Why Is the Sea So Hot?

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. By summer, an El Niño—or warm phase—had begun. Since ocean temperatures started to climb before the start of El Niño, the shift, by itself, seems insufficient to account for what’s going on. Meanwhile, the margin by which records are being shattered exceeds what’s usually seen during El Niños.

“It’s not like we’re breaking records by a little bit now and then,” Brian McNoldy, a hurricane researcher at the University of Miami, said. “It’s like the whole climate just fast-forwarded by fifty or a hundred years. That’s how strange this looks.” It’s estimated that in 2023 the heat content in the upper two thousand metres of the oceans increased by at least nine zettajoules. For comparison’s sake, the world’s annual energy consumption amounts to about 0.6 zettajoules.

A variety of circumstances and events have been cited as possible contributors to the past year’s anomalous warmth. One is the January, 2022, eruption of an underwater volcano in the South Pacific called Hunga Tonga–Hunga Ha‘apai. Usually, volcanoes emit sulfur dioxide, which produces a temporary cooling effect, and water vapor, which does the opposite. Hunga Tonga–Hunga Ha‘apai produced relatively little sulfur dioxide but a fantastic amount of water vapor, and its warming effects, it’s believed, are still being felt.

Another factor is the current solar cycle, known as Solar Cycle 25. Solar activity is ramping up—it’s expected to peak this year or next—and this, too, may be producing an extra bit of warming.

Yet another is a change in the composition of shipping fuel. Regulations that went into effect in 2020 reduced the amount of sulfur in the fuel used by supertankers. This reduction, in turn, has led to a decline in a type of air pollution that, through direct and indirect effects, reflects sunlight back to space. It’s thought that this change has led to an increase in the amount of energy being absorbed by the seas, though quantifying the effect is difficult.

Can all of these factors together account for what’s going on? Climate scientists say it’s possible. There’s also a lot of noise in the climate system. “This could end up just being natural variability,” Susan Wijffels, a senior scientist at the Woods Hole Oceanographic Institution, said.

But, possibly, something else is going on—something that scientists haven’t yet accounted for. This spring, ENSO is expected to transition into what scientists call “neutral” conditions. If precedent holds, then when this occurs ocean temperatures should start to run more in line with long-term trends.

“I think the real test will be what happens in the next twelve months,” Wijffels said. “If temperatures remain very high, then I would say more people in the community will be really alarmed and say ‘O.K., this is outside of what we can explain.’ ”

In 2023, which was by far the warmest year on record on land, as well as in the oceans, many countries experienced record-breaking heat waves or record-breaking wildfires or record-breaking rainstorms or some combination of these. (Last year, in the United States, there were twenty-eight weather-related disasters that caused more than a billion dollars’ worth of damage—another record.) If the climate projections are accurate, then the year was a preview of things to come, which is scary enough. But, if the projections are missing something, that’s potentially even more terrifying, though scientists tend to use more measured terms.

“The other thing that this could all be is, we are starting to see shifts in how the system responds,” Schmidt observed. “All of these statistics that we’re talking about, they’re taken from the prior data. But nothing in the prior data looked like 2023. Does that mean that the prior data are no longer predictive because the system has changed? I can’t rule that out, and that would obviously be very concerning.” ♦

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