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2009 Literature Review Archives - Climate Trends
Arendt, A., J. Walsh and W. Harrison. Changes of Glaciers and Climate in Northwestern North America during the Late Twentieth Century. Journal of Climate Vol 22 Aug 1, 2009 pp 4117-4134; DOI: 10.1175/2009JCLI2784.1.
A study of 46 glaciers in Alaska and Canada found that 75% have been losing mass at an increasing rate over the period 1950-2002.
American scientists studied the mass balance of 46 inland and coastal glaciers in Alaska, Yukon and Northwest Territories over the 1950-2002 period. They used 1) climate records for daily maximum and minimum temperature and total daily precipitation from 67 NOAA and Environment Canada stations, 2) aircraft altimetry to calculate time variations in mass balance, and 3) a mass balance model with which to explore sensitivity parameters and assess primary drivers of glacier variations. For the period 1950-2002, winter and summer temperatures increased 2.0 ± 0.8°C and 1.0 ±0.4°C respectively along with an overall increase in precipitation for low elevation stations. Interior locations generally had the largest temperature increases. Although there are complications in linking glacier responses to climate changes (including glacier dynamics and the role of large synoptic-scale climatic conditions), the authors are confident in stating that they have observed a coherent signal of glacier mass loss over the region. Changes in climatology best predicted changes in glacier mass balance, as observed through altimetry measurements, for low-elevation glaciers. While increased summer temperatures can account for the observed glacier changes, more research is needed as to the role of the observed increases in winter temperatures. Confirmation of decreased winter snow accumulation for maritime glaciers will require measurements of climatic conditions in associated mountain regions.
Axford, Y. et al., 2009, Recent changes in a remote Arctic lake are unique within the past 200,000 years, PNAS,published online before print October 19, 2009, doi:10.1073/pnas.0907094106.
Paleo records from a lake in the Canadian Arctic show that 20th century environmental changes, including warming, are exceptional within the past 200,000 years.
At team of Canadian and American scientists used four well preserved sediment cores from a lake on Baffin Island, to evaluate how recent environmental and climatic changes compare with long-term natural variability over the last 200,000 years, including three interglacial periods. In comparison, the oldest intact ice core record in the Arctic comes from the Greenland Ice Sheet and spans 120,000 years. Aquatic invertebrate and algal communities preserved in the sediment cores, along with sediment biogeochemistry, were used as proxies for past climatic changes. The sediment cores show that the past three interglacials were characterized by similar trends in temperature and lake biology which in these time periods were driven by changes in summer solar insolation (i.e. natural orbital forcing). However, the 20th century is the only period of the record for which all proxies show trends consistent with warming despite declining orbital forcing, which, under natural conditions, would cause climatic cooling. Furthermore, the sediment record reveals the truly exceptional nature of the 20th century changes since this lake is not simply returning to conditions that prevailed during previous warm periods. Recent warm decades are shown to be ecologically unique over the past 200,000 years. The 20th century environmental changes found in this study are consistent with biological changes documented in a number of other Arctic lakes and ponds. This study now puts these in context within a longer record encompassing multiple interglacial periods. On the basis of this evidence, the authors conclude that ecosystems and environmental conditions in many lakes and ponds in the Arctic may now be outside the range of natural Quaternary variability.
Box, J.E., L. Yang, D.H. Bromwich and L,-S. Bai, 2009, Greenland Ice Sheet Surface Air Temperature Variability: 1840-2007, J. of Climate, vol. 22, pp. 4029-4049. DOI: 10.1175/2009JCLI2816.1.
A new analysis shows that although Greenland experienced a warming trend over the period 1994 to 2007, the annual mean warming was 1-1.5°C cooler than that of the Northern Hemisphere as a whole. Therefore, further melting of the ice sheet and mass deficits are expected as Greenland's climate catches up with the NH warming trend.
