Earth’s oceans caught a fever in March 2023 that has yet to break. Since then, the bathwater-like conditions have killed corals in a record-breaking mass bleaching event, fuelled hurricanes, and collapsed entire fisheries.

The two years of heat have created a scientific mystery, Grist reports, with 450 straight days of record high global sea surface temperatures from April 2023 to July 2024—a streak that exceeded climate scientists’ predictions even when accounting for climate change and the natural climate pattern known as El Niño. A study published late last month by researchers at the University of Reading helps solve the puzzle and points to one prominent culprit: the sun.

The study in the journal Environmental Research Letters found that the rate of ocean warming has more than quadrupled over the past 40 years, driven by Earth’s growing energy imbalance—accounting for roughly 44% of the extra heat in recent El Niño years. Thanks to heat-trapping greenhouse gases and a decrease in reflectivity, the planet is absorbing more energy from the sun than is escaping back into space. Since 2010, according to the study, that disparity has doubled.

There’s been an uptick in that imbalance and that has led to an uptick in the rate of ocean warming,” said lead author Christopher Merchant, a professor of ocean and earth observation at the University of Reading in the U.K.

By looking back through satellite observations since 1985 and developing a statistical model that isolated the trends in both ocean warming and Earth’s energy imbalance, the researchers found they were escalating in lockstep. According to Merchant, the study is possibly the first to connect the two phenomena over recent decades. “It’s a very tight correlation,” he said.

This relationship is bad news for the oceans, which have absorbed some 90% of the excess warming from human activity. Some of that heat will continue to seep down into the planet’s depths, while some will cycle back up toward the surface and escape into the atmosphere. According to the study, the next 20 years could warm up the oceans more than the last 40.

If you think of the oceans as a bath, Merchant says, it’s like the hot tap was only a trickle in the 1980s—but now, it’s been cranked up. “And what’s turning the tap more open, making the warming pick up speed, is an increase in greenhouse gases—carbon dioxide and methane—which are both still rising, largely from the fossil fuel industry,” he said.

There are other factors turning up the heat. The El Niño pattern that began in 2023 added around 0.1 or 0.2°C, before the inverse La Niña pattern took over in December 2024.

Another piece of the puzzle is the planet’s diminishing reflectivity, said Brian McNoldy, an atmospheric scientist at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science. The ocean’s dark surface helps it absorb heat, whereas white clouds and aerosol particles in the atmosphere help bounce the sun’s radiation back into space. In 2020, the International Maritime Organization adopted a new rule to cut back on sulphur pollution from shipping fuel, but because the aerosol particles in emissions acted as a seed for clouds, the regulation had the unintended effect of dimming the marine layer of clouds that blanket the ocean.

“So you get rid of a lot of those, and now more of the sun’s energy can be absorbed in the ocean instead of reflecting off clouds,” McNoldy said. According to Merchant, efforts to curb air pollution from factories in countries like China also had the side effect of cutting back reflective aerosols.

The excess ocean warmth has had wide-ranging consequences. In April 2024, as the oceans started simmering, 77% of the world’s coral reefs became imperilled in the most extensive bleaching event on record, threatening the livelihoods of a billion people and a quarter of marine life. Changing ocean temperatures also shift weather patterns, potentially intensifying droughts, downpours, and storms alike.

“Hurricanes love warmer water. So all other things be equal, a warmer ocean can produce stronger hurricanes with maybe more frequent instances of rapid intensification,” McNoldy said. Last September, Hurricane Helene slammed into Florida’s Gulf Coast after surging from a Category 1 to a Category 4 storm in a single day.

“The oceans really set the pace for global warming for the Earth as a whole,” Merchant explained. So the knock-on effects—like wildfires, drought, and floods—will continue to escalate, too.

“That really needs to be understood, but it also needs to filter through to governments that changes might be coming down the line faster than they’re currently assuming.”

This story originally appeared on Grist and is part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.