“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 a lead author of the new study.

The researchers say 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. 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, told a press conference at the meeting:  “There is a new consensus that Antarctica is losing mass.” Seaward flow of ice from Greenland is also accelerating. However, projections of how far sea levels might rise are “highly uncertain,” says the report, as researchers cannot say whether such losses will continue at the same rates.

“We have no smoking gun saying that humans are causing the current changes. But the past is a cautionary tale,” Cook told the press conference. “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.” Cook added: “If the system tips over, that would have catastrophic effects on human activities and populations over wide areas.”

The panel said two other systemic changes seem less imminent, but are still of concern. Vast quantities of methane, a potent greenhouse gas, have long been locked up in ocean sediments, wetlands and permafrost. These could be destabilized by climate change, leading to blowouts of gas, and thus even more abrupt temperature shifts. The panel said blowouts appear unlikely in the next 100 years—but that steady emissions could double, especially in the north, as land and water warm up. The panel also looked at the continuous circulation of the Atlantic Ocean, which sends warm water northward and cold water southward, controlling the climate of western Europe and beyond. Some scientists say this circulation could collapse if enough northern ice melts and dilutes the salty water. The panel found this scenario unlikely in the short term, but warned that the circulation’s strength might decline 25% to 30% by 2100.

“Abrupt climate change presents potential risks for society that are poorly understood,” the researchers write. [There is an] urgent need for committed and sustained monitoring of those components [that] are particularly vulnerable.”