‘Transforming before our eyes’: Seas are getting warmer; why should we care

The world’s seas are getting hotter, higher and less hospitable, with a record 23 zettajoules of heat absorbed by the oceans last year – almost 40 times more than all the energy humanity used during the year.

The findings come from the 10th edition of the Copernicus ocean state report, which documents far-reaching changes in oceans that are fundamental to life on Earth.

According to a report in The Guardian, sea levels have risen by about 3.8 mm a year since 1999, accelerating to 4.2 mm annually between 2012 and 2025. The global ocean has also become 17% more acidic over the past 40 years.

Oceans have absorbed about one-third of the carbon dioxide released by humanity and around 90% of the additional heat trapped in the Earth’s system by greenhouse gas pollution.

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“The ocean is transforming before our eyes,” said Pierre Bahurel, director general of Mercator Ocean International, which published the report.

Why should we care?

Warmer seas are already affecting marine ecosystems and intensifying extreme weather. In Europe, the Mediterranean and Black Seas have experienced record marine heatwaves, with elevated temperatures persisting deep below the surface.

The Mediterranean has warmed by 0.32C per decade, driving more marine heatwaves, particularly in recent years. The report also found an acceleration in sea-level rise in the western Mediterranean.

Hot coastal waters can worsen heatwaves on land by releasing additional heat during nights that would otherwise provide cooling. Western Europe is experiencing an unusually warm September after a fiery summer, with temperatures reaching 40C in parts of Spain and Portugal last week.

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The warming is also damaging marine life. Between January 2023 and September 2025, 84% of the world’s coral reefs experienced bleaching-level heat stress. Nearly 5% of the ocean has experienced persistent changes in phytoplankton growth patterns over the past 30 years.

In Antarctica, four emperor penguin colonies recorded total breeding failures in 2022 during periods of record-low sea ice.

The report also highlighted a link between rising temperatures and air pollution. In the first eight months of 2026, just 5% of the world’s population was exposed to “good” levels of ground-level ozone under World Health Organization guidelines.