A set of 12 coastal and 40 inland ice surface air temperature records were used, in combination with climate model output, to reconstruct a long term (1840-2007) record of near surface air temperature variability over the Greenland ice sheet. The main results of the analysis show that over the period 1940-2007, annual means are dominated by winter variability, which is 5 times greater than summer temperature variability. Two strong warming trends occurred over the full period: 1919-1932 and 1994-2007. Consistent with previous studies, the 1920s annual warming trend is found to be about 30% larger in magnitude than the 1994-2007 warming. In the earlier warming period, the strongest positive anomalies were found in spring whereas during the 1994-2007 warming, stronger temperature anomalies occurred in autumn and winter. Two multidecadal cooling periods were also observed, in 1861-1919 and 1963-1984, that coincide with periods of multiple major volcanic eruptions. For most of the period of record the authors find that the GIS temperature anomalies are in phase with NH anomalies. However, they note that Greenland temperatures were still cooling during the 1970s and 1980s, years after NH temperatures began warming. This highlights the higher sensitivity of Greenland to volcanic (sulfate) cooling, especially in winter and in the western ice sheet margin, which is consistent with earlier studies. In addition, they find that, in contrast to the 1920s, the average annual warming over Greenland during 1994-2007 has not surpassed the NH anomaly, with temperature being 1.0° - 1.5°C cooler. They thus expect that the ice sheet melt rates and mass deficit will continue to grow in the early 21st century as Greenland's climate catches up with the NH warming trend and the strong regional Arctic climate warming projected by global climate models.
Chen, J. L., Wilson, C. R., Blankenship, D. and Tapley, B. D. 2009. Accelerated Antarctic ice loss from satellite gravity measurments. Nature Geoscience 2:859-862
New satellite data using gravity measurements indicate that ice sheet volume is currently declining over East Antarctica as well as West Antarctica. These results independently support similar conclusions of a previous study using radar data, but contradict earlier results that implied that the East Antarctic ice sheet was stable or slightly increasing in volume.
In 2008, researchers using satellite-based radar technology concluded that there has been a decline in ice volume of both the East Antarctica and West Antarctica ice sheets in recent years. Their results contradicted those from earlier studies that suggested that, while West Antarctica was losing ice, the East Antarctica ice sheet volume was stable or increasing slightly. In a recent issue of Nature Geoscience, researchers from the University of Texas report on new results that independently support the evidence from the radar data. These researchers employed a satellite based gravity measuring instrument to monitor the change in total ice mass over Antarctica since 2002. The results show that, between April 2002 and January 2009, West Antarctica experienced a net average annual loss of 132 billion tonnes of ice. However, East Antarctica also lost a net average of 57 billion tonnes of glacial ice per year, mostly from coastal regions. Most of the loss occurred after 2006. These results suggest that melting Antarctic ice sheets may be contributing more to global sea level rise than previously estimated.
Easterling, D.R. and Wehner, M.F. 2009. Is the climate warming or cooling? GRL 36, L08706, doi:10.1029/GL037810, 2009.
As the Earth warms, it will continue to also experience short intervals of little change or cooling.
While average global temperature increased rapidly during the 1990s, this trend appears to have stopped during recent years. Skeptics have argued that this indicates that the Earth is no longer warming, and that human influences on the climate system have been exaggerated. In a recent research paper published in the scientific journal Geophysical Research Letters, two American scientists challenge this conclusion by reiterating that the climate system includes internal natural variability that constantly causes climate to vary about its mean state, causing one decade to be warmer, and another colder than the mean state. They demonstrate through model simulations of the climate response to future changes in greenhouse gas and aerosol concentrations that such natural variability will continue to affect the climate system as it warms. Hence, during the next century, as the climate undergoes a long-term warming, there will be shorter time periods of a decade or two when it will not show a trend or may even temporarily cool. These will be offset by other periods when warming is temporarily faster than the long term average Therefore, the lack of trend during the past few years is neither a surprise nor an indication that the climate is no longer warming, but fully consistent with the strong long term warming trend.
Ettema, J., et al., 2009. Higher surface mass balance of the Greenland ice sheet revealed by high-resolution climate modeling, Geophysical Research Letters, vol. 36, L12501, doi:10.1029/2009GL038110.
A new study finds that between 1958 and 2007 considerably more mass accumulated on the Greenland ice sheet (GrIS) than reported previously. However, since 1990 a steady 3% per year increase in surface melt and runoff has led to a rapid decrease in the surface mass balance of the GrIS.
In the last few years, many studies have tried to estimate the rate at which the Greenland ice sheet (GrIS) is currently losing mass and how this is contributing to sea level rise (SLR). In this new study, the authors quantify the surface mass balance (SMB) the GrIS for the period 1958-2008, using a regional climate model at unprecedented high horizontal resolution (~ 11km) coupled with a snow model. Their results show that total annual precipitation on the GrIS is between 7 to 24% higher, and total accumulation (precipitation minus sublimation) is 40% higher, than recent model-based estimates. Subtracting total runoff and evaporation / sublimation from total snowfall and rainfall, they obtain a total ice sheet SMB that is 32 to 63% larger than recent model based estimates for the same period. The authors also found that while the SMB over the full 1958-2007 period reveals the classic pattern expected in a warming climate, with increased snowfall in the interior and enhanced runoff from the marginal ablation zone, in the period 1990-2007, total runoff increased significantly, by a steady 3% per year, especially in southeast Greenland. This has led to a rapid decrease in the GrIS SMB since 1990.
