Since 2018, a group of researchers from around the world have crunched the numbers on how much heat the world’s oceans are absorbing each year. In 2025, their measurements broke records once again, making this the eighth year in a row that the world’s oceans have absorbed more heat than the years before. While some of the excess heat warms the ocean’s surface, it also slowly travels further down into deeper parts of the ocean, aided by circulation and currents. Global temperature calculations—like the ones used to determine the hottest years on record—usually only capture measurements taken at the ocean’s surface. While sea surface temperatures have risen since the industrial revolution, thanks to our use of fossil fuels, these measurements don’t provide a full picture of how climate change is affecting the oceans.