Arctic ice retreating 30 years ahead of projections

Arctic sea ice is melting much more quickly than projected by even the most advanced computer models, a new government funded study has found. Comparing actual ice observations with climate models, the scientists conclude that the Arctic could be seasonally free of sea ice as early as 2020. Scientists at the National Center for Atmospheric Research and the University of Colorado's National Snow and Ice Data Center have demonstrated that the Arctic's ice cover is retreating more rapidly than estimated by any of the 18 computer models used by the Intergovernmental Panel on Climate Change in preparing its 2007 assessments. Whereas the computer models indicate that about half of the ice loss from 1979 to 2006 was due to increased greenhouse gases, and the other half due to natural variations in the climate system, the new study indicates that greenhouse gases may be playing a significantly greater role. The study, "Arctic Sea Ice Decline: Faster Than Forecast?" will appear on May 1 in the online edition of "Geophysical Research Letters." It was led by Julienne Stroeve of the National Snow and Ice Data Center and funded by the National Science Foundation and by NASA. The authors arrived at their conclusions by comparing model simulations of past Arctic climate and sea ice conditions with observations by satellites and other instruments. Satellites have flown over the Arctic and looked at sea ice since 1978. Some sea ice melts in the summer every year, even in the Arctic, where temperatures are still near freezing. But in 2002, satellites showed that the springtime melting of sea ice started earlier than normal. Satellites helped scientists learn that there was about 502,000 square miles less sea ice each September since 2001 than there typically was in previous Septembers. September marks the yearly minimum of sea ice in the Arctic. Stroeve and her team found that, on average, the models simulated a loss in September ice cover of 2.5 percent per decade from 1953 to 2006. The fastest rate of September retreat in any individual model was 5.4 percent per decade. But newly available data sets, blending early aircraft and ship reports with more recent satellite measurements that are considered more reliable than the earlier records, show that the September ice actually declined at a rate of about 7.8 percent per decade during the 1953-2006 period. The study indicates that, because of the disparity between the computer models and actual observations, the shrinking of summertime ice is about 30 years ahead of the climate model projections. As a result, the Arctic could be seasonally free of sea ice earlier than the time frame projected by the Intergovernmental Panel on Climate Change, which said the ice would disappear any time from 2050 to well beyond 2100. March is typically the month when Arctic sea ice is at its most extensive. The researchers found that although the loss of ice for March is far less dramatic than the September loss, the models underestimate it by a wide margin as well. The study concludes that the actual rate of sea ice loss in March, which averaged about 1.8 percent per decade in the 1953-2006 period, was three times larger than the mean from the computer models. The Arctic is especially sensitive to climate change partly because regions of sea ice, which reflect sunlight back into space and provide a cooling impact, are disappearing. In contrast, darker areas of open water, which are expanding, absorb sunlight and increase temperatures. This feedback loop has played a role in the increasingly rapid loss of ice in recent years, which accelerated to 9.1 percent per decade from 1979 to 2006, according to satellite observations. The authors speculate that the computer models may fail to capture the full impact of increased carbon dioxide and other greenhouse gases in the atmosphere.