Howell, S.E.L., Duguay, C.R. and Markus, T. 2009. Sea ice conditions and melt season duration variability within the Canadian Arctic Archipelago: 1979-2008. GRL 36, L10502, doi:1029/2009GL037681, 2009; Sou, T. and Flato, G. 2009. Sea ice in the Canadian Arctic Archipelago: Modeling the past (1950-2004) and the future (2041-60). J. Climate 22:2181-2197.
Recent Canadian studies indicate that the amount of ice in the Canadian Arctic Archipelago is decreasing, but suggest that the region will continue to be an important refuge for in situ and Arctic Ocean multi-year ice throughout at least the next half century.
Two recent studies have provided some new insights into trends and projections for sea ice conditions within the various channels of the Canadian Arctic Archipelago. The first, published in Geophysical Research Letters by a team of researchers headed by Stephen Howell of the University of Waterloo, notes that melt season duration in the region has increased by an average 7 days per decade since 1979 and that average multi-year ice concentrations in late summer has been declining by 6.4%/decade. However, invasion of old ice from the Arctic Ocean into the region replaces much of the disappearing multi-year ice, and the authors suggest that the key shipping channels will therefore continue to be susceptible to its presence until the Arctic Ocean becomes entirely ice free in late summers. The second study, undertaken by Canadian modelers Tessa Sou and Greg Flato and published in the Journal of Climate, uses a high resolution ice-ocean regional model to examine how ice conditions in the archipelago region respond to climate projections generated by the Canadian GCM under the IPCC A2 greenhouse gas emission scenario. Results suggest that, by mid-century, the region's waterways will continue to be ice covered in winter, but that summer concentrations will decrease by 45%. Mean ice thickness will decrease by 17% in winter and 36% in summer. Thus, while reduced summer ice cover would facilitate commercial shipping through the channels, a completely ice free archipelago by 2050 is unlikely.
Kaufman, D.S. et al 2009, Recent warming reverses long-term Arctic cooling. Science, Vol 325, doi:10.1126/science.1173983.
A multi-proxy reconstruction of Arctic summer temperatures suggests that late 20th century temperatures are the warmest of the past 2000 years.
Kaufman et al. (2009) outline the development of the longest reconstruction of Arctic temperatures available to date. The 2000 year decadally-resolved reconstruction is based on warm-season paleotemperature information archived in tree-ring chronologies, ice core records and lake sediment records from 23 Arctic locations (poleward of 60°N). Reconstructed Arctic summer temperatures showed a steady decline from year 1 to 1900 C.E. (-0.22o ±0.06°C per 1000 years) after which time temperatures have increased. The authors note that this decline in proxy-summer temperature is coincident with an orbitally-driven decline in summer insolation at high northern latitudes that was likely amplified by albedo-related positive feedbacks. Unlike the Arctic reconstruction, previous reconstructions of mean Northern Hemisphere (NH) temperatures do not show this millennial trend. The Arctic reconstruction does share centennial-scale variations with the mean NH records, deviating below the linear trend from years 450-700, above the trend from 900-1050, and with the coldest conditions reconstructed for 1600-1850. Over the late 20th century, the reconstructed summer temperatures for the Arctic are the warmest of the past two millennia (despite continued decreases in summer insolation) and, the authors conclude, they appear to have emerged above the natural variability.
1) Kwok, R., G. F. Cunningham, M. Wensnahan, I. Rigor, H. J. Zwally, and D. Yi. 2009. Thinning and volume loss of the Arctic Ocean sea ice cover: 2003-2008, JGR 114, C07005, doi:10.1029/2009JC005312. 2) R. Kwok and D. A. Rothrock. 2009. Decline in Arctic sea ice thickness from submarine and ICESat records: 1958-2008. GRL Vol. 36, L15501, doi:10.1029/2009GL039035, 2009.
Satellite and submarine measurements show a dramatic overall decline in ice thickness and a recent loss of ice volume.
