In an assessment led by the U.S. Geological Survey, a team of federal and academic scientists found evidence that some abrupt changes in climate are likely or may be happening. They reviewed published research, which was based both on computer climate models and observations in the field. In this case, "abrupt" means something that could occur in the matter of several decades or less, and could significantly disrupt nature and human activities.
(The above photos, not part of the report, depict the retreat of ice at Muir Inlet in Glacier Bay National Park and Preserve in Alaska. The picture at left was taken in 1892; the one at right, 2005. Photos by the USGS' Bruce Molnia.)
The report finds "no clear evidence" of human-induced climate change in the amount of rain and snow in North America. But recent climate modeling indicates drying is likely to intensify in the subtropics and spread north into the U.S. Southwest, increasing the chances of severe drought there. If the models are right, the report adds, the drying may already have begun - though it's hard to be sure given wide natural swings in climate in that region.
Among the report's other findings:
The northward flow of warm water in the Atlantic Ocean is likely to weaken significantly, the report says, but not enough to collapse by century's end. So no need to hunker down for another ice age, a la "The Day After Tomorrow."
Relatively rapid rise in sea level is possible, but still hard to predict based on current climate modeling. Scientists noted sea level rise from melting glaciers and ice sheets has accelerated in Greenland and Antarctica, but said they couldn't tell whether the changes were "a short-term natural adjustment or a response to recent climate change."
It's unliikely that rising temperatures will trigger a catastrophic release of methane into the atmosphere, but the rate at which the powerful greenhouse gas escapes from the sea floor, permafrost and wetlands is likely to increase, potentially accelerating the planet's warming.
Naturally enough, the report's authors call for further research to reduce the uncertainties about such troubling prospects. But its findings, hedged as they are, are likely to add to the pressure on the incoming Obama administration and Congress to craft new policy responses.