In early 2023, climate scientists—and anyone else paying attention to the data—started to notice something strange. At the beginning of March, sea-surface temperatures began to rise. By April, they’d set a new record: the average temperature at the surface of the world’s oceans, excluding those at the poles, was just a shade under seventy degrees. Typically, the highest sea-surface temperatures of the year are observed in March, toward the end of the Southern Hemisphere’s summer. Last year, temperatures remained abnormally high through the Southern Hemisphere’s autumn and beyond, breaking the monthly records for May, June, July, and other months. The North Atlantic was particularly bathtub-like; in the words of Copernicus, an arm of the European Union’s space service, temperatures in the basin were “off the charts.”
Since the start of 2024, sea-surface temperatures have continued to climb; in February, they set yet another record. In a warming world, ocean temperatures are expected to rise and keep on rising. But, for the last twelve months, the seas have been so feverish that scientists are starting to worry about not just the physical impacts of all that heat but the theoretical implications. Can the past year be explained by what’s already known about climate change, or are there forces at work that haven’t been accounted for? And, if it’s the latter, does this mean that projections of warming, already decidedly grim, are underestimating the dangers?
“We don’t really know what’s going on,” Gavin Schmidt, the director of nasa’s Goddard Institute for Space Studies, told me. “And we haven’t really known what’s going on since about March of last year.” He called the situation “disquieting.”
Last winter, before ocean temperatures began their record run, the world was in the cool—or La Niña—phase of a climate pattern that goes by the acronym enso.
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