Climate leaders are talking about ‘overshoot’ into warming danger zone. Here’s what it means

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BELEM, Brazil (AP) — The world’s climate leaders are conceding that Earth’s warming will shoot past a hard limit they set a decade ago in hopes of keeping the planet out of a danger zone. But they’re not conceding defeat.

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United Nations officials, scientists, and analysts are pinning their hopes on eventually forcing global temperatures back below the red line they set in the 2015 Paris Agreement, which sought to limit warming to 1.5 degrees Celsius (2.7 Fahrenheit) since preindustrial times.

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Busting that limit and then coming back down is called “overshoot.” In the way climate science uses the term, it doesn’t mean zooming by a red line and never looking back — it’s all about seeing the line in the rear view mirror and making a U-turn to get back to lower temperatures.

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After years of considering the 1.5 mark a strict no-go situation, officials have in the past few weeks started talking about limiting the time and magnitude of Earth’s stay in the danger zone.

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The 1.5 figure is based on temperatures averaged over a decade.

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‘It’s not a target, it’s not a goal, it’s a limit’

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Many scientists have said it’s inevitable the 1.5 degree mark will be passed. The mark won’t be considered breached until Earth goes beyond it over a 10-year average. It stands now at about 1.3 degrees Celsius (2.3 degrees Fahrenheit), and last year alone actually exceeded the 1.5 mark.

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It may be inevitable, but it won’t be pretty, they say.

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“We have a real risk of triggering irreversible changes in Earth systems when we breach 1.5,” said Johan Rockstrom, director of Germany’s Potsdam Institute for Climate Research and a science advisor to the U.N.’s annual climate conference currently being held in the Brazilian Amazon city of Belem.

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Those risks include global extinction of coral reefs and exponential growth of killer heat waves. There’s also a risk of triggering tipping points for irreversible changes, such as drying out the Amazon rainforest, melting ice sheets in Greenland and Antarctica and maybe even shutting down the entire Atlantic ocean current system, said Rockstrom and Climate Analytics CEO Bill Hare. Similar concerns were raised in a special 2018 U.N. report that showed how 1.5 starts the danger zone.

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“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,” Rockstrom told The Associated Press. “Go beyond it, we increase suffering of people, and we increase risk of crossing tipping points.”

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Likely to be a breach

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For the past few years, scientists have said while it’s technically possible for the world to stay at or below 1.5, it’s not realistic. They calculate the planet is on pace for 2.6 degrees Celsius (4.7 degrees Fahrenheit) of warming since the mid-1800s, which ushered in the Industrial Revolution and a massive uptick in burning of fossil fuels like coal, oil and gas.