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April 19, 2004

Climate study shows disappearing Arctic sea ice could reduce water availability in western U.S.

By Tim Stephens

The sea ice covering much of the Arctic Ocean is melting, a trend that may have dramatic consequences for the western United States. UCSC researchers recently looked at the long-term effects of reduced Arctic sea ice on the global climate, and their most striking finding was a significant reduction in rain and snowfall in the American West.

According to a NASA study, Arctic perennial sea ice has been decreasing at a rate of 9 percent per decade since the 1970s. These images show the minimum sea ice concentration over the Arctic Circle in 1973, above, and 2003, below.
Images courtesy Scientific Visualizations Studio, NASA GSFC

The study highlights the vulnerability of western states, which depend on winter precipitation for their water supplies, to changes in the regional climate.

The results also show the surprising ways in which a small change in one component of the global climate system can affect particular regions, said Lisa Sloan, professor of Earth sciences.

"We were surprised at the result ourselves, but it shows how interconnected the climate system is. Here we are reducing Arctic sea ice, and the biggest climatic response is felt in an entirely different part of the world," she said.

Sloan and graduate student Jacob Sewall used powerful computers running a global climate model developed by the National Center for Atmospheric Research to simulate the effects of reduced Arctic sea ice. Their findings were published online by the journal Geophysical Research Letters on March 24 and will appear in a subsequent print issue of the journal.

Sewall and Sloan based their study on projections of recent trends in Arctic sea ice cover published by a NASA researcher in 2002. Taking the projected ice cover in the year 2050 as a starting point, the researchers ran the model to see how the global climate would behave.

What they found was a change in atmospheric circulation patterns that caused a small northward shift in the paths of winter storms over western North America. This shift in winter storm tracks resulted in significantly reduced winter precipitation from southern British Columbia to the Gulf of California. In some areas, average annual precipitation dropped by as much as 30 percent. The reductions were greatest along the West Coast, with lesser changes further inland. But even as far inland as the Rocky Mountains, winter precipitation fell by 17 percent.

The sea ice acts like a lid over the ocean surface during the winter, blocking the transfer of heat from the ocean to the atmosphere, Sewall explained. Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air. The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice in the Greenland Sea and a few other locations, Sewall said.

"The projected reduction in sea ice cover during the winter is small compared to the reduction in sea ice during the summer, but it ends up having a big effect on North America," he said.

Sewall noted that the study only looked at the direct climate response to a reduction in Arctic sea ice and did not take into account additional climate effects that may result from increasing levels of greenhouse gases. Increased greenhouse gases such as carbon dioxide are a major factor driving global warming, a trend that is expected to continue well into the future.

"In a scenario with increased greenhouse gases, we would expect to see other effects on the climate that would interact with the effects of reduced sea ice," Sewall said.

Higher temperatures due to global warming, for example, would increase the rate of evaporation and exacerbate the effects of decreased precipitation on the water supply. But there could also be effects on the climate system that might counteract the influence of reduced Arctic sea ice on winter precipitation in the West, Sewall said.

Arctic sea ice has been declining gradually over the past century, but the pace of the decline has picked up during the past two decades. The cause of the decline remains unclear, and it is not certain that the trend will continue.

Nevertheless, the new study serves as a warning that climate change can have small effects in one location that propagate through the system to become big effects somewhere else, Sloan said.

"As the climate changes, the effects will vary a lot from one region to another, and it may be hard to predict where the effects will be felt most. What we saw in this study is not something one would have predicted in advance," she said.

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