Climate leaders accept a likely "overshoot" of the 1.5°C limit — and explain what that means

BELEM, Brazil — Top climate officials and scientists are increasingly accepting that global temperatures will likely pass a strict limit they set a decade ago, yet they insist that such a breach is not the end of the road.

United Nations representatives, researchers and analysts are now focusing on the prospect of drawing temperatures back down once an overshoot occurs, aiming to return to the threshold established in the 2015 Paris Agreement: limiting warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels.

The term "overshoot" describes the scenario where temperatures exceed that red line and are later brought back below it. In climate science usage, overshoot doesn’t mean passing the limit and never reversing course — it means seeing the boundary behind us and taking action to reduce temperatures again.

After years of treating 1.5°C as an absolute boundary, officials in recent weeks have openly discussed strategies to limit both how far and how long the planet remains above that mark.

That 1.5°C measure is calculated as a decade-long average.

Many scientists now say crossing 1.5°C is likely. The threshold is only considered exceeded when the 10-year average goes beyond it. Current estimates put the global average warming at about 1.3 degrees Celsius (2.3°F), and last year’s single-year value actually topped 1.5°C.

But inevitability does not mean harmlessness.

“We have a real risk of triggering irreversible changes in Earth systems when we breach 1.5,” said Johan Rockström, director of Germany’s Potsdam Institute for Climate Research and a science adviser to the U.N. climate meeting taking place in Belem. He warned those changes could include the global collapse of coral reefs and far more frequent and intense deadly heat waves.

Rockström and Bill Hare, CEO of Climate Analytics, also cautioned about possible tipping points: the drying of the Amazon rainforest, major melting of Greenland and Antarctic ice sheets and even disruption of the Atlantic ocean current system. Those dangers were highlighted in a 2018 U.N. report that marked 1.5°C as the start of a danger zone.

“In Belem, we have more scientific evidence than we had 10 years ago that 1.5 is a real limit. It’s not a target, it’s not a goal, it’s a limit, it’s a boundary,” Rockström told the Associated Press. “Go beyond it, we increase suffering of people, and we increase risk of crossing tipping points.”

For several years scientists have argued that while keeping warming at or below 1.5°C remains technically possible, it is increasingly unrealistic. Current pathways point toward about 2.6°C (4.7°F) of warming since the mid-1800s, the period that marks the start of the Industrial Revolution and the rapid escalation of fossil fuel use.

The U.N. has long insisted 1.5°C remains a meaningful benchmark. Yet officials have recently acknowledged that the world will likely exceed it in coming years or decades.

“The science is clear: We can and must bring temperatures back down to 1.5 Celsius after any temporary overshoot,” U.N. climate chief Simon Stiell said as the conference opened.

U.N. Secretary-General António Guterres was even more direct in Geneva last month: “Overshooting is now inevitable, which means that we are going to have a period, bigger or smaller, with higher or lower intensity, above 1.5 degrees in the years to come,” he said, adding that an overshoot does not mean the target is permanently lost.

Maintaining the 1.5°C goal even after a breach remains important to U.N. officials because it guides policies and ambition.

The overshoot concept assumes temperatures will climb beyond 1.5°C and then decline as emissions fall. The expectation is that when humanity sharply reduces emissions from burning coal, oil and gas, natural carbon sinks — such as forests and oceans — will absorb more carbon, and future carbon-removal technologies might extract additional CO2 from the atmosphere.

As concentrations of atmospheric carbon fall, temperatures should follow, though much depends on large-scale deployment of technologies that do not yet exist or operate at required scale.

“Without carbon dioxide removal it is simply impossible to manage the overshoot scenario,” said Ottmar Edenhofer, chief economist at the Potsdam Institute and chair of Europe’s Scientific Advisory Board on Climate Change.

Scientists still lack precise knowledge about when and where the greatest risks occur during an overshoot, or whether a longer stay slightly above 1.5°C is more damaging than a brief spike to much higher temperatures. They do, however, expect the world will likely remain in the danger zone for decades.

Analysis by Climate Action Tracker finds that even if nations do everything technically possible to cut emissions — something history shows hasn’t happened — the world would likely cross the 1.5°C threshold around 2030, peak near 1.7°C (3.1°F), and not drop back below 1.5°C until the 2060s. Under current policies, though, the trajectory is worse: temperatures could continue rising through 2100.

“Ten years ago we had a more orderly pathway for staying away from 1.5 Celsius entirely, basically with low or no overshoot,” Rockström said. “Now we are 10 years later, we have failed.”