The surface waters of the oceans are getting warmer at a rate of 0.27°C (0.50°F) a decade, new research shows. This compares with just 0.06°C (0.11°F) in the late 1980s, when we first noticed there was a problem. Although people are more directly affected by temperature rises on land, higher ocean temperatures drive bigger storms and can cause ecosystem collapse. Hotter surface waters make only a small contribution to sea level rise, except where they come in contact with ice sheets, but if deeper waters get warm they expand, and that has to go somewhere.

For a long time our knowledge of ocean surface temperatures was based on data collected by ships that had considerable inaccuracies, and largely ignored areas not on major shipping routes. That changed when we developed satellites that could take the temperature of the entire surface.

The satellite data revealed that the whole planet’s ocean averages had considerable variation based on cycles, of which the El Niño Southern Oscillation is the most powerful. Cycles like that can disguise an overall trend for a while, but eventually it shows through.

For 450 days between April 2023 and July 2024, sea temperatures averaged 0.18°C (0.32°F) degrees higher than they had ever been before, peaking 0.31°C (0.56°F) above the previous record. A team led by Professor Christopher Merchant of the University of Reading sought to identify how much of this can be attributed to long-term trend, how much was cyclical and how much reflects a recent ongoing acceleration.

They looked at sea temperatures worldwide over five-year intervals, comparing the 1985-89 period, the first for which good data is available, with that from 2019-2023. They conclude there was more to the extraordinary hot period than just a steady human-induced rise combining with an El Niño; the rapid acceleration caused almost half the extreme temperatures.

“If the oceans were a bathtub of water, then in the 1980s, the hot tap was running slowly, warming up the water by just a fraction of a degree each decade. But now the hot tap is running much faster, and the warming has picked up speed. The way to slow down that warming is to start closing off the hot tap, by cutting global carbon emissions and moving towards net-zero,” Merchant said in a statement. 

There’s a step between the release of gases and higher temperatures, which the team worked to understand. Noting that greenhouse gases increase the Earth’s energy imbalance, so that more arrives than leaves, Merchant and co-authors state, “We quantify that GMSST (lobal mean sea surface temperature) has increased by 0.54 ± 0.07 K for each GJ m^–2 of accumulated energy.”

Comparing the 2023-24 spike in ocean temperatures with one from eight years earlier the authors write, “44 percent […] of the +0.22 k difference […] is unexplained unless the acceleration of the GMSST is accounted for.” The peak in the solar cycle also contributed, but to a much smaller extent. The previous extreme El Niño was in 1997-98; temperatures rose more in the 8 years prior to the recent peak than the 18 before the previous one.

This also means that things are going to get worse a lot faster than it would appear if we just project the average rate of warming over the last 40 years into the future. For example, the authors expect more warming in the next 20 years than in the last 40.

The study is published open access in Environmental Research Letters.