The rate of ocean warming has more than quadrupled over the past four decades, a new study has shown. Ocean temperatures were rising at about 0.06 degrees Celsius per decade in the late 1980s, but are now increasing at 0.27 degrees Celsius per decade.

Published 28 January 2025 in Environmental Research Letters, the study helps explain why 2023 and early 2024 saw unprecedented ocean temperatures.

Professor Chris Merchant, lead author at the University of Reading, said, “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.”

Energy imbalance

This accelerating ocean warming is driven by the Earth’s growing energy imbalance—whereby more energy from the sun is being absorbed in the Earth’s system than is escaping back to space. This imbalance has roughly doubled since 2010, in part due to increasing greenhouse gas concentrations, and because the Earth is now reflecting less sunlight to space than before.

Global ocean temperatures hit record highs for 450 days straight in 2023 and early 2024. Some of this warmth came from El Niño, a natural warming event in the Pacific.

When scientists compared it to a similar El Niño in 2015–16, they found that the rest of the record warmth is explained by the sea surface warming up faster in the past 10 years than in earlier decades; 44% of the record warmth was attributable to the oceans absorbing heat at an accelerating rate.

Expect more warming

The findings show that the overall rate of global ocean warming observed over recent decades is not an accurate guide to what happens next: it is plausible that the ocean temperature increase seen over the past 40 years will be exceeded in just the next 20 years.

Because the surface oceans set the pace for global warming, this matters for the climate as a whole. This accelerating warming underscores the urgency of reducing fossil fuel burning to prevent even more rapid temperature increases in the future and to begin to stabilize the climate.