The USGS Assessment of Abrupt Climate Change

A group of scientists led by the U.S. Geological Survey (USGS) have released a report discussing the current knowledge of abrupt climate change.

You can download the executive summary here. The report analyzes the likelihood of "abrupt climate change," which it defines as:

"A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems."

I further quote from the executive summary:

"This report considers progress in understanding four types of abrupt change in the paleoclimatic record that stand out as being so rapid and large in their impact that if they were to recur, they would pose clear risks to society in terms of our ability to adapt: (1) rapid change in glaciers, ice sheets, and hence sea level; (2) widespread and sustained changes to the hydrologic cycle; (3) abrupt change in the northward flow of warm, salty water in the upper layers of the Atlantic Ocean associated with the Atlantic Meridional Overturning Circulation (AMOC); and (4) rapid release to the atmosphere of methane trapped in permafrost and on continental margins."

The report reflects a greater understanding of abrupt climate change compared with five years ago, and goes into greater detail on this topic than did the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report.

The gist of the report is that:

1) the melt waters from the Greenland and Antarctic ice sheets are likely to contribute to greater sea-level rises than previous assessments have stated;

2) the U.S. Southwest could experience more intense droughts;

3) the Atlantic "conveyor belt" of warm water flowing from the southern latitudes to the North Atlantic and cold water flowing south, or AMOC, is likely to decrease from 25 to 30 percent during the 21st century, but not collapse; and

4) abrupt methane releases from heavily laden sources such as permafrost are unlikely but cannot be discounted (methane has more powerful heat-trapping effects than carbon dioxide, and one of the catastrophic feedback mechanisms that scientists warn about are massive natural methane releases from permafrost or methane hydrates in the oceans, releases that are precipitated by manmade greenhouse gases).