What Happens After Warming Hits 1.5C? A Guide to Climate Overshoot

The diplomats and world leaders now gathered in Egypt for the annual United Nations climate summit are tasked, in some sense, with holding the global average temperature below 1.5 degrees Celsius of warming. That's one of the key objectives around which the 2015 Paris Agreement was formed, and so it has become a shorthand for the success of every subsequent climate summit.

Talks in Glasgow last year at COP26 ended with the conference leader saying the limit of 1.5C is "alive but its pulse is weak." Ahead of COP27 in Sharm El-Sheikh, UN-backed scientists on the Intergovernmental Panel on Climate Change contributed to the darkening prognosis by projecting that the world is likely to pass the 1.5C mark in the 2030s. Obituaries for 1.5C have followed, even if politicians speaking from the ongoing summit in Egypt haven't quite given it up for dead.

"If we retain the spirit of creative optimism - Promethean, creative optimism that we saw at Glasgow - then I think we can keep alive the hope of restricting the rise in temperatures to 1.5," said former British Prime Minister Boris Johnson on Monday. He went on to repeat a slogan invoked often when he hosted COP26: "Keep 1.5 alive."

Every tenth of a degree matters, which is why countries codified in the Paris Agreement their plan for "holding the increases in the global average temperature to well below 2C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5C." In other words, there's no precipice or event horizon on the other side of the line. It's an organizing principle.

"The thing that's obviously really important to say is that the 1.5C limit is a political limit," says David Keith, a Harvard University physicist and an adviser to the Climate Overshoot Commission, an expert group that suggests ways to reduce risk once the world exceeds these warming targets. "Whether it matters or not depends on how it matters politically. It's not like there's some scientific magic at 1.5C."

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