Oceans Set New Heat Record in 2025, Study Finds, Even as Surface Temperatures Ease

In a year when global surface temperatures eased slightly from an unprecedented peak, the planet’s largest heat reservoir quietly set a record of its own.

Oceans’ hidden record

A new international study finds that in 2025, the upper 2,000 meters of the world’s oceans contained more heat than at any time since modern measurements began in the late 1950s. The extra energy stored in the seas compared with 2024 is vast — about 23 zettajoules (23 followed by 21 zeros) — roughly equal to 37 years of current global energy consumption.

The analysis, published Jan. 9 in Advances in Atmospheric Sciences, concludes that 2025 marked the ninth year in a row that global ocean heat content has reached a new record high in the upper 2 kilometers. Because the oceans absorb more than 90% of the excess heat trapped by greenhouse gases, scientists say this deep‑ocean metric has become one of the clearest gauges of how fast the climate is changing.

“2025 saw the highest recorded ocean heat content to date, underscoring the oceans’ ongoing accumulation of heat,” the Chinese Academy of Sciences said in an English‑language statement announcing the findings. “As long as the Earth’s heat continues to increase, ocean heat content will continue to rise and records will continue to fall.”

Who did the research — and how

The study was led by climate scientist Yuying Pan at the Institute of Atmospheric Physics, part of the Chinese Academy of Sciences in Beijing. More than 50 researchers from 31 institutions — including China’s IAP, the U.S. National Oceanic and Atmospheric Administration, the European Union’s Copernicus Marine Service, Italy’s National Research Council and the U.S. National Center for Atmospheric Research — contributed.

Using four independent data products, the team reconstructed heat stored from the surface down to 2,000 meters between 1958 and 2025. They relied on observations from the global Argo network of profiling floats, ship‑based measurements, moored instruments and historical archives, combined with ocean reanalyses.

All the datasets showed the same result: 2025 was the hottest year on record for the upper global ocean.

From 2024 to 2025, the IAP analysis estimated that ocean heat content increased by about 23 zettajoules, with an uncertainty of ±8 zettajoules. The World Meteorological Organization, which highlighted the study in its own climate summary, described that amount of heat as “about 200 times the world’s total electricity generation in 2024.”

To make the scale more tangible, some scientists note that 23 zettajoules is roughly equivalent to the energy released by about a dozen Hiroshima‑size atomic bombs exploding in the oceans every second for an entire year.

Where the heat concentrated

The heat gain was not evenly distributed. The study found that about 16% of the global ocean area experienced record‑high heat content in 2025. Roughly one‑third of the seas recorded their top three warmest years since 1958, and more than half fell within their top five.

Hotspots included the tropical and South Atlantic, the Mediterranean Sea, the North Indian Ocean, parts of the North Pacific and the Southern Ocean encircling Antarctica. Many of those regions also saw exceptional marine heatwaves at the surface.

In the Mediterranean, June 2025 brought the warmest June sea‑surface temperatures on record, according to the Copernicus Marine Service. Average temperatures reached nearly 24°C (about 75°F), and 62% of the basin’s surface was classified as being under “strong or higher” marine heatwave conditions — the largest extent ever recorded there.

Globally, surface waters were also extremely warm, though not quite as hot as in 2024. Data from multiple agencies show that 2025 sea‑surface temperatures, excluding polar regions, were about 0.5°C above the 1981–2010 average, making the year the third‑warmest for the oceans’ surface after 2024 and 2023.

On land and at sea, 2025 was the third‑warmest year on record for overall global surface temperatures, behind only 2024 and 2023, according to the World Meteorological Organization and the European Union’s Copernicus Climate Change Service. Copernicus reported that the average global temperature from 2023 through 2025 was about 1.5°C above the 1850–1900 pre‑industrial baseline, the first time a three‑year period has crossed that threshold.

Why ocean heat content matters — especially under La Niña

What made the new ocean results striking to many researchers is that 2025 started and ended under La Niña, a natural climate pattern that typically cools the eastern tropical Pacific and tends to suppress global surface temperatures.

