GISS and Hadley data. Data from the NOAA reconstructed sea surface temperature show similar results. In the lower panel the final bold-face points (they lie on top of each other) are the preliminary values for 2009 based on data up to and including August.
For example, the IPCC gave the 25-year trend as 0.177 ? 0.052 ?C per decade for the period ending 2006 (based on the HadCRUT data). Updating this by including the last two years (2007 and 2008), the trend becomes 0.187 ? 0.052 ?C per decade for the period ending 2008. The recent observed
climate trend is thus one of ongoing warming, in line with IPCC predictions.
Year-to-year differences in global average temperatures are unimportant in evaluating long-term climate trends. During the warming observed over the 20th century, individual years lie above or below the long-term trend line due to internal climate variability (like 1998); this is a normal and natural phenomenon.
For example, in 2008 a La Ni?a occurred, a climate pattern which naturally causes a temporary dip in the average global temperature. At the same time, solar output was also at its lowest level of the satellite era, another temporary cooling influence. Without anthropogenic warming these two factors should have resulted in the 2008 temperature being among the coolest in the instrumental era, while in fact 2008 was the
9th warmest on record. This underpins the strong greenhouse warming that has occurred in the atmosphere over the past century. The most recent ten-year period is warmer than the previous ten-year period, and the longer-term warming trend is clear and unambiguous (Figure 3).
Is the Warming Natural or Human-Induced?
Our understanding of the causes of the recent century-scale trend has improved further since the IPCC AR4. By far the greatest part of the observed century-scale warming is due to human factors. For example, Lean and Rind (2008) analyzed the role of natural factors (e. g., solar variability, volcanoes) versus human influences on temperatures since 1889. They found that the sun contributed only about 10% of surface warming in the last century and a negligible amount in the last quarter century, less than in earlier assessments. No credible scientific literature has been published since the AR4 assessment that supports
alternative hypotheses to explain the warming trend.
Is Warming Occurring Higher up in the Atmosphere?
The IPCC AR4 noted a remaining uncertainty in temperature trends in the atmosphere above the lowest layers near the Earth?s surface. Most data sets available at that time showed weaker than expected warming in the atmospheric region referred to as the tropical upper troposphere, ten to fifteen kilometers above the surface. However, the observations suffered from significant stability issues especially in this altitude region. Researchers have since performed additional analyses of the same data using more rigorous techniques, and developed a new method of assessing temperature trends from wind observations (Allen and Sherwood 2008). The new observational estimates show greater warming than the earlier ones, and the new, larger set of
estimates taken as a whole now bracket the trends predicted by the models (Thorne 2008). This resolves a significant ambiguity expressed in AR4 (Santer et al. 2008).
Water Vapor, Rainfall and the Hydrological Cycle
New research and observations have resolved the question of whether a warming climate will lead to an atmosphere containing more water vapor, which would add to the greenhouse effect and enhance the warming. The answer is yes, this amplifying feedback has been detected: water vapor does
become more plentiful in a warmer atmosphere (Dessler et al. 2008). Satellite data show that atmospheric moisture content over the oceans has increased since 1998, with greenhouse emissions being the cause (Santer at al. 2007).
No studies were cited in IPCC AR4 linking observed rainfall trends on a fifty-year time scale to anthropogenic climate change. Now such trends can be linked. For example, Zhang et al. (2007) found that rainfall has reduced in the Northern Hemisphere subtropics but has increased in middle latitudes, and that this can be attributed to human-caused global warming. Models project that such trends will amplify as temperatures
continue to rise. Recent research has also found that rains become more intense in already-rainy areas as atmospheric water vapor content increases (Wentz et al. 2007; Allan and Soden 2008). Their
conclusions strengthen those of earlier studies. However, recent changes have occurred even faster than predicted, raising the possibility that future changes could be more severe than predicted. This is a common theme from the recent science: uncertainties existing in AR4, once resolved, point to a more rapidly changing and more sensitive climate than we previously believed.
Has global warming recently slowed down or paused?
No. There is no indication in the data of a slowdown or pause in the human-caused climatic warming trend. The observed global temperature changes are entirely consistent with the climatic warming trend of ~0.2 ?C per decade predicted by IPCC, plus superimposed short-term variability (see Figure 4). The latter has always been ? and will always be ? present in the climate system. Most of these short-term variations are due to internal oscillations like El Ni?o ? Southern Oscillation, solar variability (predominantly the 11-year Schwabe cycle) and volcanic eruptions (which, like Pinatubo in 1991, can cause a cooling lasting a few years).
If one looks at periods of ten years or shorter, such short-term variations can more than outweigh the anthropogenic global warming trend. For example, El Ni?o events typically come with global-mean temperature changes of up to 0.2 ?C over a few years, and the solar cycle with warming or cooling of 0.1 ?C over five years (Lean and Rind 2008). However, neither El Ni?o, nor solar activity or volcanic eruptions make a significant contribution to longer-term climate trends. For good reason the IPCC has chosen 25 years as the shortest trend line they show in the global temperature records, and over this time period the observed trend agrees very well with the expected anthropogenic warming. Nevertheless global cooling has not occurred even over the past ten years, contrary to claims promoted by lobby groups and picked up in some media. In the NASA global temperature data, the past ten 10-year trends (i.e. 1990-1999, 1991-2000 and so on) have all been between 0.17 and 0.34 ?C warming per decade, close to or above the expected anthropogenic trend,
with the most recent one (1999-2008) equal to 0.19 ?C per decade. The Hadley Center data most recently show smaller warming trends (0.11 ?C per decade for 1999-2008) primarily due to the fact that this data set is not fully global but leaves out the Arctic, which has warmed particularly strongly in recent years.
It is perhaps noteworthy that despite the extremely low brightness of the sun over the past three years (see next page); temperature records have been broken during this time (see NOAA, State of the Climate, 2009). For example, March 2008 saw the warmest global land temperature of any March ever measured in the instrumental record. June and August 2009 saw the warmest land and ocean temperatures in the Southern Hemisphere ever recorded for those months. The global ocean surface temperatures in 2009 broke all previous records for three consecutive months: June, July and August. The years 2007, 2008 and 2009 had the lowest summer Arctic sea ice cover ever recorded, and in 2008 for the first time in living memory the Northwest Passage and the Northeast Passage were simultaneously ice-free. This feat was repeated in 2009. Every single year of this century (2001-2008) has been among the top ten warmest years since instrumental records began.
Figure 4. Global temperature according to NASA GISS data since 1980. The red line shows annual data, the red square shows the preliminary value for 2009, based on January-August. The green line shows the 25-year linear trend (0.19 ?C per decade). The blue lines show the two most recent ten-year trends (0.18 ?C per decade for 1998-2007, 0.19 per decade for 1999-2008) and illustrates that these recent decadal trends are entirely consistent with the long-term trend and IPCC predictions. Misunderstanding about warming trends can arise if only selected portions of the data are shown, e.g. 1998 to 2008, combined with the tendency to focus on extremes or end points (e.g. 2008 being cooler than 1998) rather than an objective trend calculation. Even the highly ?cherry-picked? 11-year period starting with the warm 1998 and ending with the cold 2008 still shows a warming trend of 0.11 ?C per decade.
Can solar activity or other natural processes explain global warming?
No. The incoming solar radiation has been almost constant over the past 50 years, apart from the well-known 11-year solar cycle (Figure 5). In fact it has slightly decreased over this period. In addition, over the past three years the brightness of the sun has reached an all-time low since the beginning of satellite measurements in the 1970s (Lockwood and Fr?hlich 2007, 2008). But this natural cooling effect was more than a factor of ten smaller than the effect of increasing greenhouse gases, so it has not noticeably slowed down global warming. Also, winters are warming more rapidly than summers, and overnight minimum temperatures have warmed more rapidly than the daytime maxima ? exactly the opposite of what would be the case if the sun were causing the warming.
Other natural factors, like volcanic eruptions or El Ni?o events, have only caused short-term temperature variations over time spans of a few years, but cannot explain any longer-term climatic trends (e.g., Lean and Rind 2008).
Figure 5. Time-series of solar irradiance alongside the net effect of greenhouse gas emissions (the latter relative to the year 1880; using Meehl et al. 2004) calculated in terms of total estimated impact on global air temperatures; observed from 1970-2008; and projected from 2009-2030 (adapted from Lean and Rind 2009).
– Increases in hot extremes and decreases in cold extremes have continued and are expected to amplify further.
– Anthropogenic climate change is expected to lead to further increases in precipitation extremes, both increases in heavy precipitation and increases in drought.
– Although future changes in tropical cyclone activity cannot yet be modeled, new analyses of observational data confirm that the intensity of tropical cyclones has increased in the past three decades in line with rising tropical ocean temperatures. Many of the impacts of climate variations and climate change on society, the environment and ecosystems arise through changes in the frequency or intensity of extreme weather and climate events.
The IPCC Fourth Assessment Report (IPCC 2007) concluded that many changes in extremes had been observed since the 1970s as part of the warming of the climate system. These included more frequent hot days, hot nights and heat waves; fewer cold days, cold nights and frosts; more frequent heavy precipitation
events; more intense and longer droughts over wider areas; and an increase in intense tropical cyclone activity in the North Atlantic but no trend in total numbers of tropical cyclones. Temperature extremes
Recent studies have confirmed the observed trends of more hot extremes and fewer cold extremes and shown that these are consistent with the expected response to increasing greenhouse gases and anthropogenic aerosols at large spatial scales (CCSP 2008a; Meehl et al. 2007a; Jones et al. 2008; Alexander and
Arblaster 2009). However, at smaller scales, the effects of land-use change and variations of precipitation may be more important for changes in temperature extremes in some locations (Portmann et al. 2009). Continued marked increases in hot extremes and decreases in cold extremes are expected in most areas across the globe due to further anthropogenic climate change (CCSP 2008a; Kharin et al. 2007; Meehl et al. 2007a;Jones et al. 2008; Alexander and Arblaster 2009).
Precipitation extremes and drought Post IPCC AR4 research has also found that rains become more intense in already-rainy areas as atmospheric water vapor content increases (Pall et al. 2007; Wentz et al. 2007; Allan
and Soden 2008). These conclusions strengthen those of earlier studies and are expected from considerations of atmospheric thermodynamics. However, recent changes have occurred faster than predicted by some climate models, raising the possibility that future changes will be more severe than predicted.
An example of recent increases in heavy precipitation is found in the United States, where the area with a much greater than normal proportion of days with extreme rainfall amounts has increased markedly (see Figure 6). While these changes in precipitation extremes are consistent with the warming of the climate system, it has not been possible to attribute them to anthropogenic climate change with high confidence due to the
very large variability of precipitation extremes (CCSP 2008a; Meehl et al. 2007b; Alexander and Arblaster 2009).
Figure 6. An increasing area of the US is experiencing very heavy daily precipitation events. Annual values of the percentage of the United States with a much greater than normal proportion of precipitation due to very heavy (equivalent to the highest tenth percentile) 1-day precipitation amounts. From Gleason et al. (2008) updated by NOAA at /www.ncdc.noaa.gov/oa/climate/research/cei/cei.html
In addition to the increases in heavy precipitation, there have also been observed increases in drought since the 1970s (Sheffield and Wood 2008), consistent with the decreases in mean precipitation over land in some latitude bands that have been attributed to anthropogenic climate change (Zhang et al. 2007).
The intensification of the global hydrological cycle with anthropogenic climate change is expected to lead to further increases in precipitation extremes, both increases in very heavy precipitation in wet areas and increases in drought in dry areas. While precise figures cannot yet be given, current studies suggest that heavy precipitation rates may increase by 5% – 10% per ?C of warming, similar to the rate of increase of atmospheric water vapor.
The IPCC Fourth Assessment found a substantial upward trend in the severity of tropical cyclones (hurricanes and typhoons) since the mid-1970s, with a trend towards longer storm duration and greater storm intensity, strongly correlated with the rise in tropical sea surface temperatures. It concluded that a further
increase in storm intensity is likely. Several studies since the IPCC report have found more evidence
for an increase in hurricane activity over the past decades. Hoyos et al. (2006) found a global increase in the number of hurricanes of the strongest categories 4 and 5, and they identified rising sea surface temperatures (SST) as the leading cause. Warming tropical SST has also been linked to increasingly intense tropical
cyclone activity ? and an increasing number of tropical cyclones ? in the case of certain basins such as the North Atlantic (Mann and Emanuel 2006; Emanuel et al. 2008; Mann et al. 2009).
Scientific debate about data quality has continued, especially on the question of how many tropical cyclones may have gone undetected before satellites provided a global coverage of observations. Mann et al. (2007) concluded that such an undercount bias would not be large enough to question the recent rise in hurricane activity and its close connection to sea surface warming. A complete reanalysis of satellite data since 1980 (Elsner et al. 2008) confirms a global increase of the number of category 4 and 5 (i.e., the strongest) tropical
cyclones: they found a 1?C global warming corresponding to a 30% increase in these storms. While evidence has thus firmed up considerably that recent warming has been associated with stronger tropical cyclones, modeling studies (e.g. Emanuel et al. 2008; Knutson et al. 2008, Vecchi et al. 2008) have shown that
we have as yet no robust capacity to project future changes in tropical cyclone activity.
Other severe weather events
The IPCC Fourth Assessment concluded that there were insufficient studies available to make an assessment of observed changes in small-scale severe weather events or of expected future changes in such events. However, recent research has shown an increased frequency of severe thunderstorms in some regions, particularly the tropics and south-eastern US, is expected due to future anthropogenic climate change (Trapp
et al. 2007; Aumann et al. 2008; Marsh et al. 2009; Trapp et al. 2009). In addition, there have been recent increases in the frequency and intensity of wildfires in many regions with Mediterranean climates (e.g. Spain, Greece, southern California, south-east Australia) and further marked increases are expected due to anthropogenic climate change (Westerling et al. 2006; Pitman et al. 2008).
– Land cover change, particularly deforestation, can have a major impact on regional climate, but at the global scale its biggest impact comes from the CO2 released in the process.
– Observations through the 2005 drought in Amazonia suggest that the tropical forests could become a strong carbon source if rainfall declines in the future.
– Carbon dioxide changes during the Little Ice Age indicate that warming may in turn lead to carbon release from land surfaces, a feedback that could amplify 21st century climate change.
– Avoiding tropical deforestation could prevent up to 20% of human-induced CO2 emissions and help to maintain biodiversity.
How does land-use change affect climate?
Earth?s climate is strongly affected by the nature of the land-surface, including the vegetation and soil type and the amount of water stored on the land as soil moisture, snow and groundwater. Vegetation and soils affect the surface albedo, which determines the amount of sunlight absorbed by the land.
The land surface also affects the partitioning of rainfall into evapotranspiration (which cools the surface and moistens the atmosphere) and runoff (which provides much of our freshwater).
This partitioning can affect local convection and therefore rainfall. Changes in land-use associated with the spread of agriculture and urbanization and deforestation can alter these mechanisms.
Land use change can also change the surface roughness, affect emissions of trace gases, and some volatile organic compounds such as isoprene. Despite the key role of land cover change at regional scales, climate model projections from IPCC AR4 excluded anthropogenic land-cover change.
There has been significant progress on modeling the role of land cover change since the IPCC AR4 (Piekle et al. 2007), with the first systematic study demonstrating that large-scale land cover change directly and significantly affects regional climate (Pitman et al. 2009). This has important implications for understanding
future climate change; climate models need to simulate land cover change to capture regional changes in regions of intense land cover change. However, failing to account for land cover change has probably not affected global-scale projections (Pitman et al. 2009), noting that emissions from land cover change are included in projections.
Land-cover change also affects climate change by releasing CO2 to the atmosphere and by modifying the land carbon sink (Bondeau et al. 2007; Fargione et al. 2008). The most obvious example of this is tropical deforestation which contributes about a fifth of global CO2 emissions and also influences the landto-
atmosphere fluxes of water and energy (Bala et al. 2007).
Avoiding deforestation therefore eliminates a significant fraction of anthropogenic CO2 emissions, and maintains areas like the Amazon rainforest which supports high biodiversity and plays a critically important role in the climate system (Malhi et al. 2008).
Climate Change and the Amazon Rainforest
The distribution and function of vegetation depends critically on the patterns of temperature and rainfall across the globe. Climate change therefore has the potential to significantly alter land-cover even in the absence of land-use change. A key area of concern has been the remaining intact Amazonian rainforest
which is susceptible to ?dieback? in some climate models due to the combined effects of increasing greenhouse gases and reducing particulate or ?aerosol? pollution in the northern hemisphere (Cox et al. 2008). However, these projections are very dependent on uncertain aspects of regional climate change, most notably the sign and magnitude of rainfall change in Amazonia in the 21st century (Malhi et al. 2008, 2009).