Two recent papers provide estimates of Arctic sea ice decline based on satellite and submarine measurements. In the first, Kwok et al examine sea ice thickness and volume for the short 2003-2008 period based on satellite data from the recently launched ICESat. The basin-wide decline in thickness of Arctic Ocean sea ice was -0.17metres/year for the study period. The authors also found a remarkable thinning of ~0.6metres in multi-year ice coinciding with a 42% decrease in multi -year ice coverage since 2005. In winter the volume of multi-year ice has also declined by >40% since 2005. Arctic Ocean ice cover is now predominantly seasonal ice, rather than multi-year ice. The changes in sea ice thickness and volume are attributable to the thinning of multi-year ice cover and extent. An imbalance in the cycle of multi-year ice replenishment and export out of the Arctic Ocean basin plays a significant role in the loss of sea ice volume although ice export itself does not fully explain the recent record minimum years.
In the second paper Kwok and Rothrock review the ICESat data in the context of declassified U.S. Navy submarine records that cover 38% of the Arctic Ocean for the period 1958-2000 (with gaps). Comparing the two data sets at the end of the summer melt season there has been a 1.6 metre (53%) decline between the initial submarine observations for the 1958-1976 period and the satellite data for the 2003-2008 period.
The analysis of the combined data sets spanning 50 years confirms an Arctic sea ice thinning trend, characterized in the earlier years by the remarkable thinning of perennial sea ice and in later years by thinning linked to the dramatic drops in sea ice extent that occurred in 2005 and 2007.
Liu, Z, et al. 2009, Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming. Science, Vol 325, pp 310-314, doi: 10.1126/science.1171041; See also Timmermann, A. and L. Menviel. 2009. What drives climate flip-flops? Science, Vol 325, pp. 273-274, doi: 10.1126/science.1177159.
Numerical modeling study of last deglaciation offers new insight into possible causes of past abrupt climate changes.
Liu et al. (2009) describe the first attempt to use a coupled atmosphere-ocean GCM to produce transient simulations of climate evolution from the Last Glacial Maximum (~21 ka) through the cool Heinrich Event 1 (H1, ~17ka) to the Bølling-Allerød (BA, ~14.5 ka). A period of rapid warming in the North Atlantic region at the onset of the BA has been linked previously with changes in the Atlantic meridional overturning circulation (AMOC) and its associated heat transport. The state-of-the-art GCM (NCAR CCSM3) employed in this study is similar to those used to develop future climate projections but was forced with past changes in solar insolation, the observed history of GHG concentrations and with reconstructed changes in continental ice sheets, coast lines and meltwater discharge. The model results match those inferred from observations, previous experiments and paleoproxy reconstructions. In particular, the simulations produce a BA warming of roughly 15°C over Greenland and indicate that the warming may have occurred in response to a sudden termination of fresh meltwater discharge inputs to the North Atlantic (a largely linear response that 'restarted' AMOC). This result differs significantly from previous studies that attributed the resumption of AMOC during the BA warming to a non-linear response to a gradually varying forcing as a result of hysteresis in the AMOC system (i.e. the existence of two stable circulation regimes).
Mann, M.E., J.D. Woodruff, J.P. Donnelly and Z. Zhang. Atlantic hurricanes and climate over the past 1,500 years. Nature, Vol 460, doi:10.1038/nature08219). Also: Gilbert, N. Hurricane peak not unique. Nature. Doi:10.1038/news.2009.821.
Estimates of Atlantic hurricane activity over the past 1500 years show that an historical peak in activity around AD 900-1100 corresponds with tropical Atlantic warmth and La-Niña-like conditions in the Pacific.
Mann et al. (2009) describe the development of two 1500-year records of tropical cyclone activity for the Atlantic. The first is a composite record of sedimentary evidence of past landfalling hurricanes derived from eight sites across the North Atlantic basin. The second is generated using a statistical model of hurricane activity driven by proxy reconstructions of ENSO, tropical Atlantic SSTs and the North Atlantic Oscillation (NAO). Despite uncertainties with the two techniques, the two completely independent paleo-hurricane reconstructions both show a peak in hurricane activity in the medieval era (AD 900-1100) and a decline in activity after ~AD 1200. The medieval peak in hurricane activity matches or exceeds current levels of Atlantic hurricane activity suggesting that the high activity levels of the past decade are not unique. Assessment of the climate proxies used to drive the statistical model show that the increased activity in the medieval period was related to warm SSTs in the tropical Atlantic and La-Niña like conditions in the Pacific. Although there are considerable uncertainties about the future behaviour of ENSO and other components of the climate system, the first author suggests that the study attests to the robustness of the relationship between sea surface temperatures and cyclone activity lending support to the idea that rising global temperatures (if accompanied by rising SSTs) may increase hurricane activity (see NatureNews article referenced below).
