The global ocean reached a troubling new milestone in 2025, having absorbed more heat last year than at any point since modern records began some 125 years ago and reinforcing its role as the planet’s primary buffer against climate change.

A new research paper – led by the Institute of Atmospheric Physics at the Chinese Academy of Sciences that brought together more than 50 scientists from 31 institutions across the globe – has underscored the scale of the increase, suggesting the ocean gained an additional 23 zettajoules of energy in a single year.

This, according to the study published this week in Advances in Atmospheric Sciences – is roughly 200 times humanity’s global electricity consumption in 2023 or around 37 years of today’s global energy consumption.

Unsurprisingly – and by integrating observations and reanalysis products from Asia, Europe and North America – including data from Copernicus Marine, NOAA/NCEI and the Chinese Academy of Sciences – the team confirmed that global ocean heat content (OHC) climbed to its highest level on record in 2025.

This is not an isolated spike: 2025 marked the ninth consecutive year in which the ocean set a new heat record.

Long-term trends reveal a clear acceleration. Since the 1990s, heat accumulation in the upper 2,000 metres of the ocean has increased more rapidly than in previous decades, a signal of the growing imbalance in Earth’s energy budget.

At the surface, 2025 was the third warmest year ever recorded for sea-surface temperatures, sitting about 0.5°C above the 1981–2010 average. Temperatures were slightly lower than the record-breaking years of 2023 and 2024, largely due to the shift from El Niño to La Niña conditions in the Pacific.

Even so, warm seas have continued to amplify extreme weather, fuelling heavy rainfall and flooding across Southeast Asia, intensifying drought in the Middle East, and contributing to disruptive weather in Mexico and the Pacific Northwest.

These changes matter because the ocean is where the vast majority of excess heat ends up. More than 90% of the energy trapped by greenhouse gases is absorbed by seawater, making ocean heat content one of the most reliable indicators of long-term climate change. As the ocean warms, it expands – driving sea-level rise – and releases more heat and moisture into the atmosphere, strengthening heatwaves, storms and rainfall extremes.

As long as the planet continues to warm, scientists warn, the ocean will keep setting new heat records.

The findings will form part of a special issue of Advances in Atmospheric Sciences focused on changes in ocean heat content. The journal’s cover will itself reflect the gravity of the situation, featuring a sad shrimp and crab – characters suggested by the study’s corresponding author, Cheng Lijing of the Institute of Atmospheric Physics.

“The idea comes from the ‘shrimp soldiers and crab generals’ guarding the underwater palace in Journey to the West,” said Cheng. “We reimagined them not as mighty guardians, but as vulnerable creatures whose armour – their shells and scales – is under attack by ocean warming, acidification and other environmental changes.”

In a world where the ocean has become the planet’s heat sink, those vulnerable creatures – and the ecosystems they represent – are now on the frontline of a rapidly warming Earth.

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