by P. Voosen, Oct 10, 2024 in Science
For the past year, alarm bells have been going off in climate science: Last year’s average global temperature was so high, shooting up nearly 0.3°C above the previous year to set a new record, that human-driven global warming and natural short-term climate swings seemingly couldn’t explain it. Some, like famed climate scientist James Hansen, suggestedEarth is entering an ominous new phase of accelerated warming, driven by a rapid decline in sunlight-dimming air pollution. Others, like Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, said the rise might represent a “knowledge gap,” some new climate feedback that might tip the planet toward a future even warmer than models predict.
Now, a new series of studies suggests most of the 2023 jump can be explained instead by a familiar climate driver: the shifting waters of the tropical Pacific Ocean. The combination of a 3-year-long La Niña, which suppressed global temperatures from 2020 to 2022, followed by a strong El Niño could account for the unexpected temperature jump, the work suggests. “Earth can do this,” says Shiv Priyam Raghuraman, a climate scientist at the University of Illinois Urbana-Champaign, who led one study.
During La Niña, strong trade winds push warm surface water west along the equator toward Indonesia and pull up a fountain of deep, cold water in the eastern Pacific that helps cool the planet. During El Niño, the winds collapse, allowing warm water to slosh east and shut off the ocean air conditioner.
Last year, analyses suggested the combination of global warming and El Niño fell far short of explaining 2023’s heat, leading to the worries that something else might be at play. But Raghuraman and his co-authors weren’t convinced those studies captured El Niño’s full potential. Indeed, looking back, they found that 1977 was an awful lot like 2023, when temperatures rose by more than 0.25°C after a multiyear La Niña tipped into an El Niño.
But that’s just 2 years out of the 70-some for which El Niño records exist. To generate better statistics, Raghuraman and his coauthors compiled every climate model run they could find that simulated the planet in a steady state, unperturbed by humanity, totaling 58,021 years of simulations. Then they looked to see how often temperature spikes higher than 0.25°C occurred.
Their study, published today in Atmospheric Chemistry and Physics, showed such spikes were rare, happening only 1.6% of the time, nearly always during an El Niño. But when a long La Niña set the stage, the probability of a spike jumped to 10.3%. And during those model years, the geographic pattern of warming often matched what occurred last year, such as a large increase in the tropical Atlantic Ocean. The models show big El Niño jumps are rare but possible, Raghuraman says. “We’re not missing something.”
The result lines up with another study, published in August in Communications Earth & Environment, which compared sea surface temperatures in 2023 and the recent past. If global warming was accelerating, that trend would also be seen in the oceans. And although the oceans were anomalously hot in 2023, they were only a little warmer than during an El Niño in 2015 and 2016, says study co-author Marianne Tronstad Lund, research director at Norway’s Center for International Climate and Environmental Research. “We don’t find signs of any rapid acceleration,” she says.
Recent runs of a “climate pacemaker” experiment at the Scripps Institution of Oceanography also implicate El Niño as the main culprit in the globe’s extra heat. Unpublished results from the experiment, which feeds real-world Pacific temperatures into a climate model, re-created temperature patterns similar to those observed last year, says Scripps climate scientist Shang-Ping Xie, with some exceptions over the North Atlantic.
Meanwhile, several studies have found that cleaner, clearer air due to falling pollution from China and lower sulfur marine fuels made only a small contribution to last year’s temperatures. One study, submitted to Atmospheric Chemistry and Physics, found that declining pollution could raise global temperatures by 0.03°C over the next 20 years, with the strongest effect not occurring until later this decade. It’s not nothing, says study co-author Duncan Watson-Parris, an atmospheric physicist at Scripps, but far too little to explain last year. Taken together, says Mika Rantanen, a climate scientist at the Finnish Meteorological Institute, the results are “a good reminder that it was indeed El Niño that was the major player.”
Yet the timing of 2023’s heat remains odd, Schmidt says. It came on faster and stronger than in normal El Niño years, and it lingered far longer, even as Earth tips again toward a La Niña. Perhaps the biggest wild card is the increasing amount of sunlight satellites have detected hitting the planet’s surface over the past decade. Falling pollution can only explain some of the increase; the rest might be due to reduced cloudiness or changing surface reflectivity. How much the extra sunlight might have heated the planet in 2023 is unclear.
The new studies aren’t the final word on the problem, Schmidt says. Even as the eastern Pacific cools off again, the debate within climate science continues to simmer.