Park, G-H, K. Lee and P. Tishchenko. 2008. Sudden, considerable reduction in recent uptake of anthropogenic CO2 by the East/Japan Sea, GRL, Vol. 35, L23611, doi:10.1029/2008GL036118; Thomas, H. et al. 2008. Changes in the North Atlantic Oscillation influence CO2 uptake in the North Atlantic over the past 2 decades. Global Biogeochemical Cycles, Vol. 22, GB4027, doi:10.1029/2007GB003167, 2008.
Studies show that the observed recent decline in the uptake of CO2 in some oceanic regions of the Northern Hemisphere is tied to changing surface conditions over short time periods.
Oceans play a crucial role in the global carbon cycle, absorbing CO2 from the atmosphere and thereby acting as carbon sinks. Some recent studies have reported a decline in CO2 uptake, but key uncertainties remain about how this variability is tied to global warming trends. Two teams of scientists looked at the recent rapid and substantial decline in CO2 uptake in the North Atlantic (Thomas et al.) and in the East/Japan Sea (Park et al.). Park et al. used observational data from three surveys (1992, 1999 and 2007), where measurements were taken at varying depths at 19 stations. The results show a halving of the uptake rate of anthropogenic CO2 by the East/Japan Sea between the periods 1992-1999 and 1999-2007. Furthermore, nearly all the anthropogenic CO2 taken up in the more recent period was confined to waters less than 300 m in depth. The authors attribute this to a considerable weakening of the overturning circulation, which is the key means of transport of anthropogenic CO2-charged surface water to the deepest parts of the East/Japan Sea. Thomas et al. used a global ocean carbon model to investigate how carbon uptake has changed in regions of the North Atlantic from the period 1979-2004 to 1995-2004. In this case, the results show that the CO2 air-sea fluxes in the temperate North Atlantic exhibit large multiannual variability on sub-basin scales, in response to North Atlantic Oscillation (NAO) conditions. They argue that rapidly declining trends of ocean CO2 uptake, estimated recently from field data, may be heavily influenced by recent weakly positive or negative NAO conditions and suggest therefore that CO2 uptake may rebound in the eastern temperate North Atlantic during future periods of more positive NAO. This was observed during the sustained positive NAO period in the early 1990s. These studies highlight that long-term, coherent ocean carbon observing systems are important in order to detect trends in CO2 uptake associated with climate change.
Rennermalm, A.K., L.C. Smith, J.C. Stroeve and V.W. Chu, 2009. Does sea ice influence Greenland ice sheet surface-melt?, Environmental Research Letters, volume 4, doi:10.1088/1748-9326/4/2/024011.
A recent study finds that current pattern of sea ice retreat in the Arctic can enhance the surface melting on the Greenland Ice Sheet, especially in the southwestern part of it in the late summer\fall.
American scientists explore the hypothesis that the presence or absence of offshore sea ice influences melt extent on the Greenland Ice Sheet. Using microwave satellite data for the period 1979-2007, they looked at the interactions between the recent decreases in Arctic sea ice and increases in Greenland ice sheet (GIS) surface melt. Their results show that the mean sea ice edge retreats progressively northward during the summer, while the ice sheet surface-melt area is expanding. However, while the expansion of the GIS surface-melt throughout the summer shows a relatively uniform zonal inwards pattern, the pattern of sea ice retreat differs for the east and west coasts of Greenland. Along the eastern coast, the sea ice edge retreats northward and stays close to the coast till August, but along the western coastal areas, it retreats northwest as early as May, producing a growing near-shore open-water area. In line with these patterns, their analysis shows the strongest correlation between open-water and ice sheet surface-melt in western Greenland, particularly in the southwest region of Kangerlussuaq in the latter part of the melt season (August-September). In this region, ice sheet melt is influenced by a number of factors, including the arrival of westerly winds allows advection of ocean heat onto the ice sheet,, potentially enhancing ice sheet melt, These results suggest that sea ice retreat can enhance surface-melting on the Greenland ice sheet, especially in southwestern Greenland during late summer. In view of their results, the authors note that if climate model predictions for the 21st century are correct, with a sea-ice edge retreating ever further northward, the enhanced GIS surface-melt could increase the discharge from the Jakobshavn ice-stream which, currently, accounts for about 10% of Greenland mass losses.
S. S. Son, N. F. Tandon, L. M. Polvani, and D. W. Waugh. 2009. Ozone hole and Southern Hemisphere climate change. GRL, vol. 36, L15705, doi: 10.1029/2009GL038671.
Changes in stratospheric ozone levels over Antarctica are shown to have affected the entire atmospheric circulation in the Southern Hemisphere. The expected recovery of the ozone layer over the next fifty years is likely to have an impact on the future rate of climate change.