“The year 2025 started and ended with a cooling La Niña and yet it was still one of the warmest years on record globally,” WMO Secretary‑General Celeste Saulo said in a statement. Despite the slight dip at the surface compared with 2024, she noted, the planet continued to store more heat in the oceans.

Scientists say that contrast underscores why they increasingly look to ocean heat content, rather than single‑year air temperature records, to track the pace of global warming.

Because water has a much higher capacity to store heat than air or land, the oceans act as a giant buffer, soaking up excess energy over decades and centuries. That makes deep‑ocean temperatures less sensitive to year‑to‑year weather variability and to short‑lived events like El Niño and La Niña.

“Because ocean heat content reflects the long‑term accumulation of stored heat, it is widely regarded as one of the most reliable indicators of climate change,” the Chinese Academy of Sciences said.

A faster pace of heating

The new analysis also examined long‑term trends in the oceans’ heat uptake. From 1960 to 2025, the rate of heating in the upper 2,000 meters corresponded to about 0.14 watts per square meter of the Earth’s surface per decade. Since 2005, that rate has more than doubled to roughly 0.32 watts per square meter per decade.

Those numbers represent the strengthening of Earth’s energy imbalance — the difference between the solar energy the planet absorbs and the heat it radiates back into space. A positive imbalance means the climate system is gaining energy, primarily in the oceans.

Lead authors of the study and other researchers have pointed to several factors behind the acceleration. Rising concentrations of carbon dioxide, methane and other greenhouse gases continue to trap more heat. At the same time, reductions in sulfate air pollution from power plants and ships in recent decades have made the atmosphere less reflective, allowing more sunlight to reach the ocean surface.

Why it matters on coasts and in ecosystems

Whatever the causes, the consequences reach far beyond temperature charts.

One immediate effect of warming oceans is sea‑level rise. As seawater heats up, it expands, contributing to higher global sea levels alongside ice melt from Greenland, Antarctica and mountain glaciers. Because the deep ocean circulates slowly, the heat absorbed today will continue to swell the seas for centuries, even if greenhouse gas emissions are cut.

Warm oceans also help fuel more intense storms. In 2025, there were 101 named tropical storms worldwide, well above the 1991–2020 average of 88, according to NOAA. Fifty‑two of those storms reached hurricane or typhoon strength, and 24 became major storms of Category 3 or higher. In the North Atlantic, three hurricanes — Erin, Humberto and Melissa — reached Category 5 intensity, the second‑highest number of Category 5 storms in a single season on record.

NOAA reported that Hurricane Melissa tied the 1935 Labor Day hurricane as the strongest landfalling Atlantic storm ever recorded.

Higher ocean temperatures played a role in other extremes as well. Researchers linked unusually warm waters to widespread flooding across parts of Southeast Asia, drought in the Middle East, and heavy rains in Mexico and the U.S. Pacific Northwest.

At the same time, persistent marine heatwaves have taken a severe toll on marine ecosystems. By April 2025, NOAA said, about 84% of the world’s coral reefs had experienced heat stress linked to bleaching since January 2023, in what it described as the most extensive global bleaching event ever observed.

NOAA also confirmed that heat in the upper 700 meters of the ocean reached a record high in 2025, marking the fifth consecutive year of record‑breaking warmth in that shallower layer.

“The broken records in the ocean have become a broken record,” Lijing Cheng, a senior scientist at the Institute of Atmospheric Physics and a long‑time expert on ocean heat content, said last year when similar results were announced for 2024.

For coastal communities, small island nations, fisheries and the insurance industry, the trend is more than a scientific curiosity. Rising seas, stronger storm surges, more extreme rainfall and recurring marine heatwaves all translate into higher economic losses and more difficult adaptation choices.

A running tally beneath the waves

The authors of the new study emphasize that even rapid cuts in greenhouse gas emissions would not immediately cool the oceans, given the heat already stored. Instead, they say ocean heat content should be viewed as a running tally of the planet’s energy budget — one that will keep climbing until the world brings that budget back into balance.

As countries debate how fast to phase out fossil fuels and how to share the costs of adapting to a hotter world, the oceans offer a blunt assessment: while the temperatures people feel at the surface may fluctuate from year to year, the measure that matters most for the long term — the heat building up beneath the waves — is still rising, year after record‑breaking year.