On a mid-January afternoon in the Mediterranean, a lone swimmer waded into water that, by rights, should have felt wintry. It didn't. Across the sea, temperatures have climbed by 0.32C every decade, and the warming is no longer confined to the surface.

According to the 10th Copernicus ocean state report, the world's seas absorbed a record 23 zettajoules of heat last year — a quantity of thermal energy almost 40 times greater than everything humanity consumed in the same period. The consequences extend far beyond warmer swims. Sea levels, which rose at an average of 3.8mm a year since 1999, have accelerated to 4.

2mm a year since 2012. The oceans, which have taken in roughly 90% of the extra heat trapped by greenhouse gases and about a third of humanity's carbon dioxide emissions, have grown 17% more acidic over the last four decades. In the Mediterranean and Black Seas, marine heatwaves now persist for more than a month at a time, reaching deep below the surface rather than fading after a few warm days. Europe, warming at twice the global average, is already living with the fallout.

Last week, parts of Spain and Portugal saw temperatures hit 40C, an unusually fierce spike for September, arriving just weeks after a destructive summer of fires. Scientists say the overheated coastal waters make matters worse on land, releasing stored warmth overnight and preventing the cooling that would normally follow sunset. "The ocean is transforming before our eyes," said Pierre Bahurel, director general of Mercator Ocean International, which published the report.