IS RECENT ANTARCTIC CLIMATE CHANGE EXCEPTIONAL?

The Antarctic Peninsula is well known to have warmed substantially through most of the 20th century. There is now abundant evidence that continental West Antarctica has also warmed significantly during at least the last 50 years, both at the surface and throughout the troposphere, and that the relatively small changes over East Antarctica are due to suppression of warming from anthropogenic ozone depletion. Concomitant changes in atmospheric circulation and sea ice distributions over the same time period are also well documented. Are these recent changes exceptional, in the context of natural climate variability over longer time periods? Answering this question is challenging because of the paucity of instrumental records prior to the 1957 International Geophysical Year, and the very large magnitude of natural decadal variability observed in those instrumental data that do exist. However, a growing database of paleoclimate proxy data points strongly to the rate of change over the last century being atypical, if not demonstrably exceptional, relative to previous centuries. In particular, high resolution ice core data, and independent results from borehole thermometry, indicate that warming of West (and probably also East) Antarctica dates to at least 1930. Similar rates of warming over similar time periods occur at most twice in the previous thousand years, based on the new deep ice core from the West Antarctic Divide site. A parsimonious explanation for the century-scale warming trend is that it is a direct reflection of warming in the tropical Pacific, due to the strong influence of tropical sea surface temperature anomalies on atmospheric dynamics extending as far south as the Amundsen Sea. The dynamical response at high southern latitudes to continued anthropogenic warming in the tropics has not been adequately explored. However, if current trends continue, rapid warming in the glaciologically vulnerable area of the Pine Island and Thwaites glaciers could begin to influence the rate of ice discharge in the next century, as has already happened on the Antarctic Peninsula.