There have also been some doubts raised as to whether the Amazonian rainforest is as sensitive to rainfall reductions as large-scale models suggest. The drought in Western Amazonia in 2005 provided a test of this hypothesis using long-term monitoring of tree growth in the region (Phillips et al. 2009), and a massive carbon source was detected in the region in 2005 against the backdrop of a significant carbon sink in the decades
before. The forests of Amazonia are therefore sensitive to ?2005-like? droughts and these are expected to become more common in the 21st century (Cox et al. 2008).
A similar story emerges from the analysis of satellite and CO2 flux measurements during the European drought of 2003 (Reichstein et al. 2007). The IPCC AR4 tentatively suggested a link between global warming and the 2003 drought, and this analysis showed that the drought had an enormous impact on the health and functioning of both natural and managed landscapes in the region.
How large are feedbacks linking land-surface and climate?
The response of the land-surface to climatic anomalies feeds back on the climate by changing the fluxes of energy, water and CO2 between the land and the atmosphere. For example, it seems likely that changes in the state of the land-surface, which in turn changed the energy and water fluxes to the atmosphere,
played an important part in the severity and length of the 2003 European drought (Fischer et al. 2007). In some regions, such as the Sahel, land-atmosphere coupling may be strong enough to support two alternative climate-vegetation states; one wet and vegetated, the other dry and desert-like. There may be other ?hot-spot? regions where the land-atmosphere coupling significantly controls the regional climate; indeed it appears that the land is a strong control on climate in many semi arid and Mediterranean-like regions.
However, the strongest feedbacks on global climate in the 21st century are likely to be due to changes in the land carbon sink. The climate-carbon cycle models reported in the IPCC AR4 (Friedlingstein et al. 2006) reproduced the historical land carbon sink predominantly through `CO2 fertilization?. There is evidence of CO2 fertilization being limited in nitrogen-limited ecosystems (Hyvonen et al. 2007), but the first generation coupled climatecarbon models did not include nutrient cycling.
The IPCC AR4 climate-carbon cycle models also represented a counteracting tendency for CO2 to be released more quickly from the soils as the climate warms, and as a result these models predicted a reducing efficiency of the land carbon sink under global warming. There is some suggestion of a slow-down of natural carbon sinks in the recent observational record (Canadell et al. 2007), and strong amplifying land carbon-climate feedback
also seems to be consistent with records of the little ice-age period (Cox and Jones 2008).
Does the land-surface care about the causes of climate change?
Yes. Vegetation is affected differently by different atmospheric pollutants, and this means that the effects of changes in atmospheric composition cannot be understood purely in terms of their impact on global warming.
CO2 increases affect the land through climate change, but also directly through CO2-fertilization of photosynthesis, and ?CO2-induced stomatal closure? which tends to increase plant water-use efficiency. Observational studies have shown a direct impact of CO2 on the stomatal pores of plants, which regulate
the fluxes of water vapor and CO2 at the leaf surface. In a higher CO2 environment, stomata reduce their opening since they are able to take up CO2 more efficiently. By transpiring less, plants increase their water-use efficiency, which consequently affects the surface energy and water balance. If transpiration is
suppressed via higher CO2, the lower evaporative cooling may also lead to higher temperatures (Cruz et al. 2009). There is also the potential for significant positive impacts on freshwater resources, but this is still an area of active debate (Gedney et al. 2006, Piao et al. 2007, Betts et al. 2007).
By contrast, increases in near surface ozone have strong negative impacts on vegetation by damaging leaves and their photosynthetic capacity. As a result historical increases in near surface ozone have probably suppressed land carbon uptake and therefore increased the rate of growth of CO2 in the 20th century.
Sitch et al. (2007) estimate that this indirect forcing of climate change almost doubles the contribution that near-surface ozone made to 20th century climate change. Atmospheric aerosol pollution also has a direct impact on plant physiology by changing the quantity and nature of the sunlight reaching the land-surface. Increasing aerosol loadings from around 1950 to 1980, associated predominantly with the burning of sulphurous coal, reduced the amount of sunlight at the surface, which has been coined ?global dimming? (Wild et al. 2007). Since plants need sunlight for photosynthesis, we might have expected to see a slow-down of the land carbon sink during the global dimming period, but we didn?t. Mercado et al. (2009) offer an explanation for this based on the fact that plants are more light-efficient if the sunlight is ?diffuse?. Aerosol
pollution would certainly have scattered the sunlight, making it more diffuse, as well as reducing the overall quantity of sunlight reaching the surface. It seems that ?diffuse radiation fertilization? won this battle, enhancing the global land-carbon sink by about a quarter from 1960 to 2000 (Mercado et al. 2009). This implies that the land carbon sink will decline if we reduce the amount of potentially harmful particulates in the air. These recent studies since IPCC AR4 argue strongly for metrics to compare different atmospheric pollutants that go beyond radiative forcing and global warming, to impacts on the vital ecosystem services related to the availability of food and water.
Permafrost and hydrates
– New insights into the Northern Hemisphere permafrost (permanently frozen ground) suggest a large potential source of CO2 and CH4 that would amplify atmospheric concentrations if released.
– A recent increase in global methane levels cannot yet be attributed to permafrost degradation.
– A separate and significant source of methane exists as hydrates beneath the deep ocean floor and in permafrost. It has recently been concluded that release of this type of methane is very unlikely to occur this century.
As noted in the IPCC AR4 and more recent studies, the southern boundary of the discontinuous permafrost zone has shifted northward over North America in recent decades. Rapid degradation and upward movement of the permafrost lower limit has continued on the Tibetan plateau (Jin et al. 2008, Cui and Graf 2009). In addition, observations in Europe (?kerman and Johansson 2008; Harris et al. 2009) have noted permafrost
thawing and a substantial increase in the depth of the overlying active layer exposed to an annual freeze/thaw cycle, especially in Sweden.
As permafrost melts and the depth of the active layer deepens, more organic material can potentially start to decay. If the surface is covered with water, methane-producing bacteria break down the organic matter. But these bacteria cannot survive in the presence of oxygen. Instead, if the thawed soils are exposed to air, carbon dioxide-producing bacteria are involved in the decay process. Either case is an amplifying feedback to global warming. In fact, the magnitude of the feedback represents an important unknown in the science of global warming; this feedback has not been accounted for in any of the IPCC projections. The total
amount of carbon stored in permafrost has been estimated to be around 1672 Gt (1 Gt = 109 tons), of which ~277 Gt are contained in peatlands (Schuur et al. 2008; Tarnocai et al. 2009).
This represents about twice the amount of carbon contained in the atmosphere. A recent analysis by Dorrepaal et al. (2009) has found strong direct observational evidence for an acceleration of carbon emissions in association with climate warming from a peat bog overlying permafrost at a site in northern Sweden.
Whether or not recent observations of increasing atmospheric methane concentration (Rigby et al. 2008), after nearly a decade of stable levels, are caused by enhanced northern hemisphere production associated with surface warming is still uncertain. Another amplifying feedback to warming that has recently
been observed in high northern latitudes involves the microbial
transformation of nitrogen trapped in soils to nitrous oxide. By
measuring the nitrous oxide emissions from bare peat surfaces,
Repo et al. (2009) inferred emissions per square meter of the
same magnitude as those from croplands and tropical soils. They
point out that as the Arctic warms, regions of bare exposed peat
will increase, thereby amplifying total nitrous oxide emissions.
Between 500 and 10,000 Gt of carbon are thought to be stored
under the sea floor in the form methane hydrates (or clathrates),
a crystalline structure of methane gas and water molecules
(Brook et al. 2008). Another 7.5 to 400 Gt of carbon are stored
in the form of methane hydrates trapped in permafrost (Brook et
al. 2008). Some have argued that anthropogenic warming could
raise the possibility of a catastrophic release of methane from
hydrates to the atmosphere. In a recent assessment by the US
Climate Change Science Program (CCSP 2008b), it was deemed
to be very unlikely that such a release would occur this century,
although the same assessment deemed it to be very likely that
methane sources from hydrate and wetland emissions would
increase as the climate warmed. This is supported by a recent
analysis that found that the observed increase in atmospheric
methane 11,600 years ago had a wetland, as opposed to
hydrate, origin (Petrenko et al. 2009); as was also found in
studies using Earth models of intermediate complexity (Fyke and
Weaver 2006; Archer et al. 2009).
Few studies with AR4-type climate models have been
undertaken. One systematic study used the Community Climate
System Model, version 3 (CCSM3) with explicit treatment of
frozen soil processes. The simulated reduction in permafrost
reached 40% by ~2030 irrespective of emission scenario (a
reduction from ~10 million km2 to 6 million km2). By 2050, this
reduces to 4 million km2 (under B1 emissions) and 3.5 million
km2 (under A2 emissions). Permafrost declines to ~1 million km2
by 2100 under A2. In each case, the simulations did not include
additional feedbacks triggered by the collapse of permafrost
including out-gassing of methane, a northward expansion of
shrubs and forests and the activation of the soil carbon pool.
These would each further amplify warming.
Glaciers and ice-caps
– There is widespread evidence of increased melting of glaciers and ice-caps since the mid-1990s.
– The contribution of glaciers and ice-caps to global sea-level has increased from 0.8 millimeters
per year in the 1990s to be 1.2 millimeters per year today.
– The adjustment of glaciers and ice caps to present climate alone is expected to raise sea level by
Under warming conditions they may contribute as much as ~55 centimeters by 2100
Glaciers and mountain ice-caps can potentially contribute a
total of approximately 0.7 meters to global sea-level. Glaciers
and mountain ice-caps also provide a source of freshwater in
many mountain regions worldwide. The IPCC AR4 assessed
the contribution from worldwide shrinking glaciers and ice caps
to sea level rise at the beginning of the 21st Century at about
0.8 millimeters per year (Lemke et al. 2007, Kaser et al. 2006).
Since then, new estimates of the contribution from glaciers and
ice caps have been made using new data and by exploring new
These new assessments are shown in Figure 7. They show
glacier and ice cap contributions to sea level rise that are
generally slightly higher than those reported in IPCC AR4.
They also extend from 1850 up to 2006. These new estimates
show that the mass loss of glaciers and ice caps has increased
considerably since the beginning of the 1990s and now
contribute about 1.2 millimeters per year to global sea level rise.
Glaciers and ice caps are not in balance with the present climate.
Recent estimates show that adjustment to that alone will
cause a mass loss equivalent to ~18 centimeters sea level rise
(Bahr et al. 2009) within this century. Under ongoing changes
consistent with current warming trends, a mass loss of up to
~55 centimeters sea level rise is expected by 2100 (Pfeffer et al.
Figure 7. Estimates of the contribution of glaciers and ice-caps to global change in sea-level equivalent (SLE), in millimeters SLE per year.
Ice-Sheets of Greenland and Antarctica
– The surface area of the Greenland ice sheet which experiences summer melt has increased by
30% since 1979, consistent with warming air temperatures. Melt covered 50% of the ice sheet
during the record season in 2007.
– The net loss of ice from the Greenland ice sheet has accelerated since the mid-1990s and is now
contributing as much as 0.7 millimeters per year to sea level rise due to both increased melting
and accelerated ice flow.
– Antarctica is also losing ice mass at an increasing rate, mostly from the West Antarctic ice sheet
due to increased ice flow.
Antarctica is currently contributing to sea level rise at a rate nearly
equal to Greenland.
Antarctica and Greenland maintain the largest ice reservoirs on land. If completely melted, the Antarctic ice-sheet would raise global sea-level by 52.8 meters, while Greenland would
add a further 6.6 meters. Loss of only the most vulnerable
parts of West Antarctica would still raise sea level by 3.3
meters (Bamber et al,. 2009). IPCC AR4 concluded that net
ice loss from the Greenland and Antarctic ice sheets together
contributed to sea level rise over the period 1993 to 2003 at
an average rate estimated at 0.4 millimeters per year. Since
IPCC AR4, there have been a number of new studies observing
and modelling ice-sheet mass budget that have considerably
enhanced our understanding of ice-sheet vulnerabilities (Allison
et al. 2009). These assessments reinforce the conclusion that the
ice sheets are contributing to present sea level rise, and show
that the rate of loss from both Greenland and Antarctica has
increased recently. Furthermore, recent observations have shown
that changes in the rate of ice discharge into the sea can occur
far more rapidly than previously suspected (e.g. Rignot 2006).
The estimates of the net mass budget of the Greenland Ice Sheet since 1960. A negative mass budget indicates ice loss and sea level rise.
The estimates of the mass budget of the Greenland Ice Sheet since 1960. The data indicate that net ice mass loss from Greenland has been increasing since at least the early 1990s, and that in the 21st Century, the rate of loss has increased significantly. Multiple observational constraints and the use of several different techniques provide confidence that the rate of mass loss from the Greenland ice-sheet has accelerated. Velicogna (2009) used GRACE satellite gravity data to show that the rate of Greenland mass loss doubled over the period from April 2002 to February 2009.
Near-coastal surface melt and run-off have increased significantly since 1960 in response to warming temperature, but total snow precipitation has also increased (Hanna et al. 2008). The
average Greenland surface temperature rose by more than 1.5?C
over the period 2000 to 2006 and mass loss estimated from
GRACE gravity data occurred within 15 days of the initiation
of surface melt, suggesting that the water drains rapidly from
the ice sheet (Hall et al. 2008). Passive microwave satellite
measurements of the area of the Greenland ice sheet subject to
surface melt indicate that the melt area has been increasing since
1979 (Steffen et al. 2008; Figure 9). There is a good correlation
between total melt area extent and the number of melt days with
total volume of run off, which has also increased.
The pattern of ice sheet change in Greenland is one of nearcoastal
thinning, primarily in the south along fast-moving outlet
glaciers. Accelerated flow and discharge from some major outlet
glaciers (also called dynamic thinning) is responsible for much of
the loss (Rignot & Kanagaratnam 2006; Howat et al. 2007). In
southeast Greenland many smaller drainage basins, especially
the catchments of marine-terminating outlet glaciers, are also
contributing to ice loss (Howat et al. 2008). Pritchard et al.
(2009) used high resolution satellite laser altimetry to show
that dynamic thinning of fast-flowing coastal glaciers is now
widespread at all latitudes in Greenland. Greenland glaciers
flowing faster than 100 meters per year thinned by an average of
0.84 meters per year between 2003 and 2007.
Figure 9. The total melt area of the Greenland ice sheet increased by 30% between 1979 and 2008 based on passive microwave satellite data, with the most extreme melt in 2007. In general 33-55% of the total mass loss from the Greenland ice sheet is caused by surface melt and runoff. For 2007, the area experiencing melt was around 50% of the total ice sheet area. The low melt year in 1992 was caused by the volcanic aerosols from Mt. Pinatubo causing a short-lived global cooling (updated from Steffen et al. 2008).
New estimates of the mass budget of the Antarctic Ice Sheet.
Comprehensive estimates for Antarctica
are only available since the early 1990s. Several new studies
using the GRACE satellite gravity data all show net loss from the Antarctic since 2003 with a pattern
of near balance for East Antarctica, and greater mass loss from
West Antarctica and the Antarctic Peninsula (e.g. Chen et
al. 2006; Cazenave et al. 2009). The GRACE assessment of
Velicogna (2009) indicates that, like Greenland, the rate of mass
loss from the Antarctic ice sheet is accelerating, increasing from
104 Gt per year for 2002-2006 to 246 Gt per year for 2006-2009
(the equivalent of almost 0.7 millimeters per year of sea level
rise). Gravity and altimeter observations require correction for
uplift of the Earth?s crust under the ice sheets (glacial isostatic
adjustment): this is poorly known for Antarctica.
The largest losses occurred in the West Antarctic basins draining
into the Bellingshausen and Amundsen Seas. Satellite glacier
velocity estimates from 1974 imagery show that the outlet
glaciers of the Pine Island Bay region have accelerated since then,
changing a region of the ice sheet that was in near-balance to
one of considerable loss (Rignot 2008). Rignot et al. (2008b)
show that the ice discharge in this region further increased
between 1996 and 2006, increasing the net mass loss over
the period by 59%, and Pritchard et al. (2009) show from laser
altimetry that dynamic thinning in some parts of the Amundsen
Sea embayment has exceeded 9 meters per year. The recent
acceleration of ice streams in West Antarctica explains much of
the Antarctic mass loss, but narrow fast-moving ice streams in
East Antarctica are also contributing to the loss (Pritchard et al.
The Antarctic Peninsula region has experienced much greater
warming than the continent as a whole. This has led to
widespread retreat (Cook et al. 2005) and acceleration (Pritchard
& Vaughan 2007) of the tidewater glaciers in that region.
