In 2025, the world’s oceans absorbed more heat than in any year since modern records began, marking the ninth consecutive year of rising ocean heat content, according to research published in Advances in Atmospheric Sciences. Every second of the year, the oceans took in energy equivalent to roughly 12 Hiroshima-sized atomic bombs, steadily accumulating heat at an unprecedented pace. In total, ocean heat content increased by 23 zettajoules — more than 200 times humanity’s annual global electricity use. Because the oceans absorb over 90 percent of the excess heat trapped by greenhouse gases, ocean heat content has become one of the clearest signals of long-term global warming. Unlike surface temperatures, which can fluctuate from year to year, this measure reflects how much energy the planet is storing over time.

The warming is widespread, but it is not evenly distributed across the globe. In 2025, around 16 percent of the global ocean reached record-high heat levels, while roughly one-third of ocean areas ranked among the three warmest years ever recorded in their regional histories. The most intense warming occurred in parts of the tropical and South Atlantic, the North Pacific, and the Southern Ocean. Crucially, this heat is no longer limited to surface waters. Measurements show that human-driven warming now penetrates down to 2,000 meters below the surface. Because it can take roughly 25 years for heat to travel to these depths, today’s emissions are effectively locking in ocean warming that will persist for centuries.

Sea-surface temperatures in 2025 were slightly lower than in 2023 and 2024, largely due to shifting climate patterns in the tropical Pacific. Even so, 2025 still ranked as the third warmest year on record, with global average sea-surface temperatures sitting about 0.5°C above the 1981–2010 baseline. This short-term dip does not contradict the broader trend. Surface temperatures can rise and fall from year to year, but ocean heat content continues to increase steadily. As long as more energy enters the climate system than leaves it, the oceans will keep absorbing that excess heat, making ocean heat content one of the most reliable indicators of ongoing planetary warming.

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Rising ocean heat is already reshaping weather patterns worldwide. Warmer oceans increase evaporation, adding more heat and moisture to the atmosphere. This creates conditions that fuel heavier rainfall, stronger storms, and increasingly volatile weather. In 2025, long-term ocean warming was linked to devastating floods across South and Southeast Asia, flooding in Mexico and the Pacific Northwest, prolonged drought in parts of the Middle East, and intensified tropical cyclones. At the same time, warmer seawater expands, contributing directly to sea-level rise. This thermal expansion, combined with melting land ice, is steadily increasing the risks faced by coastal regions around the world.

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