AGU: Climate science report questions likelihood of abrupt climate change

by Megan Sever
Thursday, January 5, 2012

SAN FRANCISCO — A new U.S. Climate Change Science Program report states that abrupt climate change is unlikely to happen over the next century, scientists announced Tuesday at the annual meeting of the American Geophysical Union. However, the longer-term impacts of climate change could still be severe.

Abrupt climate change can occur over decades or less, but the effects can linger for decades to centuries. Should such change occur today, the results could be catastrophic, John McGeehin, a geologist at the U.S. Geological Survey in Reston, Va., said Tuesday at an AGU press conference. The synthesis report — the latest in the series of 21 climate reports that the White House requires CCSP to produce — examines four mechanisms that have triggered abrupt warming in the past and determines the likelihood of each mechanism occurring in the near future.

The researchers first looked at whether we are facing an abrupt change in ice and sea levels, said Peter Clark of Oregon State University in Corvallis, a coordinating lead author of the report. They found that both Greenland and Antarctica are losing mass rapidly, he said, and that seems to be largely due to acceleration of ice flow near the edges of the massive ice sheets. Whether these changes are short term or long term is unclear, he said, but if the ice sheets continue losing mass at the rate they are losing it, we will see a more rapid rise in sea level than any current models predict.

Secondly, the team looked at abrupt change in the hydrological cycle on land — specifically examining droughts. They found that droughts in places like the American Southwest will likely intensify and persist due to future warming, Clark said. And in fact, he said, if model results are correct, this drying may have already begun.

The third major mechanism the team examined is whether we could see major changes in the Atlantic Meridional Overturning Circulation — the current that carries warmer waters from the Southern Hemisphere northward along the Atlantic Coast of the United States and across the ocean to Europe. They found that although it is “very likely” that the circulation will decrease by 25 to 30 percent over the next century in response to rising greenhouse gases, it is unlikely to collapse, Clark said. Despite the unlikelihood of collapse, the decrease in circulation is likely to have severe consequences, such as a southward shift of tropical rainfall belts and sea level rise in the North Atlantic.

Finally, the team inquired whether there could be a sudden and catastrophic release of methane from wetlands, permafrost or even from hydrates beneath the sea. None of these causes are likely to cause such a catastrophic release of methane, the team found. Nonetheless, methane levels will likely rise this century, causing additional warming.

The new report is essentially an update to the Intergovernmental Panel on Climate Change report released in 2007, noted Eric Rignot of the University of California at Irvine and the Jet Propulsion Laboratory at Caltech in Pasadena, Calif. The 450-plus-page CCSP report synthesizes new findings since 2006 and is more comprehensive than the IPCC report, added Ed Cook of Lamont-Doherty Earth Observatory in Palisades, N.Y. It represents a new consensus on the understanding of abrupt climate change.

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