The Risk of Ice-Sheet Collapse
The largest unknown in the projections of sea level rise over the
next century is the potential for rapid dynamic collapse of ice
sheets. The most significant factor in accelerated ice discharge
in both Greenland and Antarctica over the last decade has been
the un-grounding of glacier fronts from their bed, mostly due
to submarine ice melting. Changes to basal lubrication by melt
water, including surface melt draining through moulins (vertical
conduits) to the bottom of the ice sheet, may also affect the
ice sheet dynamics in ways that are not fully understood. The
major dynamic ice sheet uncertainties are largely one-sided:
they can lead to a faster rate of sea-level rise, but are unlikely
to significantly slow the rate of rise. Although it is unlikely that
total sea level rise by 2100 will be as high as 2 meters (Pfeffer et
al. 2008), the probable upper limit of a contribution from the ice
sheets remains uncertain.
– Ice-shelves connect continental ice-sheets to the ocean. Destabilization of ice-shelves along the
Antarctic Peninsula has been widespread with 7 collapses over the past 20 years.
– Signs of ice shelf weakening have been observed elsewhere than in the Antarctic Peninsula,
e.g. in the Bellingshausen and Amundsen seas, indicating a more widespread influence of
atmospheric and oceanic warming than previously thought.
– There is a strong influence of ocean warming on ice sheet stability and mass balance via the
melting of ice-shelves.
Ice shelves are floating sheets of ice of considerable thickness
that are attached to the coast. They are mostly composed of
ice that has flowed from the interior ice sheet, or that has been
deposited as local snowfall. They can be found around 45% of
the Antarctic coast, in a few bays off the north coast of Ellesmere
Island near Greenland, and in a few fiords along the northern
Greenland coast (where they are termed ice tongues). Over the
last few years, the six remaining ice shelves (Serson, Petersen,
Milne, Ayles, Ward Hunt and Markham) off Ellesmere Island
have either collapsed entirely (Ayles on August 13, 2005 and
Markham during the first week of August, 2008) or undergone
Along the coast of Greenland, the seaward extent of the outlet
glacier Jakobshavn Isbrae provides a striking example of a floating
ice tongue in retreat (Figure 11). Holland et al. (2008) suggest
that the observed recent acceleration (Rignot and Kanagaratnam
2006) of Jakobshavn Isbrae may be attributed to thinning from
the arrival of warm waters in the region.
Figure 11. The floating ice tongue representing the seaward extent of Jakobshavn Isbr? on July 7, 2001. Changes in the position of the calving front from 1851 to 2006 are indicated. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio (http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003395/).
Destabilization of floating ice shelves has been widespread
along the Antarctic Peninsula with seven collapsing in the last
20 years. Warming along the Peninsula has been dramatic, and
on the western side has been substantially above the global
average. Most recently, in March 2009, more than 400 square
kilometers collapsed off the Wilkins Ice Shelf on the western
side of the Antarctic Peninsula. A number of mechanisms
are thought to play important roles in destabilizing floating
Antarctic ice shelves. These include: surface warming leading to
the creation of melt ponds and subsequent fracturing of existing
crevasses (van den Broeke 2005); subsurface ice shelf melting
from warming ocean waters (Rignot et al. 2008b); and internal
ice shelf stresses (Bruan and Humbert 2009). While the collapse
of a floating ice shelf does not itself raise sea level, its collapse
is followed by rapid acceleration of glacier outflow ? which does
raise sea level ? due to the removal of the ice shelf buttressing
effect (e.g. Rignot et al. 2004; Scambos et al. 2004).
There is evidence for the melting of ice shelves in the Amundsen
Sea, with impacts on the flow speed of glaciers draining this part
of West Antarctica. A recent modeling study has suggested
that the West Antarctic Ice Sheet would begin to collapse when
ocean temperatures in the vicinity of any one of the ice shelves
that surround it warm by about 5?C (Pollard and DeConto
2009). There is also evidence that these changes are not limited
to West Antarctica and may also affect the coastline of East
Antarctica, for example in Wilkes Land (Pritchard et al. 2009;
Shepherd and Wingham 2007). The widespread thinning and
acceleration of glaciers along the Antarctic coast may indicate
a significant impact of oceanic changes on glacier dynamics, a
factor that has received little attention in past IPCC reports due
to the lack of observational data on ice-ocean interactions and
how climate change might influence coastal ocean waters.
– The observed summer-time melting of Arctic sea-ice has far exceeded the worst-case
projections from climate models of IPCC AR4.
– The warming commitment associated with existing atmospheric greenhouse gas levels means
it is very likely that in the coming decades the summer Arctic Ocean will become ice-free,
although the precise timing of this remains uncertain.
– Satellite observations show a small increase of Antarctic sea-ice extent and changes to
seasonality, although there is considerable regional variability. This is most likely due to
changes in Southern Ocean winds associated with stratospheric ozone-depletion.
Arctic Sea Ice
Perhaps the most stunning observational change since the IPCC
AR4 has been the shattering of the previous Arctic summer
minimum sea ice extent record ? something not predicted by
climate models. Averaged over the five-day period leading up
to September 16, 2007, the total extent of sea ice in the Arctic
was reduced to an area of only 4.1 million square kilometers (see
Figure 12), surpassing the previous minimum set in 2005 by 1.2
million square kilometers (about the same size as France, Spain,
Portugal, Belgium and Netherlands combined). The median
September minimum sea ice extent since observations with the
current generation of multi-frequency passive microwave sensors
commenced in 1979 to 2000 was 6.7 million square kilometers.
Compared to the median, the 2007 record involved melting 2.6
million square kilometers more ice (~40% of the median).
Figure 12. Arctic sea ice extent over the five days leading up to and including September 16, 2007 compared to the average sea-ice minimum extent for the period 1979- 2006. Sourced from the NASA/Goddard Space Flight Center Scientific Visualization Studio.
The September Arctic sea ice extent over the last several decades
has decreased at a rate of 11.1 ? 3.3%/decade (NSIDC 2009).
This dramatic retreat has been much faster than that simulated
by any of the climate models assessed in the IPCC AR4 (Figure
13). This is likely due to a combination of several model
deficiencies, including: 1) incomplete representation of ice albedo
physics, including the treatment of melt ponds (e.g., Pedersen
et al. 2009) and the deposition of black carbon (e.g. Flanner et
al. 2007; Ramanathan and Carmichael 2008); and 2) incomplete
representation of the physics of vertical and horizontal mixing in
the ocean (e.g. Arzel et al. 2006). Winter Arctic sea ice extent
has also decreased since 1979, but at a slower rate than in
summer. The February extent has decreased at a rate of 2.9 ?
0.8%/decade (NSIDC 2009).
The thickness of Arctic sea ice has also been on a steady decline
over the last several decades. For example, Lindsay et al. (2009)
estimated that the September sea ice thickness has been
decreasing at a rate of 57 centimeters per decade since 1987.
Similar decreases in sea-ice thickness have been detected in
winter. For example, within the area covered by submarine sonar
measurements, Kwok and Rothrock (2009) show that the overall
mean winter thickness of 3.64 meters in 1980 decreased to only
1.89 meters by 2008 ? a net decrease of 1.75 meters, or 48%.
By the end of February 2009, less than 10% of Arctic sea ice was
more than two years old, down from the historic values of 30%.
When Will the Arctic Ocean be Ice-Free?
Due to the existence of natural variability within the climate
system, it is not possible to predict the precise year that the
Arctic Ocean will become seasonally ice free. Nevertheless, the
warming commitment associated with existing atmospheric
greenhouse gas levels very likely means that a summer ice-free
Arctic is inevitable. Evidence is also emerging to suggest that
the transition to an ice-free summer in the Arctic might be
expected to occur abruptly, rather than slowly (Holland et al.
2006), because of amplifying feedbacks inherent within the Arctic
climate system. In fact, in one of the simulations of the NCAR
Climate System Model version 3 (CCSM3) discussed in Holland
et al (2006), the Arctic summer became nearly ice-free by 2040.
As noted by Lawrence et al. (2008), an abrupt reduction in Arctic
summer sea ice extent also triggers rapid warming on land and
subsequent permafrost degradation.
Antarctic Sea Ice
Unlike the Arctic, Antarctic sea-ice extent changes have been
more subtle, with a net annual-mean area increase of ~1% per
decade over the period 1979?2006 (Cavalieri and Parkinson
2008; Comiso and Nishio 2008). There have however been
large regional changes in Antarctic sea-ice distribution: for
example, the Weddell and Ross Sea areas have shown increased
extent linked to changes in large-scale atmospheric circulation,
while the western Antarctic Peninsula region and the coast of
West Antarctica (Amundsen and Bellingshausen Seas) show a
significant decline consistent with more northerly winds and
surface warming observed there (Lefebvre et al. 2004; Turner et
al. 2009; Steig et al. 2009). These regional changes are linked to
a major change in the seasonality of the ice; that is, its duration
and the timing of the annual advance and retreat (Stammerjohn
et al. 2008).
Since Antarctica is a land mass surrounded by the vast Southern
Ocean, whereas the Arctic is a small ocean surrounded by
vast amounts of land, and as oceans respond less rapidly
than land to warming because of their thermal stability, one
would expect, and indeed climate models show, a delayed warming response around Antarctica.
Figure 13. Observed (red line) and modeled September Arctic sea ice extent in millions of square kilometers. The solid black line gives the ensemble mean of the 13 IPCC AR4 models while the dashed black lines represent their range. From Stroeve et al. (2007) updated to include data for 2008.
The 2009 minimum has recently been calculated at 5.10 million km2, the third lowest year on record, and still well below the IPCC worst case scenario.
In addition, Turner et al. (2009) note that stratospheric ozone depletion arising from
the anthropogenic release of chlorofluorocarbons (CFCs) has
led to the strengthening of surface winds around Antarctica
during December to February (summer). They argue that
these strengthened winds are in fact the primary cause for
the slight positive trend in Antarctic sea ice extent observed
over the last three decades. However, as CFCs are regulated
under the Montreal Protocol and have declining atmospheric
concentrations, the ozone hole over Antarctica is expected to
recover and hence one anticipates an acceleration of sea ice melt
in the Southern Hemisphere in the decades ahead.
There are few data available on the thickness distribution of
Antarctic pack ice, and no information on any changes in the
thickness of Antarctic sea ice.
Isn?t Antarctica cooling and Antarctic sea ice increasing?
Antarctica is not cooling: it has warmed overall over at least the past 50 years. Although the weather station at the South Pole shows cooling over this period, this single weather station is not representative. For example, there is a warming trend at Vostok, the only other long-term monitoring station in the interior of the continent. Several independent analyses (Chapman and Walsh 2008; Monaghan et al. 2008; Goosse et al. 2009; Steig et al. 2009)
show that on average, Antarctica has warmed by about 0.5?C since wide-scale measurements began in the 1957 International Geophysical Year, with particularly rapid warming around the Antarctic Peninsula region and over the West Antarctic Ice Sheet (Figure 14 shows the mean trend from 1957-2006). Furthermore, there is direct evidence from borehole measurements that warming in West Antarctica began no later than the 1930s (Barrett et al. 2009).
Since the development of the Antarctic ozone hole in the late 1970s, there has been a strengthening of the
circumpolar winds around Antarctica, which tends to reduce the amount of warmer air reaching the interior of
the continent. The stronger winds are due to cooling in the upper atmosphere, which are in turn a result of ozone
depletion caused by chlorofluorocarbons. As a consequence, much of East Antarctica has cooled in the summer and autumn seasons since the late 1970s. Ironically, human emissions of CFCs are thus helping to partly offset interior Antarctic warming, analogous to the global dimming due to sulphate aerosols. As the ozone hole gradually repairs over the coming century, the cooling offset is likely to diminish.
The factors that determine sea ice extent around Antarctica are very different from those in the Arctic, because Antarctica is a continent sited around the pole and surrounded by water, just the opposite of the Arctic geography. The extent of sea ice around Antarctica is strongly determined by the circumpolar winds which spread the ice out from the continent, and by the position of the polar front where the ice encounters warmer ocean waters. Sea ice cover in Antarctica shows a slight upward trend, consistent with the increase in circumpolar winds mentioned above. In West Antarctica, where the temperature increases are the greatest, sea ice has declined at a statistically significant rate since at least the 1970s.
– Estimates of ocean heat uptake have converged and are found to be 50% higher than previous
– Global ocean surface temperature reached the warmest ever recorded for each of June, July
and August 2009.
– Ocean acidification and ocean de-oxygenation have been identified as potentially devastating
for large parts of the marine ecosystem.
Detection of how climate change is impacting the oceans has
improved markedly since the IPCC AR4. Significant changes in
temperature, salinity and biogeochemical properties have been
measured. These changes are consistent with the observed 50-
year warming, rainfall and CO2 trends in the atmosphere. There
have also been important new analyses of the trends in a broader
range of properties since the IPCC AR4, including acidification
and oxygen. This has improved our understanding of the
changing state of the oceans and also identified new issues.
Where new estimates of ocean change exist since IPCC AR4,
they tend to be larger and also more consistent with projections
of climate change (e.g., global heat content).
There has been a long-term sustained warming trend in ocean
surface temperatures over the past 50 years (Figure 15). Satellite
measurements for the surface ocean showed 2007 to be the
warmest year ever recorded, despite the extremely strong El Ni?o
of 1997/1998. The year 2008 was cooler due to an intense
temporary La-Ni?a event, whereas ocean temperatures up until
the time of publication are tracking toward record warmth in
2009. For example, global ocean surface temperature was the
warmest ever recorded for June, for July and for August in 2009.
Increases in oceanic heat content in the upper ocean (0-700m)
between 1963 and 2003 have been found to be 50% higher than
previous estimates (Domingues et al. 2008, Bindoff et al. 2007).
The higher estimates of heat content change are now consistent
with observations of sea-level rise over the last 50 years,
resolving a long standing scientific problem in understanding
the contribution of thermal expansion to sea-level (Domingues
et al. 2008). Observations also show deep-ocean warming that
is much more widespread in the Atlantic and Southern Oceans
(Johnson et al. 2008a, Johnson et al. 2008b) than previously
Salinity and the Hydrological Cycle
More comprehensive analyses of ocean salinity show a
freshening of high latitudes, while regions of excess evaporation
over precipitation have become saltier. The salinity changes are
consistent with a strengthening of the hydrological cycle. The
patterns of salinity change are also consistent with regional
circulation and inter-basin exchanges. We now have increased
evidence that the long-term trends in patterns of rainfall over the
global ocean, as reflected in salinity, can be attributed to human
influence (Stott et al. 2008).
Climate Change and Ocean Circulation
Surprising salinity changes in Antarctic bottom waters provide
additional evidence of increased melt from the ice-sheets and ice
shelves (Rintoul 2007). The Arctic shows strong evidence for
increased precipitation and river run-off. Intermediate layers in
the Arctic Ocean have warmed notably (Polyakov et al. 2004).
Consistent with current model results, observations are yet
to detect any indication of a sustained change in the North
Atlantic Ocean circulation (e.g. Hansen and ?sterhus 2007).
Regional climate change is often organized and expressed around
the main patterns of variation such as the North Atlantic
Oscillation, El Ni?o, and the Southern Annular Mode. These
patterns themselves may be affected by greenhouse gases,
leading to either larger fluctuations, or a preferred state in
coming decades (e.g., a trend toward a different type of El Ni?o
event, Yeh et al. 2009; Latif and Keenlyside 2009). Currently the
influence of regional climate modes on ocean circulation is larger
than the underlying trends attributable to anthropogenic climate
The stability of the North Atlantic Ocean circulation is vitally
important for North American and European climate. For
example, a slowdown of these ocean currents could lead to
a more rapid rise of regional sea level along the northeast US
coast (Yin et al. 2009). The IPCC AR4 concluded that there is
greater than 90% probability of a slowdown of this ocean current
system, and less than 10% risk of a ?large abrupt transition?
by the year 2100. As noted in the Synthesis and Assessment
Project 3.4 of the US Climate Change Science Program (Delworth
et al. 2008), no comprehensive climate model projects such a
transition within this century. However, given uncertainty in
our ability to model nonlinear threshold behaviour, and the
recent suggestion that models may be too stable (Hofman and
Rahmstorf 2009) we cannot completely exclude the possibility of
such an abrupt transition.
Ocean Acidification, Carbon Uptake and Ocean De-oxygenation
The CO2 content of the oceans increased by 118 ? 19 Gt (1
Gt = 109 tons) between the end of the pre-industrial period
(about 1750) and 1994, and continues to increase by about
2 Gt each year (Sabine et al. 2004). The increase in ocean CO2
has caused a direct decrease in surface ocean pH by an average
of 0.1 units since 1750 and an increase in acidity by more than
30% (Orr et al. 2005: McNeil and Matear 2007; Riebesell, et al.
