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Tipping points are about to tip

Failure to limit global warming to the targets set by international accords will most likely set off several climate “tipping points,” a team of scientists said on Thursday, with irreversible effects including the collapse of the Greenland and West Antarctic ice sheets, abrupt thawing of Arctic permafrost and the death of coral reefs.

The researchers said that even at the current level of warming, about 1.1 degrees Celsius (2 degrees Fahrenheit) above pre-industrial levels, some of these self-sustaining changes might have already begun. But if warming reached above 1.5 degrees Celsius, the more ambitious of two targets set by the 2015 Paris Agreement, the changes would become much more certain. 

And at the higher Paris target, 2 degrees Celsius, even more tipping points would likely be set off, including the loss of mountain glaciers and the collapse of a system of deep mixing of water in the North Atlantic. 

The changes would have significant, long-term effects on life on Earth. The collapse of the Greenland and West Antarctic ice sheets, for example, would lead to unrelenting sea level rise, measured in feet, not inches, over centuries. The thawing of permafrost would release more heat-trapping gases into the atmosphere, hindering efforts to limit warming. A shutdown of ocean mixing in the North Atlantic could affect global temperatures and bring more extreme weather to Europe. 

Johan Rockström, the director of the Potsdam Institute for Climate Impact Research in Germany and one of the researchers, said the team had “come to the very dire conclusion that 1.5 degrees Celsius is a threshold” beyond which some of these effects would start. That makes it all the more imperative, he and others said, for nations to quickly and drastically cut emissions of carbon dioxide and other heat-trapping gases to curb global warming. 

The research is in line with recent assessments by the Intergovernmental Panel on Climate Change, a group of experts convened by the United Nations, that beyond 1.5 degrees of warming, the threats of climate change grow considerably. 

“It really provides strong scientific support for rapid emission cuts in line with the Paris Agreement,” said David Armstrong McKay, a climate scientist at the University of Exeter in Britain and the lead author of a paper describing the researchers’ work, published in Science. Limiting warming to 1.5 degrees “doesn’t guarantee we don’t see tipping points,” Dr. McKay said. “But it reduces the likelihood.”