The United Nations has acknowledged that Earth has lost its chance to prevent some of the worst harms of global warming and is now urging the world to plan for a period of dangerous heat before temperatures can be brought back to safer levels. A first-of-its-kind report from the UN Environment Programme, released Wednesday, concedes that the planet will soon cross the 1.5 degrees Celsius (2.7 degrees Fahrenheit) threshold agreed upon in the 2015 Paris climate accord.
Instead of abandoning the goal, scientists and UN officials recommend a strategy of limiting how far temperatures rise above the threshold, enduring a period of worsening climate impacts, and then cooling the planet back below the limit by the end of the century. The report describes this path as an "overshoot" and warns it will mean decades of increased weather disasters, rising seas, extinctions, and melting ice and glaciers.
"Humanity is speeding past the limit to avoid climate catastrophe," UN Secretary-General António Guterres said. "It's a reality check. We have to live with this warmer world," said report co-author Richard Betts, a climate scientist at the University of Exeter. "But there's still a lot we can do to limit warming."
The world is currently at about 1.4 C (2.5 F) above pre-industrial levels, based on a 20-year average. The report states the most optimistic scenario has warming peaking at 1.8 C (3.2 F) around mid-century if countries pursue ambitious climate goals, but that current government policies would push temperatures to 2.6 C (4.7 F) above pre-industrial levels by 2100.
UN officials had long maintained the 1.5-degree window was still open despite experts saying there was no way to limit warming to that level. Wednesday's report finally admitted the world will cross the threshold "in the next few years" and envisions a hump-shaped path of more warming, a peak, and then a gradual cooling to below the 1.5 mark, ideally by the end of the century.
"Let's be very clear, 1.5 target is still the goal. But now we need to approach it differently from above," said UNEP Executive Director Inger Andersen. The 1.5 limit is not "some magical climate cliff" where it's just game over, said University of Victoria climate scientist Andrew Weaver, who was not part of the study. "Every tenth of a degree we avoid still means less damage."
Limiting how high temperatures go past the threshold is key, as is reducing the amount of time spent above 1.5 degrees. The world likely will be in that overshoot period for "many, many decades," Betts said. Every five years of delay in reducing emissions adds at least a tenth of a degree to peak warming, said report co-author Joeri Rogelj, a climate scientist at Imperial College London.
"If we act now we can limit overshoot," the report said, adding that the result would be "not a safe world, but safer than remaining above" 1.5 degrees for the long term. Weaver described the document as "among the bleakest U.N. climate reports I have read," carrying "the unmistakable tone of desperation, frustration and urgency — but not resignation."
The report calls for increased efforts to adapt to worsening warming, which hits the poor hardest, "because some impacts simply cannot be avoided," Andersen said. During the overshoot period, more people will die or get sick from climate change and extreme weather, food production will drop, water shortages will worsen, economies will be hurt, and species like coral will go extinct.
The report warns of "tipping points" with irreversible and cascading effects, including the loss of the West Antarctic and Greenland ice sheets, the loss of the Amazon rain forest, and the shutdown of the Atlantic Ocean circulation system that keeps much of Europe warm. "Some locations might become more or less uninsurable or even uninhabitable," the report said.
The final phase in the overshoot strategy is cooling the planet by removing carbon dioxide from the air and storing it. The report notes that planting trees, the traditional approach, could shave at most a tenth of a degree off warming, underscoring the need for additional carbon removal technologies.