2009). Calcifying organisms and reefs have been shown to be
particularly vulnerable to high CO2, low pH waters (Fabry et al.
New in-situ evidence shows a tight dependence between
calcification and atmospheric CO2, with smaller shells evident
during higher CO2 conditions over the past 50,000 years (Moy
et al. 2009). Furthermore, due to pre-existing conditions, the
polar regions of the Arctic and Southern Oceans are expected to
start dissolving certain shells once the atmospheric levels reach
450ppm (~2030 under business-as-usual; McNeil and Matear
2008: Orr et al. 2009).
There is new evidence for a continuing decrease in dissolved
oxygen concentrations in the global oceans (Oschlies et al.
2008), and there is for the first time significant evidence that the
large equatorial oxygen minimum zones are already expanding
in a warmer ocean (Stramma et al. 2008). Declining oxygen is a
stress multiplier that causes respiratory issues for large predators
(Rosa and Seibel 2008) and significantly compromises the ability
of marine organisms to cope with acidification (Brewer 2009).
Increasing areas of marine anoxia have profound impacts on the
marine nitrogen cycle, with yet unknown global consequences
(Lam et al. 2009). A recent modeling study (Hofmann and
Schellnhuber 2009) points to the risk of a widespread expansion
of regions lacking in oxygen in the upper ocean if increases in
atmospheric CO2 continue.
Trend in ocean surface temperature (?C, 1959 ? 2008)
Long-term 50-year change in sea surface temperature (SST) during 1959-2008 calculated by fitting a linear trend to 50 years of monthly SST data at each grid point. The SST fields are from the Hadley Centre data set as described by Rayner et al. (2006).
Global Sea Level
– Satellite measurements show sea-level is rising at 3.4 millimeters per year since these
records began in 1993. This is 80% faster than the best estimate of the IPCC Third
Assessment Report for the same time period.
– Accounting for ice-sheet mass loss, sea-level rise until 2100 is likely to be at least twice as
large as that presented by IPCC AR4, with an upper limit of ~2m based on new ice-sheet
Population densities in coastal regions and on islands are about
three times higher than the global average. Currently 160 million
people live less than 1 meter above sea level. This allows even
small sea level rise to have significant societal and economic
impacts through coastal erosion, increased susceptibility to
storm surges and resulting flooding, ground-water contamination
by salt intrusion, loss of coastal wetlands, and other issues.
Since 1870, global sea level has risen by about 20 centimeters
(IPCC AR4). Since 1993, sea level has been accurately measured
globally from satellites. Before that time, the data come from
tide gauges at coastal stations around the world. Satellite and
tide-gauge measurements show that the rate of sea level rise
has accelerated. Statistical analysis reveals that the rate of rise
is closely correlated with temperature: the warmer it gets, the
faster sea level rises (Rahmstorf 2007).
Sea level rise is an inevitable consequence of global warming
for two main reasons: ocean water expands as it heats up, and
additional water flows into the oceans from the ice that melts on
land. For the period 1961-2003, thermal expansion contributed
~40% to the observed sea level rise, while shrinking mountain
glaciers and ice sheets have contributed ~60% (Domingues et
Figure 16. Sea level change during 1970-2010. The tide gauge data are indicated in red (Church and White 2006) and satellite data in blue (Cazenave et al. 2008). The grey band shows the projections of the IPCC
Third Assessment report for comparison.
Sea level has risen faster than expected (Rahmstorf et al.
2007), see Figure 16. The average rate of rise for 1993-2008 as
measured from satellite is 3.4 millimeters per year (Cazenave
et al. 2008), while the IPCC Third Assessment Report (TAR)
projected a best estimate of 1.9 millimeters per year for the same
period. Actual rise has thus been 80% faster than projected by
models. (Note that the more recent models of the 2007 IPCC
report still project essentially the same sea level rise as those of
the TAR, to within 10%.)
Future sea level rise is highly uncertain, as the mismatch
between observed and modeled sea level already suggests. The
main reason for the uncertainty is in the response of the big ice
sheets of Greenland and Antarctica.
Sea level is likely to rise much more by 2100 than the often-cited
range of 18-59 centimeters from the IPCC AR4. As noted in
the IPCC AR4, the coupled models used in developing the 21st
century sea level projections did not include representations of
dynamic ice sheets. As such, the oft-cited 18-59 centimeters
projected sea level rise only included simple mass balance
estimates of the sea level contribution from the Greenland and
Antarctic ice sheets. As a consequence of an assumed positive
mass balance over the Antarctic ice sheet in the AR4, Antarctica
was estimated to have contributed to global sea level decline
during the 21st century in that report. However, the Antarctic
Ice Sheet is currently losing mass as a consequence of dynamical
processes. Based on a number of
new studies, the synthesis document of the 2009 Copenhagen
Climate Congress (Richardson et al. 2009) concluded that
?updated estimates of the future global mean sea level rise are
about double the IPCC projections from 2007.?
Sea level will continue to rise for many centuries after global
temperature is stabilized, since it takes that much time for the
oceans and ice sheets to fully respond to a warmer climate. These
estimates highlight the fact that unchecked global warming is
likely to raise sea level by several meters in coming centuries,
leading to the loss of many major coastal cities and entire island
Future projections are from Rahmstorf (2007) and WBGU (2006), ?Delta Committee? are from Vellinga et al.,
Abrupt change and tipping points
– There are several elements in the climate system that could pass a tipping point this century
due to human activities, leading to abrupt and/or irreversible change.
– 1 ?C global warming (above 1980-1999) carries moderately significant risks of passing largescale
tipping points, and 3 ?C global warming would give substantial or severe risks.
– There are prospects for early warning of approaching tipping points, but if we wait until a
transition begins to be observed, in some cases it would be unstoppable.
What is a tipping point?
A tipping point is a critical threshold at which the future state of
a system can be qualitatively altered by a small change in forcing
(Lenton et al. 2008; Schellnhuber 2009). A tipping element is
a part of the Earth system (at least sub-continental in scale)
that has a tipping point (Lenton et al. 2008). Policy-relevant
tipping elements are those that could be forced past a tipping
point this century by human activities. Abrupt climate change
is the subset of tipping point change which occurs faster than
its cause. Tipping point change also includes transitions that
are slower than their cause (in both cases the rate is determined
by the system itself). In either case the change in state may
be reversible or irreversible. Reversible means that when the
forcing is returned below the tipping point the system recovers
its original state, either abruptly or gradually. Irreversible means
that it does not (it takes a larger change in forcing to recover).
Reversibility in principle does not mean that changes will be
reversible in practice. A tipping element may lag anthropogenic
forcing such that once a transition begins to be observed, a
much larger change in state is already inevitable.
Figure 18. Map of some of the potential policy-relevant tipping elements in the Earth?s climate system overlain on population density. Question marks indicate systems whose status as tipping elements is particularly uncertain. There are other potential tipping elements that are missing from the map, for example shallow-water coral reefs (Veron et al. 2009) threatened in part by ocean acidification (see Oceans chapter).
Are there tipping points in the Earth?s climate system?
There are a number of tipping points in the climate system,
based on understanding of its non-linear dynamics, and as
revealed by past abrupt climate changes and model behavior
(Pitman and Stouffer 2007; Schellnhuber 2009). Some models
pass tipping points in future projections, and recent observations
show abrupt changes already underway in the Arctic. Recent
work has identified a shortlist of nine potential policy-relevant
tipping elements in the climate system that could pass a tipping
point this century and undergo a transition this millennium
under projected climate change (Lenton et al. 2008). These are
shown with some other candidates in Figure 18.
Which ones are of the greatest concern? How has
this been assessed?
The tipping points of greatest concern are those that are
the nearest (least avoidable) and those that have the largest
negative impacts. Generally, the more rapid and less reversible a
transition is, the greater its impacts. Additionally, any amplifying
feedback to global climate change may increase concern, as can
interactions whereby tipping one element encourages tipping
another. The proximity of some tipping points has been assessed
through expert elicitation (Lenton et al. 2008; Kriegler et al.
2009). Proximity, rate and reversibility have been also assessed
through literature review (Lenton et al. 2008), but there is a need
for more detailed consideration of impacts. Some of the most
concerning regions and their tipping elements are now discussed:
Arctic: The Greenland ice sheet (GIS) may be nearing a tipping
point where it is committed to shrink (Lenton et al. 2008;
Kriegler et al. 2009). Striking amplification of seasonal melt was
observed in 2007 associated with record Arctic summer sea-ice
loss (Mote 2007). Once underway the transition to a smaller
Greenland ice cap will have low reversibility, although it is likely
to take several centuries (and is therefore not abrupt). The
impacts via sea level rise will ultimately be large and global, but
will depend on the rate of ice sheet shrinkage.
Antarctic: The West Antarctic ice sheet (WAIS) is currently
assessed to be further from a tipping point than the GIS, but this
is more uncertain (Lenton et al. 2008; Kriegler et al. 2009). The
WAIS has the potential for more rapid change and hence greater
impacts. The loss of ice-shelves around the Antarctic Peninsula,
such as Larsen B, followed by the acceleration of glaciers they
were buttressing, highlights a mechanism that could threaten
parts of the WAIS. The main East Antarctic ice sheet (EAIS) is
thought to be more stable than the WAIS. However, there is
evidence that changes are taking place along its marine sector,
which drains more ice than all of West Antarctica.
Amazonia: The Amazon rainforest experienced widespread
drought in 2005 turning the region from a sink to a source
(0.6 – 0.8 Gt C per year) of carbon (Phillips et al. 2009). If
anthropogenic-forced lengthening of the dry season continues
(Vecchi et al. 2006), and droughts increase in frequency or
severity (Cox et al. 2008), the system could reach a tipping point
resulting in dieback of up to ~80% of the rainforest (Cox et al.
2004; Scholze et al. 2006; Salazar et al. 2007; Cook and Vizy
2008), and its replacement by savannah. This could take a few
decades, would have low reversibility, large regional impacts, and
knock-on effects far away. Widespread dieback is expected in a
>4 ?C warmer world (Kriegler 2009), and it could be committed
to at a lower global temperature, long before it begins to be
observed (Jones et al. 2009).
West Africa: The Sahel and West African Monsoon (WAM)
have experienced rapid but reversible changes in the past
including devastating drought from the late 1960s through the
1980s. Forecast future weakening of the Atlantic thermohaline
circulation contributing to ?Atlantic Ni?o? conditions, including
strong warming in the Gulf of Guinea (Cook and Vizy 2006),
could disrupt the seasonal onset of the WAM (Chang et al.
2008) and its later ?jump? northwards (Hagos 2007) into the
Sahel. Perversely, if the WAM circulation collapses, this could
lead to wetting of parts of the Sahel as moist air is drawn in
from the Atlantic to the West (Cook and Vizy 2006; Patricola
and Cook 2008), greening the region in what would be a rare
example of a positive tipping point.
India: The Indian Summer Monsoon is probably already being
disrupted (Ramanathan et al. 2005; Meehl et al. 2008) by an
atmospheric brown cloud haze that sits over the sub-continent
and, to a lesser degree, the Indian Ocean. This haze is comprised
of a mixture of soot, which absorbs sunlight, and some reflecting
sulfate. It causes heating of the atmosphere rather than the land
surface, weakening the seasonal establishment of a land-ocean
temperature gradient which is critical in triggering monsoon
onset (Ramanathan 2005). In some future projections, brown
cloud haze forcing could lead to a doubling of drought frequency
within a decade (Ramanathan 2005) with large impacts,
although transitions should be highly reversible.
Several other candidate tipping elements and mechanisms
could become a major concern, for example, carbon loss from
permafrost. Recently it has been suggested that a region of
permafrost known as the Yedoma, which stores up to ~500 Gt
C (Zimov et al. 2006) could be tipped into irreversible breakdown
driven by internal, biochemical heat generation (Khvorostyanov
et al. 2008a, 2008b). However, the tipping point is estimated to
be relatively distant.
How do tipping points relate to amplifying feedbacks on climate change?
Tipping points are often confused with the phenomenon of
amplifying feedbacks on climate change. All tipping elements
must have some strong amplifying feedback ? detailed elsewhere
(Lenton et al. 2008) ? in their own internal or regional climate
dynamics in order to exhibit a threshold, but they need not
have an amplifying feedback to global climate change. Tipping
elements that could have an amplifying feedback to global
climate change include the Amazon rainforest (dieback would
make it a CO2 source, which could ultimately release up to ~100
Gt C), the thermohaline circulation (weakening or collapse would
lead to net out-gassing of CO2), and the Yedoma permafrost
(release of up to ~500 Gt C). Tipping elements that could have
a diminishing feedback on global climate change include boreal
forest (dieback would release CO2 but this would be outweighed
by cooling due to increased land surface albedo from unmasked
snow cover; Betts 2000), and the Sahel/Sahara (greening would
take up CO2 and probably increase regional cloud cover).
Should we be concerned about global amplifying
Amplifying feedbacks from individual tipping elements are mostly
fairly weak at the global scale. However, other (non tipping
element) amplifying feedbacks, including a potential future
switch in the average response of the land biosphere from a
CO2 sink to a CO2 source, could significantly amplify CO2 rise
and global temperature on the century timescale (Friedlingstein
et al. 2006). The Earth?s climate system is already in a state of
strong amplifying feedback from relatively fast physical climate
responses (Bony et al. 2006) (e.g. water vapor feedback). In
any system with strong amplifying feedback, relatively small
additional feedbacks can have a disproportionate impact on the
global state (in this case, temperature), because of the non-linear
way in which amplifiers work together.
Is there a global tipping point?
A global tipping point can only occur if a net amplifying feedback
becomes strong enough to produce a threshold whereby the
global system is committed to a change in state, carried by its
own internal dynamics. Despite much talk in the popular media
about such ?runaway? climate change there is as yet no strong
evidence that the Earth as a whole is near such a threshold.
Instead ?amplified? climate change is a much better description of
what we currently observe and project for the future.
Which anthropogenic forcing agents are dangerous?
The total cumulative emissions of CO2 (and other long-lived
greenhouse gases) determine long-term committed climate
changes and hence the fate of those tipping elements that
are sensitive to global mean temperature change, are slow to
respond, and/or have more distant thresholds. Key examples are
the large ice sheets (GIS and WAIS). Uneven sulfate (Rotstatyn
and Lohmann 2002) and soot (Ramanathan 2005; Ramanathan
and Carmichael 2008) aerosol forcing are most dangerous for
monsoons. Soot deposition on snow and ice (Ramanathan
and Carmichael 2008; Flanner et al. 2007) is a key danger to
Arctic tipping elements as it is particularly effective at forcing
melting (Flanner et al. 2007). Increasing soot aerosol, declining
sulfate aerosol (Shindell and Faluvegi 2009), and increasing
short-lived greenhouse gases (Hansen et al. 2007) (methane
and tropospheric ozone) have also contributed to rapid Arctic
warming, and together far outweigh the CO2 contribution. The
current mitigation of SO2 emissions and hence sulfate aerosol is
a mixed blessing for climate tipping elements, it may for example
be benefiting the Sahel region (Rotstayn and Lohmann 2002) but
endangering the Amazon (Cox et al. 2008) and the Arctic sea-ice
(Shindell and Faluvegi 2009). Land cover change may also drive
large areas of continents from being relatively robust to climate
change to being highly vulnerable.
Is there any prospect for early warning of an approaching tipping point?
Recent progress has been made in identifying and testing generic
potential early warning indicators of an approaching tipping
point (Lenton et al. 2008; Livina and Lenton 2007; Dakos et al.
2008; Lenton et al. 2009; Scheffer et al. 2009). Slowing down
in response to perturbation is a nearly universal property of
systems approaching various types of tipping point (Dakos et al.
2008; Scheffer et al. 2009). This has been successfully detected
in past climate records approaching different transitions (Livina
and Lenton 2007; Dakos et al. 2008), and in model experiments
(Livina and Lenton 2007; Dakos et al. 2008; Lenton et al.
2009). Flickering between states may also occur prior to a more
permanent transition (Bakke et al. 2009). Other early warning
indicators are being explored for ecological tipping points (Biggs
et al. 2009), including increasing variance (Biggs et al. 2009),
skewed responses (Biggs et al. 2009; Guttal and Jayaprakash
2008) and their spatial equivalents (Guttal and Jayaprakash
2009). These could potentially be applied to anticipating climate
Lessons from the past
– The reconstruction of past climate reveals that the recent warming observed in the Arctic,
and in the Northern Hemisphere in general, are anomalous in the context of natural
climate variability over the last 2000 years.
