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The world’s oceans are sending a loud and unmistakable warning. In 2025, they absorbed more heat than ever recorded, pushing global ocean temperatures to new highs and amplifying climate disasters across the planet, according to scientists tracking long-term climate data.
Oceans act as Earth’s primary heat sink, absorbing more than 90 per cent of the excess heat generated by human-caused carbon emissions. This makes them one of the most reliable and sobering measures of how far, and how fast, the climate crisis has progressed. Since the early 2000s, almost every year has set a new record for ocean heat, a trend researchers describe as deeply troubling and far from slowing down.
Warmer oceans are directly linked to heavier rainfall and more destructive flooding
This accumulated heat does not stay quietly beneath the surface. Warmer oceans are directly linked to stronger hurricanes and typhoons, heavier rainfall, and more destructive flooding events that disproportionately affect coastal communities. At the same time, prolonged marine heatwaves are devastating ocean ecosystems, bleaching coral reefs and disrupting marine life that millions depend on for food and livelihoods.
Rising ocean temperatures are also a major contributor to sea-level rise. As seawater warms, it expands—a process known as thermal expansion—adding to the threat already posed by melting glaciers and ice sheets. Together, these changes put billions of people living along coastlines at increased risk.
While accurate ocean temperature measurements only extend back to the mid-20th century, scientists believe today’s oceans are likely hotter than at any point in at least 1,000 years, and possibly warming faster than at any time in the past two millennia. This long-term perspective highlights just how unusual—and dangerous—the current trend is.
Unlike the oceans, the atmosphere stores relatively little heat and is more affected by natural climate cycles such as El Niño and La Niña. As a result, year-to-year air temperature records can fluctuate. Even so, 2025 is expected to rank among the hottest years ever recorded, roughly tying with 2023, while 2024 remains the hottest since records began in 1850. The planet’s recent shift into the cooler La Niña phase has done little to slow the underlying warming trend.
Each year, the planet is warming, breaking records
“Each year the planet is warming—setting a new record has become a broken record,” said Professor John Abraham of the University of St Thomas in Minnesota, who was part of the team analysing the latest data. He emphasised that ocean temperatures provide the clearest picture of climate change. “Global warming is ocean warming. If you want to know how much the Earth has warmed or how fast it will warm in the future, the answer is in the oceans.”
The findings, published in the journal Advances in Atmospheric Sciences, are based on temperature data gathered by a wide network of instruments across the world’s oceans. Three independent research teams analysed this data to calculate the heat content of the top 2,000 metres of seawater, where most of the excess heat is stored.
To put the scale into perspective, scientists estimate that the amount of heat absorbed by the oceans is equivalent to more than 200 times the total electricity used globally each year. The implications, they say, are profound and long-lasting.
Ocean warming is not evenly distributed
In 2025, particularly intense heating was observed in parts of the tropical and South Atlantic, the North Pacific, and the Southern Ocean. The latter, which encircles Antarctica, has raised serious concerns among scientists due to a sharp decline in winter sea ice in recent years—an alarming signal for global climate stability.
As emissions continue, researchers stress that ocean heat will keep rising until greenhouse gas output falls to zero. Until then, the seas will remain one of the clearest—and most dangerous—mirrors of humanity’s impact on the planet.
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