Why We Won’t Know When We’ve Passed the 1.5-Degree Threshold

A firefighter battles the 2018 Ferguson Fire in California.

A firefighter battles the 2018 Ferguson Fire in California. U.S. Forest Service

While the Paris Agreement aims to limit warming to 1.5 degrees C, experts won’t know when we have surpassed this threshold, a fact that could undermine global efforts to tackle climate change, scientists say.

Temperatures are creeping upwards, but they are doing so unevenly. Not every year is hotter than the last, meaning warming could reach 1.5 degrees before falling and then rising again. On a monthly scale, it already has. Warming surpassed the 1.5-degree mark for one month or more in 2016, 2017, 2019, 2020, and again in 2023.

Measured across the entire year, 2023 will conclude 1.4 degrees hotter than the preindustrial era, and at least one of the next five years is expected to surpass 1.5 degrees. But it’s unclear at what point the world will have officially breached the Paris target, as the pact offers no clear guidance on this matter.

The UN Intergovernmental Panel on Climate Change says the threshold will be surpassed when average warming exceeds 1.5 degrees over two decades. But using this yardstick, it would take years to gauge success or failure on the agreement.

Writing in Nature, scientists from the U.K. Met Office have called for a new approach that combines the last 10 years of observations with a forecast of the coming decade to determine the current level of warming.

“If adopted, this could mean a universally agreed measure of global warming that could trigger immediate action to avoid further rises,” lead author Richard Betts said in a statement. If warming surpasses 1.5 degrees, authors write, there will be pressure to reverse climate change, not just stop it.

“Clarity on breaching the Paris Agreement guardrails will be crucial,” Betts said. “Without an agreement on what actually will count as exceeding 1.5 degrees C, we risk distraction and confusion at precisely the time when action to avoid the worst effects of climate change becomes even more urgent.”


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