– New ice-core records confirm the importance of greenhouse gases for past temperatures on
Earth, and show that CO2 levels are higher now than they have ever been during the last
Reconstructing the last two millennia
Knowledge of climate during past centuries can help us to
understand natural climate change and put modern climate
change into context. There have been a number of studies to
reconstruct trends in global and hemispheric surface temperature
over the last millennium (e.g. Mann et al. 1998; Esper et al.
2002; Moberg et al. 2005), all of which show recent Northern
Hemisphere warmth to be anomalous in the context of at least
the past millennium, and likely longer (Jansen et al. 2007). The
first of these reconstructions has come to be known as the
?hockey stick? reconstruction (Mann et al. 1998, 1999). Some
aspects of the hockey stick reconstruction were subsequently
questioned, e.g. whether the 20th century was the warmest
at a hemispheric average scale (Soon and Baliunas 2003),
and whether the reconstruction is reproducible, or verifiable
(McIntyre and McKitrick 2003), or might be sensitive to the
method used to extract information from tree ring records
(McIntyre and McKitrick 2005a,b). Whilst these criticisms have
been rejected in subsequent work (e.g. Rutherford et al. 2005;
Wahl and Ammann 2006, 2007; Jansen et al. 2007) the US
National Research Council convened a committee to examine
the state of the science of reconstructing the climate of the
past millennium. The NRC report published in 2006 largely
supported the original findings of Mann et al. (1998, 1999) and
recommended a path toward continued progress in this area
Mann et al. (2008) addressed the recommendations of the NRC
report by reconstructing surface temperature at a hemispheric
and global scale for much of the last 2,000 years using a
greatly expanded data set for decadal-to-centennial climate
changes, along with recently updated instrumental data and
complementary methods that have been thoroughly tested and
validated with climate model simulations.
Figure 19. Comparison of various Northern Hemisphere temperature reconstructions, with estimated 95% confidence intervals shown (from Mann et al. 2008).
Their results extend previous studies and conclude that recent Northern Hemisphere
surface temperature increases are likely anomalous in a long-term
context (Figure 19).
Kaufman et al. (2009) independently concluded that recent
Arctic warming is without precedent in at least 2000 years
(Figure 20) reversing a long-term millennial-scale cooling trend
caused by astronomical forcing (i.e. orbital cycles). Warmth
during the peak of the ?Medieval Climate Anomaly? of roughly
AD 900-1100 may have rivalled modern warmth for certain
regions such as the western tropical Pacific (Oppo et al. 2009),
and some regions neighbouring the North Atlantic (Mann et al.
in-press). However, such regional warming appears to reflect a
redistribution of warmth by changes in atmospheric circulation,
and is generally offset by cooling elsewhere (e.g. the eastern
and central tropical Pacific) to yield hemispheric and global
temperatures that are lower than those of recent decades.
Ice Core Records of Greenhouse Gases
Changes in past atmospheric Carbon Dioxide (CO2) and Methane
(CH4) concentrations can be determined by measuring the
composition of air trapped in ice cores and through the analyses
of leaf stomata density and geochemical analyses of marine
The Dome Concordia (Dome C) ice core CO2 and CH4 records,
drilled by the European Project for Ice Coring in Antarctica
(EPICA), were published in 2004 and 2005 detailing events
back to 440,000 years and 650,000 years respectively (EPICA
community members 2004; Siegenthaler et al. 2005). In 2008
the record was extended to 800,000 years (L?thi et al. 2008;
Loulergue et al. 2008). The newly extended records reveal that
current greenhouse gas levels (~385ppm) are at least 40%
higher than at any time over the past 800,000 years. We must
travel back at least two to three million years, and perhaps as far
as fifteen million years, to the Pliocene and Miocene epochs of
geological time to find equivalent greenhouse gas levels in the
atmosphere (Haywood et al. 2007; Raymo et al. 1996; K?rschner
et al. 1996; Tripati et al. 2009).
Strong correlations of CH4 and CO2 with temperature
reconstructions are maintained throughout the new 800,000
year record (L?thi et al. 2008; Loulergue et al. 2008). Temperature
warming typically comes before increases in atmospheric CO2
over the ice-core record. This finding is consistent with the view
that natural CO2 variations constitute a feedback in the glacialinterglacial
cycle rather than a primary cause (Shackleton 2000);
something that has recently been explained in detail with the
help of climate model experiments (Ganopolski and Roche 2009).
Changes in the Earth?s orbit around the Sun are the pacemaker
for glacial-interglacial cycles (Hays et al. 1976; Berger 1978), but
these rather subtle orbital changes must be amplified by climate
feedbacks in order to explain the large differences in global
temperature and ice volume, and the relative abruptness of the
transitions between glacial and interglacial periods (Berger et al.
1998; Clark et al. 1999).
Palaeo Constraints on Climate and Earth System Sensitivity
One of the key questions for climate research is to determine
how sensitively the Earth?s climate responds to a given change
Figure 20. Blue line: estimates of Arctic air temperatures over the last 2,000 years based on proxy records from lake sediments, ice cores and tree rings. The green line shows the best fit long-term cooling trend for the period ending 1900. The red line shows the recent warming based on actual observations. (Courtesy Science, modified by the University Corporation for Atmospheric Research).
Isn?t climate always changing, even without human interference?
Of course. But past climate changes are no cause for complacency; indeed, they tell us that the Earth?s climate is very sensitive to changes in forcing. Two main conclusions can be drawn from climate history:
Climate has always responded strongly if the radiation balance of the Earth was disturbed. That suggests the same will happen again, now that humans are altering the radiation balance by increasing greenhouse gas concentrations. In fact, data from climate changes in the Earth?s history have been used to quantify how strongly a given change in the radiation balance alters the global temperature (i.e., to determine the climate sensitivity). The data confirm that our climate system is as sensitive as our climate models suggest, perhaps even more so. Impacts of past climate changes have been severe. The last great Ice Age, when it was globally 4-7 ?C colder than now, completely transformed the Earth?s surface and its ecosystems, and sea level was 120 meters lower. When the Earth last was 2-3 ?C warmer than now, during the Pliocene 3 million years ago, sea level was 25-35 meters higher due to the smaller ice sheets present in the warmer climate.
Despite the large natural climate changes, the recent global warming does stick out already. Climate reconstructions suggest that over the past two millennia, global temperature has never changed by more than 0.5 ?C in a century (e.g. Mann et al. 2008; and references therein).
in our planet?s radiation budget. This is often described by
the ?Climate Sensitivity?, defined as the equilibrium global
temperature response to a doubling of atmospheric CO2
IPCC AR4 summarizes the research aimed at characterizing
the uncertainty in climate sensitivity (e.g. Andronova and
Schlesinger 2001; Frame et al. 2005; Annan and Hargreaves
2006) by stating that ?climate sensitivity is likely to lie in the
range 2?C to 4.5?C, with a most likely value of about 3?C?. More
recent studies have agreed with this assessment (e.g. Knutti and
Hegerl 2008). These estimates of climate sensitivity have also
been used to determine the likely impacts, both environmental
and social/economic, of various CO2 stabilization scenarios,
or the level of greenhouse gas emissions consistent with
stabilization of the global mean temperature below a certain
value (e.g. Meinshausen et al. 2009; this document section
?Mitigating global warming?).
Are we just in a natural warming phase, recovering from the ?little ice age??
No. A ?recovery? of climate is not a scientific concept, since the climate does not respond like a pendulum that swings back after it was pushed in one direction. Rather, the climate responds like a pot of water on the stove: it can only get warmer if you add heat, according to the most fundamental law of physics, conservation of energy. The Earth?s heat budget (its radiation balance) is well understood. By far the biggest change in the radiation balance over the past 50 years, during which three quarters of global warming has occurred, is due to the human-caused increase in greenhouse gas concentrations (see above). Natural factors have had a slightly cooling effect during this period.
Global temperatures are now not only warmer than in the 16th-19th centuries, sometimes dubbed the ?the little ice age? (although this term is somewhat misleading in that this largely regional phenomenon has little in common with real ice ages). Temperatures are in fact now globally warmer than any time in the past 2000 years ? even warmer than in the ?medieval optimum? a thousand years ago (see Figure 19). This is a point that all global climate reconstructions by different groups of researchers, based on different data and methods, agree upon.
In climate history, didn?t CO2 change in response to temperature, rather than the other way round?
It works both ways: CO2 changes affect temperature due to the greenhouse effect, while temperature changes affect CO2 concentrations due to the carbon cycle response. This is what scientists call a feedback loop. If global temperatures are changed, the carbon cycle will respond (typically with a delay of centuries). This can be seen during the ice age cycles of the past 3 million years, which were caused by variations in the Earth?s orbit (the so-called Milankovich cycles). The CO2 feedback amplified and globalized these orbital climate changes: without the lowered CO2 concentrations and reduced greenhouse effect, the full extent of ice ages cannot be explained, nor can the fact that the ice ages occurred simultaneously in both hemispheres. The details of the lag-relationship of temperature and CO2 in Antarctic records have recently been reproduced in climate model experiments (Ganopolski and Roche 2009) and they are entirely consistent with the major role of CO2 in climate change. During the warming at the end of ice ages, CO2 was released from the oceans ? just the opposite of what we observe today, where CO2 is increasing in both the ocean and the atmosphere.
If the CO2 concentration in the atmosphere is changed, then the temperature follows because of the greenhouse effect. This is what is happening now that humans release CO2 from fossil sources. But this has also happened many times in Earth?s history. CO2 concentrations have changed over millions of years due to natural carbon cycle changes associated with plate tectonics (continental drift), and climate has tracked those CO2 changes (e.g. the gradual cooling into ice-age climates over the past 50 million years).
A rapid carbon release, not unlike what humans are causing today, has also occurred at least once in climate history, as sediment data from 55 million years ago show. This ?Paleocene-Eocene thermal maximum? brought a major global warming of ~ 5 ?C, a detrimental ocean acidification and a mass extinction event. It serves as a stark warning to us today.
– Global mean air-temperature is projected to warm 2?C ? 7?C above pre-industrial by 2100.
The wide range is mainly due to uncertainty in future emissions.
– There is a very high probability of the warming exceeding 2?C unless global emissions peak
and start to decline rapidly by 2020.
– Warming rates will accelerate if positive carbon feedbacks significantly diminish the
efficiency of the land and ocean to absorb our CO2 emissions.
– Many indicators are currently tracking near or above the worst case projections from the
IPCC AR4 set of model simulations.
There has been no new coordinated set of future climate model
projections undertaken since the IPCC AR4. Instead, much
of the new research over the past few years has focused on
preparation for the next round of IPCC simulations for AR5, and
continued evaluation of the AR4 model runs. This includes new
analyses of the observed rate of climate change in comparison to
the IPCC AR4 projections (e.g., Rahmstorf 2007; Stroeve et al.
2007), and new calculations that take existing simulations and
incorporate coupled carbon feedbacks and other processes (e.g.
Zickfeld et al. 2009; Allen et al. 2009). While models exhibit
good skill at capturing the mean present-day climate, some
recent observed changes, notably sea-level rise and Arctic sea-ice
melt, are occurring at a faster rate than anticipated by IPCC AR4.
This is a cause for concern as it suggests that some amplifying
feedbacks and processes, such as land-ice melt, are occurring
faster than first predicted.
The latest estimates of global mean air temperature projected out
to 2100 are shown in Figure 21. The wide range in the projection
envelope is primarily due to uncertainty in future emissions. At
the high end of emissions, with business as usual for several
decades to come, global mean warming is estimated to reach
4-7?C by 2100, locking in climate change at a scale that would
profoundly and adversely affect all of human civilization and all
of the world?s major ecosystems. At the lower end of emissions,
something that would require urgent, deep and long-lasting cuts in
fossil fuel use, and active preservation of the world?s forests, global
mean warming is projected to reach 2-3?C by century?s end. While
clearly a better outcome than the high emissions route, global
mean warming of even just 1.5-2.0?C still carries a significant
risk of adverse impacts on ecosystems and human society. For
example, 2?C global temperature rise could lead to sufficient
warming over Greenland to eventually melt much of its ice sheet
(Oppenheimer and Alley 2005), raising sea level by over six
meters and displacing hundreds of millions of people worldwide.
Despite the certainty of a long-term warming trend in response
to rising greenhouse gases, there is no expectation that the
warming will be monotonic and follow the emissions pathway
on a year-to-year basis. This is because natural variability and
the 11-year solar cycle, as well as sporadic volcanic eruptions,
generate short-term variations superimposed on the long term
trend (Lean and Rind 2009). Even under a robust centurylong
warming trend of around 4?C, we still expect to see the
temperature record punctuated by isolated but regular ten-year
periods of no trend, or even modest cooling (Easterling and
Wehner 2009). Such decades therefore do not spell the end of
global warming ? emissions must peak and decline well before
that is to occur. In fact, the peak in global temperature might
not be reached until several centuries after emissions peak
(e.g., Allen et al. 2009). Even after emissions stop completely,
atmospheric temperatures are not expected to decline much
for many centuries to millennia (Matthews and Caldeira 2008;
Solomon et al. 2009; Eby et al. 2009) because of the long lifetime
of CO2 in the atmosphere. Furthermore, dry season rainfall
reductions in several regions are expected to become irreversible
(Solomon et al. 2009).
Mitigating global warming
While global warming can be stopped, it cannot easily be
reversed due to the long lifetime of carbon dioxide in the
atmosphere (Solomon et al. 2009; Eby et al. 2009). Even
a thousand years after reaching a zero-emission society,
temperatures will remain elevated, likely cooling down by only
a few tenths of a degree below their peak values. Therefore,
decisions taken now have profound and practically irreversible
consequences for many generations to come, unless affordable
ways to extract CO2 from the atmosphere in massive amounts
can be found in the future. The chances of this do not appear to
The temperature at which global warming will finally stop
depends primarily on the total amount of CO2 released to the
atmosphere since industrialization (Meinshausen et al. 2009,
Allen et al. 2009, Zickfeld et al. 2009). This is again due to the
long life-time of atmospheric CO2. Therefore if global warming
is to be stopped, global CO2 emissions must eventually decline
to zero. The sooner emissions stop, the lower the final warming
will be. From a scientific point of view, a cumulative CO2 budget
for the world would thus be a natural element of a climate
policy agreement. Such an agreed global budget could then be
distributed amongst countries, for example on the basis of equity
principles (e.g., WBGU 2009).
The most widely supported policy goal is to limit global warming
to at most 2 ?C above the preindustrial temperature level (often
taken for example as the average 19th Century temperature,
although the exact definition does not matter much due to the
small variations in preindustrial temperatures). Many nations
have publically recognized the importance of this 2?C limit.
Furthermore, the group of Least Developed Countries as well as
the 43 small island states (AOSIS) are calling for limiting global
warming to only 1.5?C. The Synthesis Report of the Copenhagen
climate congress (Richardson et al. 2009), the largest climate
science conference of 2009, concluded that ?Temperature rises
above 2 ?C will be difficult for contemporary societies to cope
with, and are likely to cause major societal and environmental
disruptions through the rest of the century and beyond.?
A number of recent scientific studies have investigated in detail
what global emissions trajectories would be compatible with
limiting global warming to 2 ?C. The answer has to be given in
terms of probabilities, to reflect the remaining uncertainty in
the climate response to elevated CO2, and the uncertainty in
the stability of carbon stored in the land and ocean systems.
Meinshausen et al. (2009) found that if a total of 1000 Gigatons
of CO2 is emitted for the period 2000-2050, the likelihood of
exceeding the 2-degree warming limit is around 25%. In 2000-
2009, about 350 Gigatons have already been emitted, leaving only 650 Gigatons for 2010-2050.
Figure 21. Reconstructed global-average temperature relative to 1800-1900 (blue) and projected global-average temperature out to 2100 (the latter from IPCC AR4). The envelopes B1, A2, A1F1 refer to the IPCC AR4 projections using those scenarios. The reconstruction record is taken from Mann et al. (2008).
At current emission rates this budget would be used up within 20 years.
An important consequence of the rapidly growing emissions
rate, and the need for a limited emissions budget, is that any
delay in reaching the peak in emissions drastically increases the
required rapidity and depth of future emissions cuts (see Figure
22 and also England et al. 2009). In Figure 22, emissions in the
green exemplary path are 4 Gt CO2 in the year 2050, which, with
a projected world population of around 9 billion, would leave
only less than half a ton per person per year. While the exact
number will depend strongly on the path taken, the required
decline in emissions combined with a growing population will
mean that by 2050, annual per capita CO2 emissions very likely
will need to be below 1 ton.