Over the past several years, the increase in GHGs and the associated warming have been linked to a suite of changes observed in the atmospheric circulation in the Southern Hemisphere (SH). These include an increase in the tropopause height, a poleward intensification of the westerly jet, a poleward shift in storm tracks, and a poleward expansion of the Hadley cell. Depletion of the stratospheric ozone layer by anthropogenic chemicals has also been shown to affect the Southern Hemisphere climate system through changes in sea-level pressure including the Southern Annular Mode (SAM) and the position of the mid-latitude jet stream. This study investigated whether changes in stratospheric ozone levels might have a more pervasive impact on Southern Hemisphere climate than previously thought. Results from past and future climate simulations by over 20 GCMs were examined, with future scenarios driven by moderate GHG forcing (emission scenario A1B). The method with which ozone was represented in the models was the key discriminating factor among the models. Some models used an ozone depletion and recovery scheme while others used a simple monthly climatological field (which does not change from year to year). By comparing results from models with constant or changing ozone forcing, the authors showed that depletion of stratospheric ozone has had far-reaching impacts on the SH summer climate system. Ozone depletion has likely contributed to decreasing lower-stratospheric temperatures, increased tropopause heights, poleward shifting of the westerly jet in mid-latitudes, expansion of the Hadley cell poleward, increasing high-latitude precipitation and increasing the SAM index, indicating that many of the effects attributed to GHGs may be partially in response to changes in stratospheric ozone. Ozone depletion effects on the climate system will likely be reversed over the next 50 to 60 years with the anticipated recovery of the ozone layer as a result of controls on ozone depleting substances under the Montreal Protocol. With the end to the ozone depletion forcing, the models' projections indicated a slower pace of change within the Antarctic climate circulation system resulting from only increased GHGs. At the surface, we may observe an increase in the pace of warming over the Antarctica.
Steig, E.J., D.P. Schneider, S.D. Rutherford, M.E. Mann, J.C. Comiso, and D.T. Shindell. 2009. Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year. Nature Vol 457 22 January, 2009, pp459-463.
A new assessment of temperature trends over Antarctica provides strong evidence that the interior of the continent is warming in addition to the Antarctic Peninsula. This result updates earlier research that suggested a slight cooling in the interior of the continent.
The Antarctic Peninsula is one of the most rapidly warming regions on Earth. Elsewhere on the continent, however, mainly insignificant trends over recent decades have been reported. It is widely acknowledged though that obtaining a clear picture of Antarctic temperature trends has been hampered by the lack of observations for most of the interior of the continent. This new study combined data from weather stations with that from satellites to form a 50-year reconstruction of climate for the continent. The results paint a different picture of continental changes over this time period than has prevailed up until now. Significant warming is shown, not only along the Peninsula, but extending over most of West Antarctica as well. In fact, the rate of warming has been greater (0.17 ± 0.06°C/decade) across the West Antarctic as a whole than it has been across the Peninsula (0.11 ± 0.04°C/decade) over the past 50 years. Significant warming was also observed in East Antarctica, although cooling was apparent in the autumn. The continent wide trend is 0.12 ± 0.07°C/decade over the 1957-2006 period. The authors state that this warming trend is difficult to explain without accounting for the effect of increases in atmospheric greenhouse gas concentrations.
Stine, A.R., P. Huybers and I.Y. Fung, 2009. Changes in the phase of the annual cycle of surface temperature. Nature, Vol 457, doi:10.1038/nature07675.
Observations show a decrease in amplitude of the annual cycle of surface temperature and a shift toward earlier seasons. However, few of the global climate models presented in the IPCC Fourth Assessment Report can replicate these trends.
In a recent paper, three American scientists looked at the annual cycle of global mean surface temperature to analyse trends in the phase (timing of seasons) and the amplitude of that cycle, over the period 1954-2007. They used gridded 5°X5° temperature records from the University of East Anglia's Climate Research Unit for land and ocean. They found that land surface temperatures show trends predominantly towards earlier seasons over the past 54 years (1.7 days earlier on average). For the same period, the ocean trends are large (up to 5.0 days) but regionally disparate with trends towards later seasons north of 50°N and primarily toward earlier seasons south of 50°N. When compared with earlier records (1900-1953, then back to 1850), they found that the 1954-2007 trend is anomalous and not consistent with the structure of natural variability found in the earlier records. They also found a decrease, globally, in the amplitude of the annual cycle on land, with a mean decrease of 2.5°C over the period. That decrease represents the well-known amplification of winter warming in northern regions. However, in some regions like Western Europe and the Middle East, the amplitude has increased, associated with a greater warming in summer than in winter. To better understand these changes, the authors analyzed the results of 72 global climate model simulations of twentieth-century climate. They found that the observed decrease in amplitude of the land temperature cycle has a larger magnitude than that of the simulated climate in all but six of the model simulations, and that no model reproduced the observed shift towards earlier seasons. Therefore, the authors conclude that the mechanisms behind the observed changes are not captured by the current generation of models.
