WASHINGTON — The United States faces the possibility of much more rapid climate change by the end of the century than previous studies have suggested, according to a report led by the U.S. Geological Survey.
The study, which was commissioned by the U.S. Climate Change Science Program and issued this month, expands on the 2007 findings of the United Nations Intergovernment Panel on Climate Change. Looking at factors such as rapid sea ice loss in the Arctic and prolonged drought in the Southwest, the new assessment suggests that earlier projections may have underestimated the climatic shifts that could take place by 2100.
However, the assessment also suggests that some other feared effects of global warming are not likely to occur by the end of the century, such as an abrupt release of methane from the seabed and permafrost or a shutdown of the Atlantic Ocean circulation system that brings warm water north and colder water south. But the report projects an amount of potential sea level rise during that period that may be greater than what other researchers have anticipated, as well as a shift to a more arid climate pattern in the Southwest by midcentury.
Thirty-two scientists from federal and nonfederal institutions contributed to the report, which took nearly two years to complete. The Climate Change Science Program, which was established in 1990, coordinates the climate research of 13 federal agencies.
Tom Armstrong, senior advisor for global change programs at the U.S. Geological Survey, said the report "shows how quickly the information is advancing" on potential climate shifts. The prospect of abrupt climate change, he said, "is one of those things that keeps people up at night, because it's a low-probability but high-risk scenario. It's unlikely to happen in our lifetimes, but if it were to occur, it would be life-changing."
In one of the report's most worrisome findings, the agency estimates that in light of recent ice sheet melting, global sea levels could rise as much as 4 feet by 2100. The intergovernment panel had projected a rise of no more than 1.5 feet by that time, but satellite data over the last two years show the world's major ice sheets are melting much more rapidly than previously thought. The Antarctic and Greenland ice sheets are losing an average of 48 cubic miles of ice a year, equivalent to twice the amount of ice in the Alps.
Konrad Steffen, who directs the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder and was lead author on the report's chapter on ice sheets, said the models the intergovernment panel used did not factor in some of the dynamics that scientists now understand about ice sheet melting. Steffen and his collaborators have identified, among other things, a process of "lubrication," in which warmer ocean water gets underneath coastal ice sheets and accelerates melting.
"This has to be put into models," said Steffen, who organized a conference during the summer in St. Petersburg, Russia, as part of an effort to develop more sophisticated ice sheet models. "What we predicted is sea level rise will be higher, but I have to be honest, we cannot model it for 2100 yet."
Still, Armstrong said, the report "does take a step forward from where the [intergovernment panel] was," especially in terms of ice sheet melting.
Scientists also looked at the prospect of prolonged drought over the next 100 years. They said it was impossible to determine yet whether human activity is responsible for the drought the Southwestern United States has experienced over the last decade, but every indication suggests the region will become consistently drier in the next several decades. Richard Seager, a senior research scientist at Columbia University's Lamont-Doherty Earth Observatory, said that nearly all of the 24 computer models the group surveyed projected the same climatic conditions for the North American Southwest, which includes Mexico.
"If the models are correct, it will transition in the coming years and decades to a more arid climate, and that transition is already underway," Seager said, adding that such conditions would probably include prolonged droughts lasting more than a decade.
The current models cover broad swaths of landscape, and Seager said scientists needed to work on developing versions that can make projections on a much smaller scale.
Armstrong said the need for "downscaled models" was one of the challenges facing the federal government. When it comes to abrupt climate shifts, he said, "we need to be prepared to deal with it in terms of policymaking. . . . There are really no policies in place to deal with abrupt climate change."
Richard Moss, who worked for the Climate Change Science Program from 2000 to 2006 and now serves as vice president and managing director for climate change at the World Wildlife Fund-U.S., welcomed the report but called it "way overdue."