Deadly heat in the Southwest. Hot-tub temperatures in the Atlantic Ocean. Sweltering conditions in Europe, Asia and South America.

That 2023 was Earth’s hottest year on record was in some ways no surprise. For decades, scientists have been sounding the alarm about rapidly rising temperatures driven by humanity’s relentless burning of fossil fuels.

But last year’s sudden spike in global temperatures blew far beyond what statistical climate models had predicted, leading one noted climate scientist to warn that the world may be entering “uncharted territory.”

“It’s humbling, and a bit worrying, to admit that no year has confounded climate scientists’ predictive capabilities more than 2023 has,” wrote Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, in a recent article in the journal Nature.

Now, he and other researchers are scrambling to explain why 2023 was so anomalously hot. Many theories have been proposed, but “as yet, no combination of them has been able to reconcile our theories with what has happened,” Schmidt wrote.

Last year’s global average temperature of 58.96 degrees was about a third of a degree warmer than the previous hottest year in 2016, and about 2.67 degrees warmer than the late 1800s pre-industrial period against which global warming is measured.

While human-caused climate change and El Niño can account for much of that warming, Schmidt and other experts say the extra three or four tenths of a degree are harder to account for.

Theories for the increase include a 2020 change in aerosol shipping regulations designed to help improve air quality around ports and coastal areas, which may have had the unintended consequence of enabling more sunlight to reach the planet.

The 2022 euption of the Hunga Tonga-Hunga Ha’apai volcano also shot millions of tons of water vapor into the stratosphere, which scientists say helped to trap some heat. What’s more, a recent uptick in the 11-year solar cycle may have contributed about a tenth of a degree of additional warning.

But these factors alone cannot explain what’s happening, Schmidt said.

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