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 Antarctic climate change - a position statement

Is a changing Antarctic climate signposting more ominous changes to come elsewhere? Like the ozone hole is it giving humanity early warning that more precipitous changes are to come? Or do the observations of ice retreat and warming temperatures lie within expected bounds in a highly variable climatic regime? These are the questions that spur on the meteorologists, glaciologists, geologists and atmospheric physicists who study the Antarctic environment. They summarise here their current views on these important issues. This statement, first issued in October 1999, was last updated in September 2002 .

The polar regions are an important part of the Earth's climate system and can exert strong controls on how global climate may change as a result of the accumulation of "greenhouse" gases in the atmosphere. The presence of ice, particularly sea ice, makes the climate of the polar regions particularly sensitive to warming by introducing a strong "feedback" mechanism into the climate system. Warming of Antarctica is of concern because the continental Antarctic ice sheets contain vast reserves of water and increased melting of this ice in a warmer climate could contribute to global sea level rise.

Global climate model predictions of how the Antarctic climate may change over the next 100 years differ in detail from model to model. Most models, however, indicate relatively modest temperature rises around Antarctica over the next 50 years and, over this time period, increased snowfall over the continent should more than compensate for increased melting of Antarctic ice and will thus partially offset the rise in sea level resulting from thermal expansion of the oceans and melting of icecaps and glaciers elsewhere in the world. However, many processes occurring in the polar regions are not well represented in climate models at present and further research is needed to improve our confidence in these predictions. This is particularly true for predictions beyond 50 years, when Antarctica may start to warm enough to have a significant impact on the ice sheets.

Few Antarctic stations have climate records extending back longer than 50 years so it is difficult to say whether temperature changes in Antarctica reflect those in the global record, which shows an overall warming trend of about 0.5°C between the late nineteenth century and the present Antarctic temperature records are characterised by a very high level of interannual variability, which makes the determination of trends from short records problematical. At most Antarctic stations, temperature trends are small and statistically insignificant. The magnitudes (and even the signs) of the trends are highly dependent on the exact period studied. The observing station network is sparse and there are large areas of the interior of the continent that have no representative climate records. It is thus not possible to say definitively whether Antarctica as a whole is warming or cooling. The extent of winter sea ice around Antarctica is thought to be a very sensitive indicator of climatic change but it has only been possible to observe this since suitable instruments were deployed on satellites in the early 1970s. Like the temperature records, the sea ice record exhibits a great deal of interannual variability. In recent years, reducing sea ice extent in some regions has been balanced by increasing extent in others and there is no evidence for a decline in overall Antarctic sea ice extent.

One region of Antarctica where detectable climatic change does seem to be occurring is the central and southern parts of the west coast of the Antarctic Peninsula. Climate records from this region extend back 50 years and, over this period, annual mean temperatures have risen by nearly 3°C - a far larger rise than seen elsewhere in the Southern Hemisphere. Although climate model predictions do indicate an enhanced response to future global warming in some parts of the polar regions, the Antarctic Peninsula is not one of these areas. The lack of a clear modelled association between Peninsula warming and global warming means that it is premature to attribute warming in the Peninsula to an enhanced "greenhouse" effect. However, climate models are currently unable to reproduce the warming observed over the past 50 years in the Peninsula (while they simulate global changes over this period quite well). Given this weakness in current model performance, future climate scenarios for the region must be treated with some caution and a link between Peninsula warming and the enhanced "greenhouse" effect cannot be ruled out completely at present. Whatever the case, we know that the climate of the region is highly sensitive as a result of complex interactions between atmosphere, oceans and sea-ice and studying it can tell us much about polar climate processes. Recent research also shows that the climate of this region is strongly influenced by climate variations in the subtropical and tropical South Pacific, such as those associated with El Niño - Southern Oscillation (ENSO). While such "teleconnections" are responsible for much of the short-term variability in climate seen in this region, their role in driving longer-term (decadal to century scale) change remains to be clarified.

The observed warming has already had a significant impact in the region and is believed to have caused the disintegration of both the Wordie Ice Shelf and the northern part of the Larsen Ice Shelf. Warmer conditions in recent years have also led to increased colonisation by plants at certain sites in the region. The collapse of the ice shelf that formerly occupied Prince Gustav Channel provided BAS scientists with a unique opportunity to study seabed sediments from beneath the former ice shelf and reconstruct a history of its extent. The sediment record suggests that an ice shelf has existed in this area for much of the past 10 000 years. However, there is evidence from iceberg-rafted rock debris in sediment cores that the ice shelf disintegrated about 5 000 years ago and re-formed some 2 000 years ago. The recent disintegration (and, by inference, the recent warming) is thus unusual, but not unprecedented in the context of the past 10 000 years.

Changes have also occurred in the upper atmosphere over Antarctica. Measurements made over the Antarctic Peninsula and the Falkland Islands show that the level of peak electron concentration in the ionospheric F-region (at about 300 km altitude) has fallen by about 8 km over 38 years. Unlike the surface temperature trends, these changes can be attributed to increased greenhouse gas concentrations with some level of confidence. While the lower atmosphere warms in response to increasing concentrations of greenhouse gases, the upper atmosphere cools. Theoretical studies indicate that the observed fall in the height of the F-region is compatible with expected temperature changes in the thermosphere.

Some of the issues covered in this position statement are discussed at greater length in a recent review paper by BAS staff: Vaughan, D. G., G. J. Marshall, W. M. Connolley, J. C. King, and R. M. Mulvaney, 2001: “Devil in the detail” . Science, 293, 1777-1779.

Dr John King and members of the Physical and Geological Sciences Divisions of the British Antarctic Survey. E-mail: jcki@bas.ac.uk

Revised by JCK, 10/9/2002


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