Faster Climate Change Feared

Ice sheets in the Antarctic and Greenland, above, are losing 48 cubic miles per year, pushing up sea level worldwide. (By John Mcconnico -- Associated Press)

Armstrong said the need for "downscaled models" is one of the challenges facing the federal government, along with better coordination among agencies on the issue of climate change. When it comes to abrupt climate shifts, he said, "We need to be prepared to deal with it in terms of policymaking, keeping in mind it's a low-probability, high-risk scenario. That said, there are really no policies in place to deal with abrupt climate change."

Richard Moss, who directed the Climate Change Science Program's coordination office between 2000 and 2006 and now serves as vice president and managing director for climate change at the World Wildlife Fund-U.S., welcomed the new report but called it "way overdue."

"There is finally a greater flow of climate science from the administration," Moss said, noting that the report was originally scheduled to come out in the summer of 2007. "It really is showing the potential for abrupt climate change is real."

The report is reassuring, however, on the prospects for some potentially drastic effects -- such as a huge release of methane, a potent heat-trapping gas, that is now locked deep in the seabed and underneath the Arctic permafrost. That is unlikely to occur in the near future, the scientists said.

"It's unlikely that we're going to see an abrupt change in methane over the next hundred years, but we should worry about it over a longer time frame," said Ed Brook, the lead author of the methane chapter and a geosciences professor at Oregon State University. "All of these places where methane is stored are vulnerable to leaking."

By the end the century, Brook said, the amount of methane escaping from natural sources such as the Arctic tundra and waterlogged soils in warmer regions "could possibly double," but that would still be less than the current level of human-generated methane emissions. Over the course of the next thousand years, he added, methane hydrates stored deep in the seabed could be released: "Once you start melting there, you can't really take it back."

In the near term, Brook said, more precise monitoring of methane levels worldwide would give researchers a better sense of the risk of a bigger atmospheric release. "We don't know exactly how much methane is coming out all over the world," he said. "That's why monitoring is important."

While predictions remain uncertain, Steffen said cutting emissions linked to global warming represents one of the best strategies for averting catastrophic changes.

"We have to act very fast, by understanding better and by reducing our greenhouse gas emissions, because it's a large-scale experiment that can get out of hand," Steffen said. "So we don't want that to happen."