Released at the American Geophysical Union Fall Meeting in San Francisco last month, the snappily titled Synthesis and Assessment Product 3.4: Abrupt Climate Change acts like a mini-, up-to-date IPCC report for North America. The 2007 IPCC report included publications from 2006, while this new document includes papers published just six months ago.

“It’s a snapshot of the science, what we know and don’t know,” explained John McGeehin of the US Geological Survey.

Some of the latest research indicates that drying of the American West may already have begun. “There is no clear evidence of a human-induced effect but since the IPCC report, models indicate that aridity in the south-west is likely to intensify,” said Peter Clark of Oregon State University. “It’s too early to know.”

According to Edward Cook of the Lamont-Doherty Earth Observatory, there is abundant evidence of megadroughts in the past. “The climate system has the capacity to lock into periods of long drought,” he explained.

As well as looking at the hydrologic cycle, the latest report assesses the likelihood of abrupt changes to the Atlantic Meridional Overturning Circulation, a rapid release of methane to the atmosphere, and abrupt sea-level rise.

While it’s likely that the northward flow of warm water in the upper layers of the Atlantic Ocean will decrease by around 25–30%, the report reckons that it’s unlikely that the circulation will collapse or that the weakening will occur abruptly.

Similarly, it’s likely that the pace of methane emissions from wetlands, permafrost and seafloor methane hydrates will increase but it's unlikely to be an abrupt release. The increased emissions will in turn feed climate change as methane is a powerful greenhouse gas.

With regards to sea level, an abrupt change is possible but predictions are highly uncertain because of shortcomings in existing climate models.

“There is now a big consensus that the West Antarctic Ice Sheet is losing mass,” said Eric Rignot of the University of California, Irvine. “Antarctica is particularly implicated in sea-level rise right now.”

The news for Arctic sea ice is particularly bleak: climate-model simulations and observations suggest that rapid and sustained September Arctic sea-ice loss is likely this century. And according to Peter Clark of Oregon State University, “the rapid decline in Arctic sea ice may be faster than models are suggesting.”

The report recommends additional research, such as improving observation systems of glaciers and ice sheets to make more certain estimates of mass balance; ice-sheet modelling to predict sea-level rise better; ameliorating drought forecasting; understanding long-term oceanic changes; observation and prediction of Atlantic Meridional Overturning Circulation; monitoring methane abundance; and modelling processes that release methane.