A newly proposed method for calculating the global temperature suggests that countries may have already failed their main climate goal: to limit warming to below 1.5 degrees Celsius. Scientists using the method say the world was 1.49 C hotter than in pre-industrial times by the end of 2023. Conventional accounting puts that number at around 1.3 C.
The U.K.-based scientists behind the new approach, who published it Monday in the journal Nature Geoscience, say it simplifies the tracking of climate change, is easier to use and provides a more accurate assessment of warming to date.
The global average surface temperature is a key metric that countries use to evaluate climate progress, or lack thereof, as the planet heats up. Dozens of countries and cities and hundreds of companies have pledged to help honor the 1.5 C goal, a centerpiece of the 2015 Paris Agreement and a likely focus of discussions at the COP29 summit that starts Monday in Baku, Azerbaijan.
The 1.5 C goal is diplomatic and largely symbolic — nothing dramatic happens in the atmosphere when it’s crossed — and reaching it for just a month or a year says more about short-term weather than the long-term climate trend. Even so, every extra increment of heat risks worsening climate impacts like powerful storms and drought. This year is expected to be the hottest year on record, and the EU’s Copernicus Climate Change Service predicts it will also be the first year ever to breach 1.5 C of warming.
Surprisingly, the scientific community has never specifically defined this central metric, write Andrew Jarvis of Lancaster University and Piers Forster of University of Leeds, so it is "necessarily still open to change."
By convention, scientists measure this temperature increase against a pre-industrial baseline, the 1850-1900 average. The authors observe, as others have, that industrialization started before 1850, and the current baseline doesn’t reflect that. They can run their analysis according to the standard baseline — which suggests 1.4 C of warming to date — or shift it back to 1700, to capture the emissions from early industrialization. Their method also strips out variability caused by transient, natural weather, thus providing a clear reading of human-induced warming.
Richard Betts is a University of Exeter scientist and head of climate impacts at the U.K. Met Office’s Hadley Center. He called the paper "important and useful" for its "clear and simple way to give up-to-date estimates of the current level of human-induced global warming." Betts read the paper in draft as an independent peer reviewer for Nature Geoscience.
Shifting the temperature record baseline from the late 19th century to 1700 "highlights a complex and potentially controversial issue," said Betts, who wrote in Nature in December about how to recognize when the world has passed 1.5 C. "There’s a danger of accusations of ‘moving the goalposts.’"
But there is also a risk, he said, in continuing apace when "it’s clear that current warming is much closer to 1.5 C than expected."
Jarvis and Forster said that the new analysis has several advantages over the standard approach. It’s faster — the data are easily managed and updated monthly in a simple spreadsheet, a factor that could be of use to delegations at United Nations negotiations and for teachers in the classroom. Jarvis said he has explained it on a napkin to non-scientists in pubs.
Despite the complexity of the atmosphere, oceans, land and the life swarming about them, there’s a direct, stepwise relationship between the amount of carbon dioxide in the atmosphere and the resulting temperature rise. In other words, the CO2-to-temperature trend is a straight line.
"I think there’ll be incredulity that we can have the nerve to boil the climate response to such a simple feature," Jarvis said in a press conference before the paper’s release.
Many scientists are concerned about unpredictable changes in the pace of warming, including potential "tipping points," beyond which warming begets more warming. The new analysis may have a diagnostic approach, too. Because the CO2-heat relationship is linear, a deviation from it could indicate a new source of emissions, such as methane release from permafrost thawing, Jarvis and Forster write.
Kim Cobb, a climate scientist at Brown University, said the approach is "compelling" and joins useful previous work that looks at an earlier start date for the temperature record.
There are tradeoffs between the new math and the conventional method, she said. The latter relies on research models that demonstrate how fossil-fuel pollution causes warming. The new analysis shows how CO2 and heat are correlated. It may not be able to splice out some low-grade natural fluctuations that are critical for determining where precisely on the warming curve we might be, she said.
The paper "is a new approach to an age-old and critical question, reminding us how very close we are to breaching the 1.5 C threshold (if we haven’t crossed it already)," she said by email.
The idea of changing the world’s climate scorecard may be a heavy lift for the already difficult challenge of communicating science to policymakers and the public.
"This question of warming since 1700 is of academic interest, but I think the Paris Agreement — even if it doesn’t have a formal definition of ‘pre-industrial’ — has largely come to be interpreted as warming since 1850-1900," said Zeke Hausfather, climate research lead at Stripe.
Hausfather also said that people already place too much emphasis on the Paris targets of 1.5C and the higher 2 C mark. "Nothing magic happens between 1.49 C and 1.51 C," he said. The Paris goal assumes a long-term trend of 20 or 30 years above 1.5 C.
Giving temperature targets more significance than they have could contribute to people becoming dispirited or rushing into untested geoengineering, he said.
Whether the new approach ever gains wide adoption or not, Jarvis emphasized during the press conference his surprise "that this wasn’t done 30 years ago."
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