New Report Points to Accelerated Melting, Longer Drought
Thursday, December 25, 2008; Page A02
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 new report led by the U.S. Geological Survey.
The survey -- 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 mid-century.
Thirty-two scientists from federal and non-federal 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 different federal agencies.
Tom Armstrong, senior adviser for global change programs at USGS, 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 level rise could be as much as four feet by 2100. The IPCC had projected a sea level rise of no more than 1.5 feet by that time, but satellite data over the past two years show the world's major ice sheets are melting much more rapidly than previously thought. The Antarctic and Greenland ice sheets are now losing an average of 48 cubic miles of ice a year, equivalent to twice the amount of ice that exists 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 IPCC used did not factor in some of the dynamics that scientists now understand about ice sheet melting. Among other things, Steffen and his collaborators have identified a process of "lubrication," in which warmer ocean water gets in underneath coastal ice sheets and accelerates melting.
"This has to be put into models," said Steffen, who organized a conference last 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 IPCC 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 is impossible to determine yet whether human activity is responsible for the drought the Southwestern United States has experienced over the past 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 project 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 need to work on developing versions that can make projections on a much smaller scale. "That's what the water managers out there really need," he said. Current models "don't give them the hard numbers they need."
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