Ocean Warming Breaks Record for Ninth Straight Year

Every second of last year, the Earth’s oceans absorbed the equivalent in energy to 12 Hiroshima-sized atomic bombs. 

Global ocean heat content (OHC) increased for the ninth consecutive year in 2025, according to a report released Friday in Advances in Atmospheric Sciences. The study—a collaboration involving more than 50 scientists from 31 international institutions—measured temperature fluctuations in the upper 2,000 meters of the planet’s waters, finding the greatest increases in the Southern Atlantic, the Northern Pacific and the Southern Ocean. Warming waters are linked to increasingly extreme weather patterns, coral reef die-offs and sea level rise. 

Earth’s oceans act as the planet’s main thermal energy sink. Absorbing more than 90 percent of the excess heat trapped by greenhouse gases, ocean temperatures serve as a critical indicator of long-term climate change. “Global OHC is the single best indicator that the planet is warming,” said Kevin Trenberth, a co-author and scholar at the National Center for Atmospheric Research. 

“Measuring the ocean heat content is probably the best way to measure global heating as a whole,” added Michael Mann, director of the University of Pennsylvania’s Center for Science, Sustainability and the Media. Last year’s total ocean energy increase amounted to 23 Zetta Joules—more than 200 times the electrical energy usage of the entire planet. 

The warming was widespread, with 16 percent of the ocean reaching a record high. A further 33 percent of areas ranked among the top three hottest years in recorded history, and 57 percent within the top five years of data. This unabated warming was driven not only by increased greenhouse gas concentrations, but also by a recent reduction in sulfate aerosols that have historically acted as a reflective sunscreen for the planet.

While warming of the top 500 meters of oceans was visible as early as the late 1970s, man-made heat now penetrates as deep as 2,000 meters below the surface. According to Trenberth, it takes roughly 25 years for heat to travel to such depths, creating a warming effect that will likely persist for centuries. Though the mean sea-surface temperature was lower than in 2023 and 2024, it remained the third warmest year on record. 

“A warming ocean leads to warmer, wetter air—which, in turn, leads to stronger storms,” said John Abraham, a professor of thermal sciences at the University of St. Thomas in Minnesota. As ocean temperatures increase, so too does evaporation, leading to more moisture in the atmosphere. “Think of ‘weather on steroids.’ We can expect our weather to become more extreme and unpredictable.” 

Intense monsoon rains that killed more than 1,350 people in South and Southeast Asia, catastrophic flash floods along Texas’ Guadalupe River that caused 138 fatalities, and over 1,200 wildfires that burned more than five million hectares in Canada, can all be linked back to long-term ocean heat accumulation, according to the paper. Meanwhile, the increased frequency and intensity of marine heatwaves has driven record-low mass levels for both the Greenland and Antarctic ice sheets, in turn adding to global sea level rise. 

Researchers warn warming oceans also pose a serious threat to coral reefs. “When seawater temperature exceeds the thermal tolerance threshold of corals, the symbiotic system collapses,” said Lijing Cheng, an oceanographer from the Chinese Academy of Sciences and co-author of the study.

Critical to marine ecology, reefs support more than 25 percent of all known marine species despite covering less than 0.1 percent of the seabed. “Corals expel the algae, lose their colorful pigments, and appear ‘bleached’,” said Cheng, referencing how hotter water is a key stress indicator for reefs. 

The forecast remains unchanged: Global ocean heat content is expected to increase year-on-year until net-zero greenhouse gas emissions are achieved. Because of the quantity of thermal energy being stored in the Earth’s oceans, Trenberth warns change is irreversible on a human timescale. 

Though a complete reduction in carbon dioxide would not immediately lead to the same pre-1970 ocean temperatures, researchers emphasized the ability to revert course. “While these latest data convey the urgency of climate action, we nonetheless still have agency,” said Mann, noting surface warming could stabilize if emissions stopped.

Abraham agreed, suggesting the fix to ocean heat increases is a matter of will rather than capability. “We can solve this problem today, with today’s technology,” he said. “There is real optimism among scientists.”