How Will Climate Scientists Know When the World Gets to the 1.5 C Mark?
As the world gets closer to the mark 1.5 degrees Celsius in Paris climate agreement, scientists are racing to establish a single way to monitor current warming
Climate activists gather with signs for a demonstration calling upon the G20 conference to adhere to limit global temperature rise to 1.5 degrees Celsius compared to pre-industrial levels, at the Sharm el-Sheikh International Convention Centre, in Egypt’s Red Sea resort city of the same name, during the COP27 climate conference, on November 15, 2022.
Fayez Nureldine/AFP via Getty Images
CLIMATEWIRE | The world can’t seem to agree on when the planet will exceed a key temperature threshold in the Paris climate agreement.
Nearly 200 nations committed back in 2015 to pursuing efforts to keep global temperatures from exceeding 1.5 degrees Celsius above preindustrial levels. But there is no official metric for determining when the world has crossed that line into increasingly catastrophic impacts.
Enter an international team of scientific experts.
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The World Meteorological Organization first convened the team of around 10 experts last summer to look at the different methodologies and devise a more accurate way to measure current warming. Some of their preliminary findings are detailed in the WMO’s latest State of Climate report, which estimates that current global warming is somewhere between 1.34 degrees and 1.41 degrees compared with the 1850-1900 average.
The expert group is still working to come up with a single way to monitor current warming, said Chris Hewitt, director of the climate services branch of the WMO. Then they’ll work on an estimate for when the world will likely exceed the 1.5-degree threshold.
While it’s generally accepted that 1.5 degrees refers to a long-term average, the Paris agreement doesn’t go into more scientific detail on how to measure that, Hewitt said.
The latest assessment by the Intergovernmental Panel on Climate Change, the world’s top authority on global warming, defines future warming in terms of 20-year averages relative to the average from 1850-1900, the WMO says. But that doesn’t provide an up-to-date assessment of current warming and could lead to delays in recognizing when 1.5 has been exceeded — which in turn could delay the responses needed not just to keep warming from rising but to reverse it.
“If we look just back in the past, we won’t know we’ve reached 1.5 until five or 10 years after it’s happened, and for policymakers that’s probably not very helpful to be told, ‘Oh, so sorry, this happened five or 10 years ago’,” said Hewitt.
Experts began discussing the need for a better metric for long-term warming ahead of global climate talks in 2023, when different analyses began showing that the world was getting exceedingly close to the 1.5-degree mark, with reports of monthly averages exceeding that threshold. The world needs to agree on the current level of warming — and how to define 1.5 degrees of warming — before it actually exceeds the threshold, experts say.
“Reaching 1.5 degrees of global warming may well be a trigger for policy responses, or at least public responses,” said Richard Betts, head of climate impacts research at the U.K.’s Met Office Hadley Centre and University of Exeter. “And we want to avoid confusion, and some people saying that we’re at 1.5 and some people say we’re not.”
The expert team assembled by WMO — which includes Betts — used several new approaches referred to in Wednesday’s report to come up with a more accurate metric for current warming. For example, one combines the average temperatures over the past 10 years with projections for the next decade. They’re looking at a wider range of methods with the aim of narrowing down uncertainties.
The report also found that the global mean temperature in 2024 was 1.55 degrees Celsius (plus or minus 0.13 degrees) above the planet’s preindustrial levels. That’s up from the 1.45 degree mean temperature in 2023.
A breach of a single year doesn’t mean that the Paris goal is dead, experts say.
But “it is a wake-up call that we are increasing the risks to our lives, economies and to the planet,” WMO Secretary-General Celeste Saulo wrote in the forward to the report.
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