Although CO2 is the most important anthropogenic climate
forcing, other greenhouse gases as well as aerosols also play a
non-negligible role. Successful limitation of the non- CO2 climate
forcing would therefore create more leeway in the allowable CO2
emissions budget. Studies have shown that attractive options
for particularly rapid and cost-effective climate mitigation are the
reduction of black carbon (soot) pollution and tropospheric lowlevel
ozone (Wallack and Ramanathan 2009). In contrast to CO2,
these are very short-lived gases in the atmosphere, and therefore
respond rapidly to policy measures.
Figure 22. Examples of global emission pathways where cumulative CO2 emissions equal 750 Gt during the time period 2010-2050 (1 Gt CO2 = 3.67 Gt C). At this level, there is a 67% probability of limiting global warming to a maximum of 2?C. The graph shows that the later the peak in emissions is reached, the steeper their subsequent reduction has to be. The figure shows variants of a global emissions scenario with different peak years: 2011 (green), 2015 (blue) and 2020 (red). In order to achieve compliance with these
curves, maximum annual reduction rates of 3.7 % (green), 5.3 % (blue) or 9.0 % (red) would be required (relative to 2008). (Source: German Advisory Council on Global Change; WBGU 2009).
?kerman, H. J. & M. Johansson, (2008) Thawing permafrost and thicker
active layers in sub-arctic Sweden. Permafrost and Periglacial
Processes 19, 279-292.
Alexander, L. V. & J. M. Arblaster, (2009) Assessing trends in observed
and modelled climate extremes over Australia in relation to
future projections. International Journal of Climatology 29,
Allan, R. P. & B. J. Soden, (2008) Atmospheric warming and the
amplification of precipitation extremes. Science 321, 1481-
Allen, R. J. & S. C. Sherwood, (2008) Warming maximum in the tropical
upper troposphere deduced from thermal winds. Nature
Geoscience 1, 399-403.
Allen, M. R. et al., (2009) Warming caused by cumulative carbon
emissions toward the trillionth tonne. Nature 458, 1163-
Alley, R. B. et al., (2003) Abrupt Climate Change. Science 299, 2005-
Allison, I. et al., (2009) Ice sheet mass balance and sea level. Antarctic
Science, 21, 413-426.
Andronova, N. & M. E. Schlesinger, (2001) Objective estimation of the
probability distribution for climate sensitivity. Journal of
Geophysical Research 106, 22605-22612.
Annan, J. D. & J. C. Hargreaves, (2006) Using multiple observationallybased
constraints to estimate climate sensitivity. Geophysical
Research Letters 33, L06704.
Archer, D. et al., (2009) Ocean methane hydrates as a slow tipping
point in the global carbon cycle. Proceedings of the National
Academy of Sciences, doi:10.1073/pnas.0800885105.
Arzel, O. et al., (2006) Sea ice evolution over the 20th and 21st
centuries as simulated by the current AOGCMs. Ocean
Modelling 12, 401-415.
Aumann, H. H. et al., (2008) Frequency of severe storms and global
warming. Geophysical Research Letters 35, L19805.
Bahr, D. B. et al., (2009) Sea-level rise from glaciers and ice caps: A lower
bound. Geophysical Research Letters 36, L03501.
Bakke, J. et al., (2009) Rapid oceanic and atmospheric changes during the
Younger Dryas cold period. Nature Geoscience 2.
Bala, G. et al., (2007) Combined climate and carbon-cycle effects of largescale
deforestation. Proceedings of the National Academy of
Sciences 104, 6550-6555.
Barnett, T. P. et al., (2008) Human induced changes in the hydrology of
the western United States. Science 319, 1080-1083.
Barrett B. E. et al., (2009) Rapid recent warming on Rutford Ice Stream,
West Antarctica, from borehole thermometry, Geophysical
Research Letters, 36, L02708.
Benestad, R. E. & G. A. Schmidt, (2009) Solar trends and global warming.
Journal of Geophysical Research 114, D14101.
Berger, A., (1978) Long-term variations of daily insolation and
Quaternary climatic changes. Journal of the Atmospheric
Sciences 35, 2362-2367.
Berger, A. & M. F. Loutre, (1991) Insolation values for the climate of
the last 19 million years. Quaternary Science Reviews 10,
Berger, A. et al., (1998) Sensitivity of the LLN climate model to the
astronomical and CO2 forcings over the last 200 ky. Climate
Dynamics 14, 615-629.
Betts, R. A., (2000) Offset of the potential carbon sink from boreal
afforestation by decreases in surface albedo. Nature 408,
Betts, R. A. et al., (2007) Projected increases in continental river runoff
due to plant responses to carbon dioxide. Nature 448, 1037-
Biggs, R. et al., (2009) Turning back from the brink: Detecting an
impending regime shift in time to avert it. Proceedings of the
National Academy of Sciences 106, 826-831, doi:10.1073/
Bindoff, N. L. et al., (2007) Observations: Oceanic Climate Change and
Sea Level. In Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change S.
Solomon et al. eds (Cambridge University Press).
Bondeau, A. et al., (2007) Modelling the role of agriculture for the 20th
century global terrestrial carbon balance. Global Change
Biology 13, 679-706.
Bony, S. et al., (2006) How well do we understand and evaluate climate
change feedback processes? Journal of Climate 19, 3445?
Booth, B. B. et al., (2009 submitted) Global warming uncertainties due to
carbon cycle feedbacks exceed those due to CO2 emissions.
Braun, M. & A. Humbert, (2009) Recent retreat of Wilkins Ice Shelf
reveals new insights in ice shelf breakup mechanisms. IEEE
Geoscience and Remote Sensing Letters 6, 263-267.
Brewer, P. G., (2009) A changing ocean seen with clarity. Proceedings of
the National Academy of Sciences 106, 12213-12214.
Brook, E. et al., (2008) Potential for abrupt changes in atmospheric
methane. In Abrupt Climate Change. A report by the U.S.
Climate Change Science Program and the Subcommittee on
Global Change Research. 360-452 (U.S. Geological Survey).
Canadell, J. G. et al., (2007) Contributions to accelerating atmospheric
CO2 growth from economic activity, carbon intensity,
and efficiency of natural sinks. Proceedings of the National
Academy of Sciences 104, 18866-18870.
Cavalieri, D. J. & C. L. Parkinson, (2008) Antarctic sea ice variability and
trends, 1979-2006. Journal of Geophysical Research 113,
Cazenave, A. et al., (2009) Sea level budget over 2003-2008: A
reevaluation from GRACE space gravimetry, satellite altimetry
and ARGO. Global and Planetary Change 65, 83-88.
CCSP, (2008a) Weather and Climate Extremes in a Changing Climate.
Regions of Focus: North America, Hawaii, Caribbean, and U.S.
Pacific Islands. A Report by the U.S. Climate Change Science
Program. (Department of Commerce, NOAA?s National
Climatic Data Center, Washington, D.C., USA).
CCSP, (2008b) Abrupt Climate Change. A report by the U.S. Climate
Change Science Program and the Subcommittee on Global
Change Research. (U.S. Geological Survey, Reston VA).
Chang, P. et al., (2008) Oceanic link between abrupt change in the North
Atlantic Ocean and the African monsoon. Nature Geoscience
Chapman W.L. & J.E. Walsh, (2007) A synthesis of Antarctic
temperatures. Journal of Climate, 20, 4096-4117.
Chen, J. et al., (2006) Antarctic mass rates from GRACE. Geophysical
Research Letters 33, L11502.
Church, J. A. & N. J. White, (2006) A 20th century acceleration in global
sea-level rise. Geophysical Research Letters 33, L01602.
Clark, P. U. et al., (1999) Northern Hemisphere ice-sheet influences on
global climate change. Science 286, 1104-1111.
Cogley, J. G., (2009) Geodetic and direct mass-balance measurements:
comparison and joint analysis. Annals of Glaciology 50, 96-
Comiso, J. C. & F. Nishio, (2009) Trends in the sea ice cover using
enhanced and compatible AMSR-E, SSM/I and SMMR data.
Journal of Geophysical Research 113, C02S07.
Cook, A. et al., (2005) Retreating glacier-fronts on the Antarctic
Peninsula over the last 50 years. Science 22, 541-544.
Cook, K. H. & E. K. Vizy, (2006) Coupled Model Simulations of the West
African Monsoon System: Twentieth- and Twenty-First-
Century Simulations. Journal of Climate 19, 3681-3703.
Cook, K. H. & E. K. Vizy, (2008) Effects of Twenty-First-Century Climate
Change on the Amazon Rain Forest. Journal of Climate 21,
Cox, P. M. et al., (2004) Amazonian forest dieback under climate-carbon
cycle projections for the 21st century. Theoretical and Applied
Climatology 78, 137-156.
Cox, P. M. et al., (2008) Increasing risk of Amazonian drought due to
decreasing aerosol pollution. Nature 453, 212-216.
Cox, P. M. & C. D. Jones, (2008) Data from the past illuminates the
modern dance of climate and carbon dioxide. Science 321,
Cruz, F. et al., (2009 in press) Probabilistic simulations of the impact of
increasing leaf-level atmospheric carbon dioxide on the global
land surface. Climate Dynamics, doi:10.1007/s00382-008-
Cui, X. & H. F. Graf, (2009) Recent land cover changes on the Tibetan
Plateau: a review. Climatic Change 94, 47-61.
Curry, R. et al., (2003) A change in the freshwater balance of the Atlantic
Ocean over the past four decades. Nature 426, 826-829.
Dakos, V. et al., (2008) Slowing down as an early warning signal for
abrupt climate change. Proceedings of the National Academy of
Sciences 105, 14308-14312.
Delworth, T. L. et al., (2008) The potential for abrupt change in the
Atlantic Meridional Overturning Circulation. In Abrupt
Climate Change. A report by the U.S. Climate Change Science
Program and the Subcommittee on Global Change Research.
Dessler, A. E. et al., (2008) Water-vapor climate feedback inferred from
climate fluctuations, 2003-2008. Geophysical Research Letters
Domingues, C. M. et al., (2008) Improved estimates of upper-ocean
warming and multi-decadal sea-level rise. Nature 453, 1090-
Dorrepaal, E. et al., (2009) Carbon respiration from subsurface peat
accelerated by climate warming in the subarctic. Nature 460,
Easterling, D. R. & M. F. Wehner, (2009) Is the climate warming or
cooling? Geophysical Research Letters 36, L08706.
Eby, M. et al., (2009) Lifetime of anthropogenic climate change: millennial
time scales of potential CO2 and surface temperature
perturbations. Journal of Climate 22, 2501-2511.
Eisenman, I. & J. S. Wettlaufer, (2009) Nonlinear threshold behavior
during the loss of Arctic sea ice. Proceedings of the National
Academy of Sciences 106, 28-32.
Elsner, J. B. et al., (2008) The increasing intensity of the strongest tropical
cyclones. Nature 455, 92-95.
Emanuel, K. et al., (2008) Hurricanes and Global Warming: Results from
Downscaling IPCC AR4 Simulations. Bulletin of the American
Meteorological Society 89, 347-367.
England, M. H. et al., (2009) Constraining future greenhouse gas
emissions by a cumulative target Proceedings of the National
Academy of Sciences 106, 16539-16540.
EPICA community members, (2004) Eight glacial cycles from an Antarctic
ice core. Nature 429, 623-628.
Esper, J. et al., (2002) Low-frequency signals in long tree-ring
chronologies for reconstructing past temperature variability.
Science 295, 2250-2253.
Fabry, V. J. et al., (2008) Impacts of ocean acidification on marine fauna
and ecosystem processes. ICES Journal of Marine Science 65,
Fargione, J. et al., (2008) Land clearing and the biofuel debt. Science 319,
Fischer, E. M. et al., (2007) Contribution of land-atmosphere coupling to
recent European heat waves. Geophysical Research Letters 34,
Flanner, M. G. et al., (2007) Present-day climate forcing and response
from black carbon in snow. Journal of Geophysical Research
Frame, D. J. et al., (2005) Constraining climate forecasts: The role of prior
assumptions Geophysical Research Letters 32, L09702.
Frederick, E. et al., (2004) Greenland Ice Sheet: increased coastal thinning
Geophysical Research Letters 31, L24402.
Friedlingstein, P. et al., (2006) Climate-carbon cycle feedback analysis:
Results from the C4MIP model intercomparison. Journal of
Climate 19, 3337-3353.
Fyke, J. G. & A. J. Weaver, (2006) The effect of potential future climate
change on the marine methane hydrate stability zone. Journal
of Climate 19, 5903-5916.
Galloway, J. N. et al., (2008) Transformation of the nitrogen cycle: recent
trends, questions and potential solutions. Science 320, 889-
Ganopolski, A. & D. M. Roche, (2009 in press) On the nature of
lead?lag relationships during glacial?interglacial climate
transitions. Quaternary Science Reviews, doi:10.1016/j.
Goosse, H. et al., (2009) Consistent past half-century trends in the
atmosphere, the sea ice and the ocean at high southern
latitudes. Climate Dynamics, 33, 999-1016
Gedney, N. et al., (2006) Detection of a direct carbon dioxide effect in
continental river runoff records. Nature 439, 835-838.
Gleason, K. L. et al., (2008) A Revised U.S. Climate Extremes Index.
Journal of Climate 21, 2124-2137.
Guan, D. et al., (2009) Journey to world top emitter: An analysis of
the driving forces of China?s recent CO2 emissions surge.
Geophysical Research Letters 36, L04709.
Guttal, V. & C. Jayaprakash, (2008) Changing skewness: an early warning
signal of regime shifts in ecosystems. Ecology Letters 11, 450-
Guttal, V. & C. Jayaprakash, (2009) Spatial variance and spatial skewness:
leading indicators of regime shifts in spatial ecological
systems. Theoretical Ecology 2, 3-12.
Hagos, S. M. & K. H. Cook, (2007) Dynamics of the West African
Monsoon Jump. Journal of Climate 20, 5264-5284.
Hall, D. K. et al., (2008) Greenland ice sheet surface temperature, melt
and mass loss: 2000-06. Journal of Glaciology 54, 81-93.
Hanna, E. et al., (2008) Increased runoff from melt from the Greenland Ice
Sheet: a response to global warming. Journal of Climate 21,
Hanna, E. et al., (2009) Hydrologic response of the Greenland Ice Sheet:
the role of oceanographic forcing. Hydrological Processes 23,
Hansen, B. & S. ?sterhus, (2007) Faroe Bank Channel overflow 1995 –
2005. Progress in Oceanography 75, 817-856.
Harris, C. et al., (2009) Permafrost and climate in Europe: monitoring
and modelling thermal, geomorphological and geotechnical
responses. Earth-Sciecne Review 92, 117-171.
Hays, J. D. et al., (1976) Variations in the Earth?s orbit: Pacemaker of the
ice ages. Science 194, 1121-1132.
Haywood, A. M. et al., (2007) The mid-Pliocene Warm Period: a test-bed
for integrating data and models. In Deep-time perspectives
on climate change: marrying the signal from computer models
and biological proxies M. Williams et al. eds 443-458
(The Micropalaeontological Society, Special Publication, The
Geological Society, London).
Hock, R. et al., (2009) Mountain glaciers and ice caps around Antarctica
make a large sea-level rise contribution. Geophysical Research
Letters 36, L07501.
Hofmann, M. & H. J. Schellnhuber, (2009) Oceanic acidification affects
marine carbon pump and triggers extended marine oxygen
holes. Proceedings of the National Academy of Sciences 106,
Hofmann, M. & S. Rahmstorf, (2009) On the stability of the Atlantic
Meridional Overturning Circulation. Proceedings of the
National Academy of Sciences. doi:10.1073/pnas.0909146106
Holland, M. M. et al., (2006) Future abrupt reductions in the summer
Arctic sea ice. Geophysical Research Letters 33, L23503.
Holland, D. M. et al., (2008) Acceleration of Jakobshavn Isbrae triggered
by warm subsurface ocean waters. Nature Geoscience 28,
House, J. I. et al., (2008) What do recent advances in quantifying climate
and carbon cycle uncertainties mean for climate policy?
Environmental Research Letters 3, 044002.
Howat, I. M. et al., (2007) Rapid changes in ice discharge from Greenland
outlet glaciers. Science 315, 1559-1561.
Howat, I. M. et al., (2008) Rates of southeast Greenland ice volume loss
from combined ICESat and ASTER observations. Geophysical
Research Letters 35, L17505.
