A record-breaking 23 zettajoules of heat was sunk into the world’s seas last year, leaving ocean waters hotter, higher and less hospitable, a report has found.

The explosive addition of thermal energy, which was almost 40 times greater than all the energy humanity used that year, comes amid far-reaching changes to the deep water bodies fundamental to life on Earth.

Sea levels have risen by about 3.8mm per year since 1999, and at a faster rate of 4.2mm per year between 2012 and 2025, according to the 10th edition of the Copernicus ocean state report. It found the global ocean has become 17% more acidic in the last 40 years.

Oceans have absorbed about one-third of the carbon dioxide that humanity has spewed and about 90% of the extra heat that greenhouse gas pollution has trapped in the Earth’s system.

“The ocean is transforming before our eyes,” said Pierre Bahurel, director general of Mercator Ocean International, which published the report. “Warming, rising seas and ecosystem degradation are interconnected signs of significant and long-term change.”

In Europe, which is heating twice as fast as the global average, the Mediterranean and Black Seas have been plagued by record-breaking marine heatwaves – with elevated temperatures that persist deep below the surface – and bouts of extreme heat that have lasted more than a month, the scientists found.

A Mediterranean swimmer in mid-January: the entire sea has warmed by .32C per decade. Photograph: Hassan Ammar/AP

The scientists found a clear acceleration of sea level rise in the western Mediterranean, even as the eastern part showed uneven gains. The entire sea has warmed by 0.32C per decade, the report found, driving up marine heatwaves, particularly in the last few years. A joule, the unit used to measure heat, is the amount of electricity required to run a one watt device for one second; a zettajoule is a billion trillion joules.

The report comes as western Europe experiences an unusually warm September right after its ruinously fiery summer, with temperatures hitting 40C highs in parts of Spain and Portugal last week. Hot coastal waters exacerbate heatwaves on land by adding heat during nights that would otherwise cool down.

Ángel Borja, a marine ecologist at AZTI, a Basque research institute, who was not involved in the report, said the effects have already been seen for several years and will certainly increase.

“But it seems to be a problem that society tends to downplay. For example, the combined problems of rising sea levels and extreme events, which are eroding the Mediterranean coast – and yet some people insist on rebuilding in places that will undoubtedly disappear.”

The report documented extensive damage to marine life resulting from the heat. Between January 2023 and September 2025, 84% of the world’s coral reefs experienced bleaching-level heat stress. Over the past 30 years, nearly 5% of the ocean has seen a persistent shift in growth patterns of phytoplankton, tiny creatures that underpin food webs and regulate carbon flows. In the Antarctic, four emperor penguin colonies saw total breeding failures in 2022 during periods of record-low sea ice.

Water-sprinklers spray mist to suppress dust and help curb air pollution at the BRICS summit in New Delhi, India, in September 2026. Photograph: Mukhtar Khan/AP

The findings were published alongside separate Copernicus research that found this year’s deadly cocktail of extreme heat and dirty air subjected more than one-third of humanity to dangerous levels of ground-level ozone pollution.

The phenomenon, known as the “ozone-climate penalty”, is the result of chemical changes that occur when certain pollutants are exposed to heat and sunlight. In the first eight months of 2026, the report found, just 5% of the world’s population has been exposed to “good” levels of ozone that fall within the World Health Organization’s guidelines.

Florian Pappenberger, director-general of the European Centre for Medium-Range Weather Forecasts, which conducted the analysis, said the findings show that “air pollution knows no border”.

“Wildfire smoke, ozone and dust can travel thousands of kilometres, affecting communities far from their source,” he said. “As the international community prepares for Cop31, these findings are a timely reminder that climate change, air quality and public health are deeply interconnected challenges that require coordinated global action.”