Tripati, A. K., C. D. Roberts and R. A. Eagle. 2009. Coupling of CO2 and ice sheet stability over major climate transitions of the last 20 million years. Science December 4. Vol 326 No. 5958, pp 1394-1397, doi:10.1126/science.1178296.
Recent study finds that the close coupling between CO2 and climate extends through major climate transitions of the past 20 million years and that CO2 levels have not been as high as present (387 ppmv) since the Mid-Miocene (~15 million years ago).
Tripati et al. employ a technique based on the ratio of boron (B) to calcium (Ca) in foraminifera (single celled marine algae) from sites in the western tropical Pacific Ocean to study CO2 levels during the major climate transitions of the last 20 million years. Over the past 800, 000 years, their results coincide with atmospheric CO2 concentrations derived from Antarctic ice cores showing concentrations ranged between ~180 ppmv and ~280 ppmv prior to the industrial revolution. The only period when CO2 levels approached modern levels was during the Early and Mid-Miocene (~20-14 million years ago; Ma) when concentrations were sustained at ~400 ppmv. Climate proxies indicate that global temperatures during the Middle Miocene were ~3-6oC warmer and sea level 25-40 m higher than today. Decreases in CO2 were coincident with major episodes of glacial expansion documented in other climate proxies. For example, the B/Ca record indicates that CO2 dropped by ~200 ppmv over the period ~14-10 million years ago and by ~150 ppmv during the late Pliocene (~3.3-2.4 Ma). The results indicate that changes in CO2 were closely related to the climate evolution of the Middle and Late Miocene and the Late Pliocene and therefore likely played an important role in driving these transitions.
M. Van den Broeke, J. Bamber, J. Ettema, E. Rignot, E. Schrama, W. J. van de Berg, E. van Meijgaard, I. Velicogna, and B. Wouters. 2009. Partitioning Recent Greenland Mass Loss. Science 12 Vol. 326. no. 5955. Nov 2009.
Two independent methods for determining the rate of ice mass loss of the Greenland Ice Sheet showed that in recent years, surface processes and ice discharge have contributed equally to the loss of ice.
Recent studies have indicated that the rate at which ice is lost from the Greenland ice sheet (GIS) is accelerating but estimates of that rate have varied substantially. This study by van den Broeke at al., obtained consistent estimates of ice mass loss for the GIS for the 2003 - 2008 period using two independent methods. Results from a mass budget method, based on quantifying the individual components of ice sheet mass balance [surface mass balance (SMB) and ice discharge (D)], were validated with data from the Gravity Recovery and Climate Experiment (GRACE) satellites (which use gravity measurements to estimate changes in total ice mass). The combination of results enabled the investigators to resolve individual components of surface processes and ice dynamics that were contributing to recent ice sheet mass loss in space and time. Since 2000, the GIS mass balance has been persistently negative, caused by a simultaneous decrease in surface mass balance and an increase in ice discharge. The study found a total net mass loss of about 1500 gigatons from 2000 to 2008, equivalent to 0.46 millimeters per year of global sea rise. This was equally split between contributions from surface processes and ice discharge, a result which differs from previous work which attributed about 61% (rather than 50%) to ice discharge. Since 2006, higher summer melt rates have led to accelerated ice mass loss at a rate of 273 gigatons per year (from 166 gigatons per year over the 2000-2008 period), which has resulted in 0.75 millimeter per year of equivalent sea level rise.
Velicogna, I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters, doi:10.1029/2009GL040222.) See also related article: R.A. Kerr. Both of the world's ice sheets may be shrinking faster and faster. Science, Vol 326, p. 217. Pritchard, H.D., R.J. Arthern, D.G. Vaughan and L.A. Edwards. 2009. Extensive dynamic thinning of the margins of the Greenland and Antarctic ice sheets. Nature, doi:10.1038/nature08471.
Two studies using satellite data reveal accelerated rates of ice mass-loss over the current decade for the Greenland and Antarctic ice sheets with the greatest rates along coastal margins.
