A new U.S. government report has determined that abrupt climate shifts resulting in rising sea levels and severe droughts, may come within decades, which is sooner than previously believed.
Commissioned by the U.S. Climate Change Science Program, the report was authored by experts from the U.S. Geological Survey, Columbia University's Lamont-Doherty Earth Observatory and other leading institutions.
Many scientists are now raising the possibility that abrupt, catastrophic switches in natural systems may punctuate the steady rise in global temperatures now underway.
However, the likelihood and timing of such "tipping points," where large systems move into radically new states, has been controversial.
The new report synthesizes the latest published evidence on four specific threats for the 21st century.
It uses studies not available to the Intergovernmental Panel on Climate Change (IPCC), whose widely cited 2007 report explored similar questions.
"This is the most up to date, as it includes research that came out after IPCC assembled its data," said Edward Cook, a climatologist at Lamont-Doherty and the lead author of the new study.
The researchers said that the IPCC's maximum estimate of two feet of sea level rise by 2100 may be exceeded, because new data shows that melting of polar ice sheets is accelerating.
Among other things, there is now good evidence that the Antarctic ice cap is losing overall mass.
Also, the report contends that seas could rise rapidly if melting of polar ice continues to outrun recent projections, and that an ongoing drought in the U.S. west could be the start of permanent drying for the region.
At the time of the IPCC report, scientists were uncertain whether collapses of ice shelves into the ocean off the western Antarctica were being offset by snow accumulation in the continent's interior.
But one coauthor, remote-sensing specialist Eric Rignot of the Jet Propulsion Laboratory, said, "There is a new consensus that Antarctica is losing mass."
Seaward flow of ice from Greenland is also accelerating.
In the interior US, a widespread drought that began in the Southwest about 6 years ago could be the leading edge of a new climate regime for a wider region.
The new research also cited a 2007 Science paper that showed how changes in temperature over the Pacific have driven large-scale droughts across western North America.
According to Cook, "What this tells us is that the system has the ability to lock into periods of profound, long-lasting aridity. And there is the suggestion that these changes are related to warmer climate."
"If the system tips over, that would have catastrophic effects no human activities and populations over wide areas," he added.