Hoyos, C.D. et al. (2006) Deconvolution of the Factors Contributing to
the Increase in Global Hurricane Intensity, Science, 312, 94-
Hyv?nen, R. et al., (2007) The likely impact of elevated CO2, nitrogen
deposition, increased temperature and management
on carbon sequestration in temperate and boreal forest
ecosystems: a literature review. New Phytologist 173, 463-
IPCC, (2001) Climate Change 2001: The Scientific Basis. Contribution
of Working Group I to the Third Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC TAR). J. T.
Houghton et al. eds (Cambridge University Press, Cambridge,
UK and New York, NY, USA).
IPCC, (2007) Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change
(AR4). S. Solomon et al. eds (Cambridge University Press,
Cambridge, UK & New York, NY, USA).
IPCC, (2007) Summary for Policymakers. In Climate Change 2007: The
Physical Science Basis. Contribution of Working Group I to
the Fourth Assessment Report of the Intergovernmental Panel
on Climate Change (IPCC AR4) S. Solomon et al. eds
(Cambridge University Press).
Jansen, E. et al., (2007) Palaeoclimate. In Climate Change 2007: The
Physical Science Basis. Contribution of Working Group I to the
Fourth Assessment Report of the Intergovernmental Panel on
Climate Change S. Solomon et al. eds (Cambridge University
Jin, H.-j. et al., (2008) Changes in permafrost environments along the
Qinghai-Tibet engineering corridor induced by anthropogenic
activities and climate warming. Cold Regions Science and
Technology 53, 317-333.
Johannessen, O. et al., (2005) Recent ice-sheet growth in the interior of
Greenland. Science 310, 1013-1016.
Johnson, G. C. & N. Gruber, (2007) Decadal water mass variations
along 20?W in the Northeastern Atlantic Ocean. Progress in
Oceanography 73, 277-295.
Johnson, G. C. et al., (2008) Reduced Antarctic meridional overturning
circulation reaches the North Atlantic Ocean. Geophysical
Research Letters 35, L22601.
Johnson, G. C. et al., (2008) Warming and freshening in the abyssal
southeastern Indian Ocean. Journal of Climate 21, 5351-
Jones, G. S. et al., (2008) Human contribution to rapidly increasing
frequency of very warm Northern Hemisphere summers.
Journal of Geophysical Research 113, D02109.
Jones, K. F. & B. Light, (2008) Sunlight, water, and ice: Extreme Arctic sea
ice melt during the summer of 2007. Geophysical Research
Letters 35, L11501.
Jones, C. et al., (2009) Commited ecosystem change due to climate
change. Nature Geoscience 2, 484-487.
Joos, F. & R. Spahni, (2008) Rates of change in natural and anthropogenic
radiative forcing over the past 20,000 years. Proceedings of the
National Academy of Sciences 105, 1425-1430.
Kaser, G. et al., (2006) Mass balance of glaciers and ice caps: Consensus
estimates for 1961-2004. Geophysical Research Letters 33,
Kaufman, D. S. et al., (2009) Recent Warming Reverses Long-Term Arctic
Cooling. Science 325, 1236-1239.
Kharin, V. V. et al., (2007) Changes in temperature and precipitation
extremes in the IPCC ensemble of global coupled model
simulations. Journal of Climate 20, 1419-1444.
Khvorostyanov, D. V. et al., (2008) Vulnerability of east Siberia?s frozen
carbon stores to future warming. Geophysical Research Letters
Khvorostyanov, D. V. et al., (2008) Vulnerability of permafrost carbon to
global warming. Part I: model description and the role of heat
generated by organic matter decomposition. Tellus B 60B,
Knutti, R. & G. C. Hegerl, (2008) The equilibrium sensitivity of the
Earth?s temperature to radiation changes. Nature Geoscience
Krabill, W. et al., (2000) Greenland Ice Sheet: High-Elevation Balance and
Peripheral Thinning Science 289, 428-430.
Krabill, W. et al., (2004) Greenland Ice Sheet: Increased coastal thinning.
Geophysical Research Letters 31, L24402.
Kriegler, E. et al., (2009) Imprecise probability assessment of tipping
points in the climate system. Proceedings of the National
Academy of Sciences 106, 5041-5046.
K?rschner, W. M. et al., (1996) Oak leaves as biosensors of late Neogene
and early Pleistocene paleoatmospheric CO2 concentrations.
Marine Micropaleontology 27, 299-312.
Kwok, R. & D. A. Rothrock, (2009) Decline in Arctic sea ice thickness
from submarine and ICESat records: 1958?2008. Geophysical
Research Letters 36, L15501.
Lam, P. et al., (2009) Revising the nitrogen cycle in the Peruvian oxygen
minimum zone. Proceedings of the National Academy of
Sciences 106, 4752-4757.
Latif, M. & N. S. Keenlyside, (2009) El Ni?o/Southern Oscillation
Response to Global Warming. Proceedings of the National
Academy of Sciences, doi:10.1073/pnas.0710860105
Lawrence, D. M. & A. G. Slater, (2005) A projection of severe near-surface
permafrost degradation during the 21st century. Geophysical
Research Letters 32, L24401.
Lawrence, D. M. et al., (2008) Accelerated Arctic land warming and
permafrost degradation during rapid sea ice loss. Geophysical
Research Letters 35, L11506.
Le Qu?r?, C. et al., (2007) Saturation of the Southern Ocean CO2 sink
due to recent climate change. Science 316, 1735-1738.
Le Qu?r?, C. et al., (2009) Trends in the sources and sinks of carbon
dioxide. Nature Geosciences, 2, in press.
Lean, J. L. & D. H. Rind, (2008) How natural and anthropogenic
influences alter global and regional surface temperatures:
1889 to 2006. Geophysical Research Letters 35, L18701.
Lean, J. L. & D. H. Rind, (2009) How will Earth?s surface temperature
change in future decades? Geophysical Research Letters 36,
Lefebvre, W. et al., (2004) Influence of the southern annular mode on the
sea-ice-ocean system. Journal of Geophysical Research 109,
Lemke, P. et al., (2007) Observations: Changes in Snow, Ice and Frozen
Ground In Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change S.
Solomon et al. eds (Cambridge University Press).
Lenton, T. M. et al., (2008) Tipping Elements in the Earth?s Climate
System. Proceedings of the National Academy of Sciences 105,
Lenton, T. M. et al., (2009) Using GENIE to study a tipping point in the
climate system. Philosophical Transactions of the Royal Society
A 367, 871-884.
Letenmaier, D. P. & P. C. D. Milly, (2009) Land water and sea level. Nature
Geoscience 2, 452-454.
Lindsay, R. W. et al., (2009) Arctic sea ice retreat in 2007 follows
thinning trend. Journal of Climate 22, 165-175.
Livina, V. & T. M. Lenton, (2007) A modified method for detecting
incipient bifurcations in a dynamical system. Geophysical
Research Letters 34, L03712.
Lombard, A. et al., (2006) Perspectives on present-day sea level change.
Ocean Dynamics 56, 445-451.
Loulergue, L. et al., (2008) Orbital and millennial-scale features of
atmospheric CH4 over the past 800,000 years. Nature 453,
Lovenduski, N. et al., (2008) Toward a mechanistic understanding of the
decadal trends in the Southern Ocean carbon sink. Global
Biogeochemical Cycles 22, GB3016.
Luthcke, S. B. et al., (2006) Recent Greenland ice mass loss by drainage
system from satellite gravity observations. Science 314, 1286-
L?thi, D. et al., (2008) High-resolution carbon dioxide concentration
record 650,000-800,000 years before present. Nature 453,
Malhi, Y. et al., (2008) Climate Change, deforestation and the fate of the
Amazon. Science 319, 169-172.
Malhi, Y. et al., (2009) Exploring the likelihood and mechanism of a
climate-change induced dieback of the Amazon rainforest.
Proceedings of the National Academy of Sciences, doi:10.1073/
Mann, M. E. et al., (1998) Global-Scale Temperature Patterns and Climate
Forcing Over the Past Six Centuries. Nature 392, 779-787.
Mann, M. E. et al., (1999) Northern Hemisphere Temperatures During the
Past Millennium: Inferences, Uncertainties, and Limitations.
Geophysical Research Letters 26, 759-762.
Mann, M. E. et al., (2008) Proxy-Based Reconstructions of Hemispheric
and Global Surface Temperature Variations over the Past Two
Millennia. Proceedings of the National Academy of Sciences
Mann, M. E. et al., (2009) Atlantic hurricanes and climate over the past
1,500 years. Nature 460, 880-883.
Mann, M. E. et al., (in press) Global Signatures and Dynamical Origins of
the ?Little Ice Age? and ?Medieval Climate Anomaly?. Science.
Marsh, P. T. et al., (2009) Preliminary investigation into the severe
thunderstorm environment of Europe simulated by the
Community Climate Systems Model 3. Atmospheric Research
Matthews, H. D. & K. Caldeira, (2008) Stabilizing climate requires near
zero emissions. Geophysical Research Letters 35, L04705.
McIntyre, S. & R. McKitrick, (2003) Corrections to the Mann et al. (1998)
proxy database and northern hemispheric average temperature
series. Energy and Environment 14, 751-771.
McIntyre, S. & R. McKitrick, (2005) Hockey sticks, principal components,
and spurious significance. Geophysical Research Letters 32,
McIntyre, S. & R. McKitrick, (2005) The M&M critique of the MBH98
Northern Hemisphere climate index: Update and implications.
Energy and Environment 16, 69-99.
McNeil, B.I., and R.J. Matear (2007) Climate Change Feedbacks on
Oceanic pH. Tellus-B, 59B, 191-198.
McNeil, B. I. & R. J. Matear, (2008) Southern Ocean acidification: A
tipping point at 450-ppm atmospheric CO2. Proceedings of the
National Academy of Sciences 105, 18860-18864.
Meehl, G. A. et al., (2004) Combinations of Natural and Anthropogenic
Forcings in Twentieth-Century Climate. Journal of Climate 19,
Meehl, G. A. et al., (2007a) Global Climate Projections. In Climate
Change 2007: The Physical Science Basis. Contribution of
Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change S. Solomon et al.
eds (Cambridge University Press).
Meehl, G. A. et al., (2007b) Contributions of natural and anthropogenic
forcing to changes in temperature extremes over the U.S.
Geophysical Research Letters 34, L19709.
Meehl, G. A. et al., (2008) Effects of Black Carbon Aerosols on the Indian
Monsoon. Journal of Climate 21, 2869-2882.
Meier, M. F. & M. B. Dyurgerov, (2007) Glaciers dominate eustatic sealevel
rise in the 21st century. Science 317, 1064-1067.
Meinshausen, M. et al., (2009) Greenhouse-gas emission targets for
limiting global warming to 2?C. Nature 458, 1158-1162.
Mercado, L. M. et al., (2009) Impact of change in diffuse radiation on the
global land carbon sink. Nature 458, 1014-1017.
Metzl, N., (2009) Decadal increase of oceanic carbon dioxide in Southern
Indian surface ocean waters (1991-2007). Deep Sea Research
Part II: Topical Studies in Oceanography 56, 607-619.
Moberg, A. et al., (2005) Highly variable Northern Hemisphere
temperatures reconstructed from low- and high-resolution
proxy data. Nature 433, 613-617.
Monaghan A. J. et al., (2008) Recent variability and trends of Antarctic
near-surface temperature. Journal of Geophysical Research,
Mote, T. L., (2007) Greenland surface melt trends 1973-2007: Evidence
of a large increase in 2007. Geophysical Research Letters 34,
Moy, A. D. et al., (2009) Reduced calcification in modern Southern Ocean
planktonic foraminifera. Nature Geoscience 2, 276-280.
Nakicenovic, N. et al., (2000) IPCC Special Report on Emissions
Scenarios. (Cambridge, UK), 599 pp.
NASA Goddard Institute for Space Studies, (2009) GISS Surface
Temperature Analysis. Global Temperature Trends: 2008 Annual
Nghiem, S. V. et al., (2007) Rapid reduction of Arctic perennial sea ice.
Geophysical Research Letters 34, L19504.
Nicholls, R. J. et al., (2007) Coastal systems and low-lying areas. In
Climate Change 2007: Impacts, Adaptation and Vulnerability.
Contribution of Working Group II to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change M.
L. Parry et al. eds 315-356 (Cambridge University Press).
NOAA, (2009) State of the Climate, 2009,
NRC (National Research Council), (2006) Surface Temperature
Reconstructions for the Last 2,000 Years. (National Academies
Press, Washington, DC).
NSDIC – National Snow and Ice Data Center, (2009)
Oerlemans, J. et al., (2007) Reconstructing the glacier contribution to
sea-level rise back to 1850. The Cryosphere 1, 59-65.
Oppo, D. W. et al., (2009) 2,000-year-long temperature and hydrology
reconstructions from the Indo-Pacific warm pool Nature 460,
Orr, J. C. et al., (2005) Anthropogenic ocean acidification over the
twenty-first century and its impact on calcifying organisms.
Nature 437, 681-686.
Orr, J. C. et al., (2009) Amplified acidification of the Arctic Ocean. IOP
Conf. Series: Earth and Environmental Science 6, 462009.
Oschlies, A. et al., (2008) Simulated 21st century?s increase in oceanic
suboxia by CO2-enhanced biotic carbon export Global
Biogeochemical Cycles 22, GB4008.
Pall, P. et al., (2007) Testing the Clausius-Clapeyron constraint on
changes in extreme precipitation under CO2 warming.
Climate Dynamics 28, 351-363.
Patricola, C. M. & K. H. Cook, (2008) Atmosphere/vegetation feedbacks:
A mechanism for abrupt climate change over northern Africa.
Journal of Geophysical Research (Atmospheres) 113, D18102.
Pearson, P. L. & M. R. Palmer, (2000) Middle Eocene Seawater pH and
Atmospheric Carbon Dioxide Concentrations. Science 284,
Pedersen, C. A. et al., (2009) A new sea ice albedo scheme including
melt ponds for ECHAM5 general circulation model. Journal of
Geophysical Research 114, D08101.
Perovich, D. K. et al., (2007) Increasing solar heating of the Arctic Ocean
and adjacent seas, 1979-2005: Attibution and role in the icealbedo
feedback. Geophysical Research Letters 34, L19505.
Petrenko, V. V. et al., (2009) 14CH4 measurements in Greenland ice:
investigating last glacial termination CH4 sources. Science
Pfeffer, W. T. et al., (2008) Kinematic constraints on glacier contributions
to 21st-Century sea-level rise. Science, 321, 1340-1343.
Phillips, O. L. et al., (2009) Drought sensitivity of the Amazon rainforest.
Science 323, 1344-1347.
Piao, S. et al., (2007) Changes in climate and land-use have a larger
direct impact than rising CO2 on global river runoff records.
Proceedings of the National Academy of Sciences 104, 15242-
Piekle, R. A. et al., (2007) An overview of regional land-use and landcover
impacts on rainfall. Tellus B 59, 587-601.
Pitman, A. J. et al., (2007) The impact of climate change on the risk of
forest and grassland fires in Australia. Climatic Change 84,
Pitman, A. J. et al., (2009) Uncertainties in climate responses to
past land cover change: First results from the LUCID
intercomparison study. Geophysical Research Letters 36,
Pollard, D. & R. M. DeConto, (2009) Modelling West Antarctic ice sheet
growth and collapse through the past five million years.
Nature 458, 329-332.
Polyakov, I. V. et al., (2004) Variability of the intermediate Atlantic water
of the Arctic Ocean over the last 100 years. Journal of Climate
Portmann, R. W. et al., (2009) Linkages between climate change, extreme
temperature and precipitation across the United States.
Proceedings of the National Academy of Sciences 106, 7324-
Pritchard, H. D. & D. G. Vaughan, (2007) Widespread acceleration of
tidewater glaciers on the Antarctic Peninsula. Journal of
Geophysical Research 112, F03S29.
Pritchard, H. D. et al., (2009) Extensive dynamic thinning on the margins
of the Greenland and Antarctic ice sheets. Nature, 461, 971-
Rahmstorf, S., (2007) A Semi-Empirical Approach to Projecting Future
Sea-Level Rise. Science 315, 368-370.
Rahmstorf, S. et al., (2007) Recent climate observations compared to
projections. Science 316, 709.
Ramanathan, V. et al., (2005) Atmospheric brown clouds: Impacts on
South Asian climate and hydrological cycle. Proceedings of the
National Academy of Sciences 102, 5326-5333.
Ramanathan, V. & G. Carmichael, (2008) Global and regional climate
changes due to black carbon. Nature Geoscience 1, 221-227.
Raupach, M. R. et al., (2007) Global and regional drivers of accelerating
CO2 emissions. Proceedings of the National Academy of
Sciences 104, 10288-10293.
