The world’s oceans have been set to simmer, and the heat is being cranked up. Last year saw the hottest ocean temperatures in recorded history, the sixth consecutive year that this record has been broken, according to new research. The heating up of our oceans is being primarily driven by the human-caused climate crisis, scientists say, and represents a starkly simple indicator of global heating. While the atmosphere’s temperature is also trending sharply upwards, individual years are less likely to be record-breakers compared with the warming of the oceans. Last year saw a heat record for the top 2,000 meters of all oceans around the world, despite an ongoing La Niña event, a periodic climatic feature that cools waters in the Pacific. The 2021 record tops a stretch of modern record-keeping that goes back to 1955. The second hottest year for oceans was 2020, while the third hottest was 2019. “The ocean heat content is relentlessly increasing, globally, and this is a primary indicator of human-induced climate change,” said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Colorado and co-author of the research, published in Advances in Atmospheric Sciences. Warmer ocean waters are helping supercharge storms, hurricanes and extreme rainfall, the paper states, which is escalating the risks of severe flooding. Heated ocean water expands and eats away at the vast Greenland and Antarctic ice sheets, which are collectively shedding around 1tn tons of ice a year, with both of these processes fueling sea level rise. Oceans take up about a third of the carbon dioxide emitted by human activity, causing them to acidify. This degrades coral reefs, home to a quarter of the world’s marine life and the provider of food for more than 500m people, and can prove harmful to individual species of fish. As the world warms from the burning of fossil fuels, deforestation and other activities, the oceans have taken the brunt of the extra heat. More than 90% of the heat generated over the past 50 years has been absorbed by the oceans, temporarily helping spare humanity, and other land-based species, from temperatures that would already be catastrophic. The amount of heat soaked up by the oceans is enormous. Last year, the upper 2,000 meters of the ocean, where most of the warming occurs, absorbed 14 more zettajoules (a unit of electrical energy equal to one sextillion joules) than it did in 2020. This amount of extra energy is 145 times greater than the world’s entire electricity generation which, by comparison, is about half of a zettajoule. Long-term ocean warming is strongest in the Atlantic and Southern oceans, the new research states, although the north Pacific has had a “dramatic” increase in heat since 1990 and the Mediterranean Sea posted a clear high temperature record last year. The heating trend is so pronounced it’s clear to ascertain the fingerprint of human influence in just four years of records, according to John Abraham, another of the study’s co-authors. “Ocean heat content is one of the best indicators of climate change,” added Abraham, an expert in thermal sciences at University of St Thomas. “Until we reach net zero emissions, that heating will continue, and we’ll continue to break ocean heat content records, as we did this year,” said Michael Mann, a climate scientist at Penn State University and another of the 23 researchers who worked on the paper. “Better awareness and understanding of the oceans are a basis for the actions to combat climate change.”