Two recent studies employing different satellite data indicate that the rate of ice loss of the Greenland and Antarctic ice sheets is accelerating. Velicogna (2009) used satellite-based gravity measurements to estimate monthly ice mass changes of the Greenland and Antarctic ice sheets over the period April 2002 to February 2009. The results show that, over the short 7 year period, the rate of ice mass-losses accelerated significantly for both the Greenland and Antarctic ice sheets (the annual rate doubled for Greenland and more than doubled for Antarctica. The combined contribution of ice mass-loss from the two ice sheets over the study period corresponded to an equivalent acceleration in sea level rise of 0.17 ± 0.05 mm/yr2. In a separate study recently published in Nature, Pritchard et al (2009) use satellite based laser altimetry data to evaluate dynamic thinning (accelerated flow) along the entire coastal margins of the Greenland and Antarctic ice sheets over the period February 2003 to November 2007. These data are available at a much higher temporal and spatial resolution than previous studies using radar altimeters which enabled elevation changes related to fast flowing ice to be distinguished from those related to other causes. The results of the study reveal that the greatest changes in the ice sheets are currently occurring at coastal margins. In Greenland, dynamic thinning is evident at all latitudes and 81 of the 111 glaciers surveyed thinned dynamically at double the rate of slower flowing ice at the same altitude. Dynamic thinning is found to penetrate far into the interior of both the Greenland and Antarctic ice sheet and is spreading as ice sheets thin due to ocean-driven melt (i.e., it is more extensive and important than previously thought). These studies suggest that continued monitoring of ice sheet loss, particularly related to dynamic thinning which is poorly understood, is essential for constraining ice sheet models thus providing better estimates of the future contributions of ice sheet losses to sea level changes.
Vinther et al. Holocene thinning of the Greenland ice sheet. Nature, Vol 46, doi:10.1038/nature08355. Also: Smith, K. Climate change warning from Greenland. Nature, doi:10.1038/news.2009.917.
New temperature and elevation histories developed for the Greenland Ice Sheet identify an unusually warm period (~2°C above present) in this region from about 9000 to 6000 years ago that was associated with rapid melting.
Understanding the response of the Greendland Ice Sheet (GIS) to past temperature changes can provide key insights into its dynamics and sensitivity to changes in temperature (even though the climate forcings in the distant past were obviously not anthropogenic). Vinther et al. (2009) describe an updated temperature and elevation history for the GIS derived from ice core data (five cores from the GIS and one from the Agassiz ice cap on neighbouring Ellesmere Island). Previous studies of GIS temperatures derived from stable isotope data (Δ18O) from water in dated ice cores found little spatially consistent evidence of the warmer conditions documented elsewhere for the northern latitudes during the early Holocene (9,000-6,000 years ago). Vinther et al. apply a new technique whereby changes in GIS elevation (which influence Δ18O concentrations) are better accounted for. The results indicate that temperatures over this early Holocene period were homogeneous across the GIS and were ~2°C warmer than present. The corresponding elevation data suggest that the GIS responded with rapid melting and that maximum thinning occurred near the GIS margins (with some evidence that the GIS shrank by 200 km at its margins and decreased in elevation by 150 m at the summit; see related NatureNews article). In light of their results, the authors conclude that regional temperature increases of a few degrees Celsius may result in larger GIS mass loss, and therefore a larger contribution to sea level rise, than projected previously by state-of-the-art ice sheet models.
Wang, X.L., F.W. Zwiers, V.R. Swail and Y. Feng, 2009, Trends and variability of storminess in the Northeast Atlantic region, 1874-2007, Clim, Dyn., 33: 1179-1195.
Environment Canada scientists document profound decadal or longer time scale fluctuations in storminess conditions in the Northeast Atlantic region over the past 130 years.
In this study, four scientists from Environment Canada look in detail at seasonal and regional changes in storminess in the Northeast Atlantic region over the period 1874-2007. Their analysis showed that decadal or longer time-scale variability in storminess is characteristic of this region, with considerable seasonal and regional differences. The most notable differences are between winter and summer, and between the North Sea area and other parts of the region. In particular, winter storminess shows an unprecedented maximum in the early 1990s, while the summer maximum occurred around 1880, in both cases in the North Sea. In addition, winter storminess shows a steady upward trend in the north eastern region, and a decline in the western area and the Norwegian Sea, while summer storminess appears to have declined in most part of the NEAR, including the North Sea. The authors note that the maximum winter storminess in the early 1990s is consistent with severe storms in the United Kingdom during the same period. Using the North Atlantic Oscillation (NAO) index, they found that storminess conditions in the NEAR are significantly correlated with this index in all seasons but autumn. This is especially true in winter and spring where the higher the NAO index, the rougher the storminess conditions.
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