Standing at the ocean’s edge and knowing that the world’s largest heat store is located just beneath its shimmering surface—quiet, ancient, and uncaring—is almost unsettling. You can’t tap a battery. It is not a drainable reservoir. Just energy trapped in water, growing silently for decades, and now, in 2025, reaching a point that has truly unnerved researchers.
The figures from the international ocean study this year don’t quite make sense. The world’s oceans absorbed 23 zettajoules of heat in 2025 alone. That’s 23 followed by 21 zeroes, or about 37 years of all the energy that humans currently consume in a single year. Using data from three continents, more than fifty scientists from thirty-one institutions came to the same conclusion: the ocean has never been this warm since we began measuring it. Records for nine years in a row. The run is still going strong.
The ocean’s patience, in addition to its size, is what makes it such a unique heat sink. The deep ocean absorbs energy slowly and releases it even more slowly than the atmosphere, which fluctuates greatly with the seasons and weather systems. The ocean has an enormous capacity to retain heat, according to Dr. Paul Durack, a research scientist at Lawrence Livermore National Laboratory. Over 90% of the excess energy trapped by greenhouse gases does not dry out your soil or heat your city. It plunges into the ocean. To the unaided eye, it vanishes completely.
However, that disappearance is not real. The heat has been rising on what scientists refer to as an unrelenting upward curve in the two kilometers below the ocean’s surface, which is far below satellite range and beyond the scope of casual observation. The surface varies. Sea surface temperatures rise during El Niño and then slightly decline during La Niña, as was the case between 2023 and 2025. However, none of that seasonal drama is felt beneath the surface. The deep water simply keeps getting warmer. Ocean change researcher Dr. Bernadette Sloyan of Australia’s CSIRO compares the ocean to a flywheel, which has tremendous momentum, drives the system, and is difficult to stop.
Even though most people are still unaware of their origins, the effects of that momentum are already apparent. In 2025, the Middle East experienced drought, Mexico and the Pacific Northwest experienced flooding, and Southeast Asia experienced severe flooding as a result of the warming ocean. Warmer water evaporates more quickly, adds more moisture to the atmosphere, and intensifies storms. Thermal expansion, or the physical swelling of the ocean as its temperature rises, is directly responsible for about one-third of the rise in sea levels worldwide. Reefs are bleaching. Millions of people rely on the nurseries of fisheries that are disappearing along with underwater meadows and forests.
Observing all of this build up gives the impression that the ocean is continuously providing a service for which it was never intended. According to Sloyan, it has functioned as the planet’s air conditioner. However, air conditioners have their limits, and this one has been operating at maximum efficiency for many years. Storm systems that would have seemed extreme twenty years ago, rising coastlines, and bleached coral are all signs of the cost.
The ceiling is still unknown, as is whether the ocean’s absorption slows down at a certain point or if the curve just keeps rising. There is currently no indication of leveling in the data. More energy is added to this planetary reservoir with each year that fossil fuels are used; this energy cannot be turned off, drawn down, or redirected. Charged by human activity, it is the largest battery in the world and is filling up without a true off switch. It’s hot inside. Furthermore, it is not going away.
