The world's oceans recorded their highest ever average surface temperature on Saturday, reaching 21.1C outside polar regions. This rising marine heat is considered a significant indicator of the climate crisis, as over 90% of the heat trapped by carbon dioxide from fossil fuel burning is absorbed by the oceans.
Scientists warn that hotter oceans worsen sea level rise and extreme storms, which pose a threat to billions globally. They also drive marine heatwaves that harm sea life and impact coastal livelihoods. The current ocean heat spike is being intensified by a rapidly developing El Niño event, which is anticipated to be the hottest in at least a century.
Dr Samantha Burgess of the European Union’s Copernicus Climate Change Service (C3S), which provided the data, stated that the record is "another clear signal of an ocean under growing stress." She added that El Niño is adding heat on top of decades of human-driven warming, with marine heatwave days globally more than tripling since the early 1990s.
Warmer oceans contribute to sea level rise through thermal expansion, increasing flood risks for coastal communities. Higher ocean temperatures also intensify extreme weather events by transferring more energy and water vapour to the atmosphere, leading to more intense rainfall and storms. A hotter ocean is also less efficient at absorbing CO2, potentially weakening a crucial natural buffer against global heating.