Raymo, M. E. et al., (1996) Mid-Pliocene warmth: stronger greenhouse
and stronger conveyor. Marine Micropaleontology 27, 313-
Rayner, N. A. et al., (2006) Improved analyses of changes and
uncertainties in sea surface temperature measured in situ
since the mid-nineteenth century: the HadSST2 data set.
Journal of Climate 19, 446-469.
Reichstein, M. et al., (2007) Reduction of ecosystem productivity and
respiration during the European summer 2003 climate
anomaly: a joint flux tower, remote sensing and modelling
analysis. Global Change Biology 13, 634-651.
Repo, M. E. et al., (2009) Large N2O emissions from cryoturbated peat
soil in tundra. Nature Geoscience 2, 189-192.
Richardson, K. et al., (2009) Climate Change: Global Risks, Challenges
& Decisions. Synthesis Report of the Copenhagen Climate
Congress. (University of Copenhagen).
Riebesell, U. et al., (2009) Sensitivities of marine carbon fluxes to ocean
change. Proceedings of the National Academy of Sciences.
Rigby, M. et al., (2008) Renewed growth of atmospheric methane.
Geophysical Research Letters 35, L22805.
Rignot, E. et al., (2004) Accelerated ice discharge from the Antarctic
Peninsula following the collapse of Larsen B ice shelf.
Geophysical Research Letters 31, L18401.
Rignot, E., (2006) Changes in ice dynamics and mass balance of the
Antarctic ice sheet. Philosophical Transactions of the Royal
Society A 364, 1637-1655.
Rignot, E. & P. Kanagaratnam, (2006) Changes in the velocity structure
of the Greenland Ice Sheet. Science 311, 986-990.
Rignot, E., (2008a) Changes in West Antarctic ice stream dynamics
observed with ALOS PALSAR data. Geophysical Research
Letters 35, L12505.
Rignot, E. et al., (2008b) Recent Antarctic ice mass loss from radar
interferometry and regional climate modelling. Nature
Geoscience 1, 106-110.
Rignot, E. et al., (2008) Mass balance of the Greenland ice sheet from
1958 to 2007. Geophysical Research Letters 35, L20502.
Rintoul, S. R., (2007) Rapid freshening of Antarctic Bottom Water formed
in the Indian and Pacific oceans. Geophysical Research Letters
Rohling, E. J. et al., (2008) High rates of sea-level rise during the last
interglacial period. Nature Geoscience 1, 38-42.
Rosa, R. & B. A. Seibel, (2008) Synergistic effects of climate-related
variables suggest future physiological impairment in a top
oceanic predator. Proceedings of the National Academy of
Sciences 105, 20776-20780.
Rotstatyn, L. D. & U. Lohmann, (2002) Tropical Rainfall Trends and the
Indirect Aerosol Effect. Journal of Climate 15, 2103-2116.
Rutherford, S. et al., (2005) Proxy-Based Northern Hemisphere Surface
Temperature Reconstructions: Sensitivity to Method, Predictor
Network, Target Season, and Target Domain. Journal of
Climate 18, 2308-2329.
Sabine, C. L. et al., (2004) The Oceanic Sink for Anthropogenic CO2.
Science 305, 367-371.
Salazar, L. F. et al., (2007) Climate change consequences on the biome
distribution in tropical South America. Geophysical Research
Letters 34, L09708.
Santer, B. D. et al., (2007) Identification of human-induced changes in
atmospheric moisture content. Proceedings of the National
Academy of Sciences 104, 15248-15253.
Saunders, M. A. & A. S. Lea, (2008) Large contribution of sea surface
warming to recent increase in Atlantic hurricane activity.
Nature 451, 557-560.
Scambos, T. A. et al., (2004) Glacier acceleration and thinning after
ice shelf collapse in the Larsen B embayment, Antarctica.
Geophysical Research Letters 31, L18402.
Schellnhuber, H. J., (2009) Tipping elements in the Earth System.
Proceedings of the National Academy of Sciences.
Scholze, M. et al., (2006) A climate-change risk analysis for world
ecosystems. Proceedings of the National Academy of Sciences
Schuster, U. et al., (2009 in press) Trends in North Atlantic sea surface
pCO2 from 1990 to 2006. Deep Sea Research Part II: Topical
Studies in Oceanography, doi:10.1016/j.dsr2.2008.12.011.
Schuur, E. A. G. et al., (2008) Vulnerability of permafrost carbon to
climate change: Implications for the global carbon cycle.
BioScience 58, 701-714.
Shackleton, N. J., (2000) The 100,000-year ice-age cycle identified
and found to lag temperature, carbon dioxide, and orbital
eccentricity. Science 289, 1897-1902.
Sheffield, J. & E. F. Wood, (2008) Global Trends and Variability in Soil
Moisture and Drought Characteristics, 1950?2000, from
Observation-Driven Simulations of the Terrestrial Hydrologic
Cycle. Journal of Climate 21, 432-458.
Shindell, D. & G. Faluvegi, (2009) Climate response to regional radiative
forcing during the twentieth century. Nature Geoscience 2,
Siegenthaler, U. et al., (2005) Stable carbon cycle-climate relationship
during the late Pleistocene. Science 310, 1313-1317.
Sitch, S. et al., (2007) Indirect raditive forcing due to ozone effects on the
land carbon sink. Nature 448, 791-794.
Sokolov, A. P. et al., (2009) Probabilistic forecast for 21st century climate
based on uncertainties in emissions (without policy) and
climate parameters. Journal of Climate 22, 5175-5204.
Solomon, S. et al., (2009) Irreversible climate change due to carbon
dioxide emissions. Proceedings of the National Academy of
Sciences 106, 1704-1709.
Soon, W. & S. Baliunas, (2003) Proxy climatic and environmental changes
of the past 1000 years. Climate Research 23, 89-110.
Stammerjohn, S. E. et al., (2008) Trends in Antarctic annual sea ice
retreat and advance and their relation to El Ni?o-Southern
Oscillation and Southern Annular Mode variability. Journal of
Geophysical Research 113, C03S90.
Steffen, K. et al., (2008) Rapid changes in glaciers and ice sheets and
their impacts on sea level. In Abrupt Climate Change: A
Report by the U.S. Climate Change Science Program and the
Subcommittee on Global Change Research 60-142 (U.S.
Steig, E. J. et al., (2009) Warming of the Antarctic ice-sheet surface since
the 1957 International Geophysical Year. Nature 457, 459-
Stott, P. A. et al., (2008) Detection and attribution of Atlantic salinity
changes. Geophysical Research Letters 35, L21702.
Stramma, L. et al., (2008) Expanding oxygen-minimum zones in the
tropical oceans. Science 320, 655-658.
Stroeve, J. et al., (2007) Arctic sea ice decline: Faster than forecast.
Geophysical Research Letters 34, L09501.
Takahashi, T. et al., (2009) Climatological mean and decadal changes
in surface ocean pCO2, and net sea-air CO2 flux over the
global oceans. Deep Sea Research Part II: Topical Studies in
Oceanography 56, 554-577.
Tarnocai, C. et al., (2009) Soil organic carbon pools in the northern
circumpolar permafrost region. Global Biogeochemical Cycles
Thompson, D. W. J. & S. Solomon, (2002) Interpretation of recent
Southern Hemisphere Climate Change Science 296, 895-899.
Thorne, P. W., (2008) The answer is blowing in the wind. Nature
Geoscience 1, 347-348.
Trapp, R. J. et al., (2007) Changes in severe thunderstorm
environment frequency during the 21st century caused
by anthropogenically enhanced global radiative forcing.
Proceedings of the National Academy of Sciences 104, 19719-
Trapp, R. J. et al., (2009) Transient response of severe thunderstorm
forcing to elevated greenhouse gas concentrations.
Geophysical Research Letters 36, L01703.
Tripati, A. K. et al., (2009 in press) Coupling of CO2 and Ice Sheet
Stability Over Major Climate Transitions of the Last 20 Million
Years. Science, doi:10.1126/science.1178296.
Turner, J. et al., (2009) Non-annular atmospheric circulation change
induced by stratospheric ozone depletion and its role in
the recent increase of Antarctic sea ice extent. Geophysical
Research Letters 36, L08502.
van den Broeke, M., (2005) Strong surface melting preceded collapse of
Antarctic Peninsula ice shelf. Geophysical Research Letters 32,
Vecchi, G. A. et al., (2006) Weakening of tropical Pacific atmospheric
circulation due to anthropogenic forcing. Nature 441, 73-76.
Vecchi, G. A. et al., (2008) Whither hurricane activity? Science 322, 687-
Velicogna, I. & J. Wahr, (2006) Acceleration of Greenland ice mass loss in
spring 2004. Nature 443, 329-331.
Velicogna, I., (2009) Increasing rates of ice mass loss from the Greenland
and Antarctic ice sheets revealed by GRACE. Geophysical
Research Letters 36, L19503.
Vellinga, P. et al., (2008) Exploring high-end climate change scenarios for
flood protection of the Netherlands: an international scientific
assessment. (KNMI, Wageningen, the Netherlands).
Veron, J. E. N. et al., (2009) The coral reef crisis: the critical importance of
<350ppm CO2. Marine Pollution Bulletin 58, 1428-1437.
Wahl, E. R. et al., (2006) Comment on ?Reconstructing past climate from
noisy data?. Science 312, 529.
Wahl, E. R. & C. M. Ammann, (2007) Robustness of the Mann, Bradley,
Hughes reconstruction of Northern Hemisphere surface
temperatures: Examination of criticisms based on the nature
and processing of proxy climate evidence. Climatic Change
Wallack, J. S. & R. Ramanathan, (2009 accepted) Strategies for Hedging
Against Rapid Climate Change. Foreign Affairs.
WBGU – German Advisory Council on Global Change, (2006) The Future
Oceans – Warming Up, Rising High, Turning Sour. (WBGU,
WBGU – German Advisory Council on Global Change, (2009) Solving the
climate dilemma: The budget approach. (WBGU, Berlin).
Weart, S. & R. T. Pierrehumbert, (2007) A Saturated Gassy Argument,
Wentz, F. J. et al., (2007) How much more rain will global warming
bring? Science 317, 233-235.
Westerling, A. L. et al., (2006) Warming and Earlier Spring Increase
Western U.S. Forest Wildfire Activity. Science 313, 940-943.
Wild, M. et al., (2007) The impact of global dimming and brightening on
global warming. Geophysical Research Letters 34, L04702.
Wingham, D. et al., (2006) Mass balance of the Antarctic ice sheet.
Transactions of the Royal Society of London Series A 364, 1627-
Wouters, B. et al., (2008) GRACE observes small-scale mass loss in
Greenland. Geophysical Research Letters 35, L20501.
Yeh, S.-W. et al., (2009) El Ni?o in a changing climate. Nature 461, 511-
Yin, J. et al., (2009) Model projections of rapid sea-level rise on the
northeast coast of the United States. Nature Geoscience 2,
Zhang, X. et al., (2007) Detection of human influence on twentiethcentury
precipitation trends. Nature 448, 461-465.
Zickfeld, K. et al., (2009) Setting cumulative emissions targets to reduce
the risk of dangerous climate change. Proceedings of the
National Academy of Sciences 106, 16129-16134.
Zimov, S. A. et al., (2006) Permafrost and the Global Carbon Budget.
Science 312, 1612-1613.
Ian Allison is leader of the Ice Ocean Atmosphere and Climate
program in the Australian Antarctic Division, a Lead Author of
the IPCC Fourth Assessment Report and the President of the
International Association of Cryospheric Sciences.
Nathan Bindoff is Professor of Physical Oceanography at the
University of Tasmania, Australia, and a Coordinating Lead
Author of the IPCC Fourth Assessment Report.
Robert Bindschadler is Chief Scientist of the Laboratory for
Hydrospheric and Biospheric Processes at NASA Goddard Space
Flight Center, USA, a Senior Fellow of NASA Goddard, an AGU
Fellow and past President of the International Glaciological
Peter Cox is Professor and Met Office Chair in Climate System
Dynamics at the University of Exeter, UK, and a Lead Author of
the IPCC Fourth Assessment Report.
de Noblet, Nathalie
Nathalie de Noblet is a Research Scientist at the Laboratoire des
Sciences du Climat et de l?Environnement (LSCE), Gif-sur-Yvette,
Matthew England is an Australian Research Council Federation
Fellow, Professor of Physical Oceanography, an IPCC
Contributing Author and joint Director of the UNSW Climate
Change Research Centre (CCRC) at the University of New South
Jane Francis is a Senior Lecturer of Earth Sciences at Leeds
University, UK, and the Director of the Leeds University Centre
for Polar Science.
Nicolas Gruber is Professor of Environmental Physics at ETH
Zurich, Switzerland, and a contributing author of the IPCC
Fourth Assessment Report.
Alan Haywood is Reader in Palaeoclimatology at the School of
Earth and Environment, University of Leeds, UK, and a recent
recipient of the Philip Leverhulme Prize.
David Karoly is Professor of Meteorology and an ARC Federation
Fellow at the University of Melbourne, Australia, and a Lead
Author of the IPCC Third and Fourth Assessment Reports.
Georg Kaser is a glaciologist at the University of Innsbruck,
Austria, a Lead Author of the IPCC Fourth Assessment Report
and the IPCC Technical Paper on Climate Change and Water, and
the Immediate Past President of the International Association of
Le Qu?r?, Corinne
Corinne Le Qu?r? is Professor of Environmental Science at
the University of East Anglia, UK, a researcher at the British
Antarctic Survey, co-Chair of the Global Carbon Project and a
Lead Author of the IPCC Third and Fourth Assessment Reports.
Tim Lenton is Professor of Earth System Science at the University
of East Anglia, UK and the recipient of the Times Higher
Education Award for Research Project of the Year 2008 for his
work on climate tipping points.
Michael E. Mann is a Professor in the Department of Meteorology
at Penn State University, USA, Director of the Penn State Earth
System Science Center, and a Lead Author of the IPCC Third
Ben McNeil is an Australian Research Council Queen Elizabeth
II Research Fellow at the Climate Change Research Centre at the
University of New South Wales, Australia and an expert reviewer
of the IPCC Fourth Assessment Report.
Andy Pitman is joint director of the Climate Change Research
Centre at the University of New South Wales, Australia, and a
Lead Author of the IPCC Third and Fourth Assessment Reports.
Stefan Rahmstorf is Professor of Physics of the Oceans and
department head at the Potsdam Institute for Climate Impact
Research in Germany, a Lead Author of the IPCC Fourth
Assessment Report and a member of the German government?s
Advisory Council on Global Change.
Eric Rignot is a glaciologist and Senior Research Scientist at
NASA?s Jet Propulsion Laboratory, USA, a Professor of Earth
System Science at the University of California Irvine, and a Lead
Author of the IPCC Fourth Assessment Report.
Schellnhuber, Hans Joachim
Hans Joachim Schellnhuber is Professor for Theoretical Physics
and Director of the Potsdam-Institute for Climate Impact
Research, Germany, Chair of the German Advisory Council on
Global Change (WBGU) and a longstanding member of the
Intergovernmental Panel on Climate Change (IPCC).
Stephen Schneider is the Lane Professor of Interdisciplinary
Environmental Studies at Stanford University, an IPCC Lead
Author of all four Assessment and two Synthesis Reports, and
founder and Editor of the Journal Climatic Change.
Steven Sherwood is a Professor of atmospheric sciences at the
Climate Change Research Centre at the University of New South
Wales, Australia, and a contributing author to the IPCC Fourth
Richard C. J. Somerville is Distinguished Professor Emeritus at
Scripps Institution of Oceanography, University of California, San
Diego, USA and a Coordinating Lead Author of the IPCC Fourth
Konrad Steffen is Director of the Cooperative Institute for
Research in Environmental Sciences (CIRES) and Professor of
Climatology at the University of Colorado in Boulder, USA, and
the Chair of the World Climate Research Programme?s Climate
and Cryosphere (CliC) project.
Eric J. Steig is Director of the Quaternary Research Center,
and Professor of Earth and Space Sciences at the University of
Martin Visbeck is Professor of Physical Oceanography and
Deputy Director of the Leibniz Institute of Marine Sciences, IFMGEOMAR,
Germany, Chair of Kiel”s multidisciplinary research
cluster of excellence -The Future Ocean- and Co-Chair of the World Climate Research Programme’s Climate Variability and
Predictability (CLIVAR) Project.
Andrew Weaver is Professor and Canada Research Chair in
Climate Modelling and Analysis at the University of Victoria,
Canada, a Lead Author of the IPCC Second, Third, and Fourth
Assessment Reports and Chief Editor of the Journal of Climate.
Updating the World on the Latest Climate Science 2009
– 2009 UNSW Climate Change Research Centre
UNSW Sydney NSW 2052