Think of them as the Four Horsemen of the abrupt climate change apocalypse – four environmental disasters that have happened in the distant past and that could, within the scope of little more than a decade, seriously threaten human society if they occur again.

A new report from the U.S. Climate Change Science Program released Tuesday weighed the risks the four possible scenarios – rising sea levels, widespread drought, a shift in an Atlantic Ocean current that warms northern Europe and the massive release of methane, a powerful greenhouse gas, from ancient reservoirs – and found that all four require more study.

At least that's the careful assessment that scientists involved in the report laid out Tuesday at the American Geophysical Union's fall meeting in San Francisco.

But of the four scenarios, the first two – rising sea levels caused by the melting of ice sheets, and the potential for widespread and long-lasting droughts influenced by global warming – appeared to pose the greater risk, the scientists reported.

Ice sheets in Greenland and Antarctica have been losing mass since the mid-1990s, and their flow and thinning have been speeding up recently, said Eric Rignot, a professor at University of California at Irvine and senior research scientist at the Jet Propulsion Laboratory in Pasadena.

The melting of Arctic ice sheets that's been occurring for years will likely accelerate as well in the coming century, he added (see Global Warming Threatens Arctic Feedback Loops).

Because these trends "are enabled by warming climates, as the climate continues to warm, these adjustments may well become much more frequent," Rignot said.

Recent data may force scientists to revise upward projections from a 2007 report from the Intergovernmental Panel on Climate Change that the ocean could rise from 0.18 meters to 0.58 meters by century's end, he said.

As for the threat of drought, the report focused on the American Southwest, where geological records show long-lasting and severe droughts have occurred before.

Given data that shows precipitation is declining in the region again, "The conclusion is that American Southwest aridity will likely intensify and persist due to future greenhouse warming, and increasing the likelihood of severe and persistent drought there in the future," Rignot said.

In fact, it may have already begun, though the natural variations in the region's climate make that hard to say with any certainty, said Edward R. Cook, senior scholar at Columbia University's Tree Ring Laboratory at the Lamont-Doherty Earth Observatory.

Less likely as a shutdown in the Atlantic Meridional Overturning Circulation, the current that brings warm water from the Gulf of Mexico to the north Atlantic, helping Europe stay warmer than it otherwise would be.

While it's likely that current will decrease by up to 30 percent over the century because of global warming, a complete collapse of the current – rendered as Hollywood fantasy in the film "The Day After Tomorrow" – is very unlikely, the report found.

As for a catastrophic release of methane from frozen seabed deposits, arctic permafrost and tropical wetlands, causing runaway global warming, the report found that was very unlikely as well.

Still, climate change will probably accelerate emissions from those sources, said Edward Brook, a professor with the Department of Geosciences at Oregon State University.