All European seas have warmed considerably since 1870 and particularly since the late 1970s, with recent years been among the warmest on record. According to climate projections, sea surface temperature in European basins are expected to increase by 2-6°C by 2100 under the high emissions scenario. The frequency and magnitude of marine heatwaves have increased significantly both globally and in European seas. This is projected to continue, with increasing impacts on climate and ecosystems expected.

Being the planet’s greatest carbon sink, oceans are absorbing more heat which results in an increase in sea surface temperature (SST) and rising sea levels. This affects species’ metabolism, distribution and phenology, with many marine species and habitats being highly sensitive to changes in SST. Increases in mean SST can also lead to increases in atmospheric water vapour over the oceans, influencing entire weather systems and eventually global climate. The EU is committed to mitigating global warming and its negative impacts, including on oceans, and adapting to climate change .

All five European seas have warmed considerably since 1870, particularly since the late 1970s. Between 1991 and 2022, SST increased by between around 0.3°C per decade in the North Sea and around 0.6°C per decade in the Black Sea.

Over the past century (1925-2016), the increase in SST has been accompanied by an increase in the frequency and intensity of marine heatwaves, both globally and in European seas, with an approximate doubling from 1982 to 2016 . This has had considerable ecological impacts, including promoting harmful algal blooms, with increased risks to human health, ecosystems and aquaculture . For example, recent marine heatwaves led to unprecedented levels of vibriosis infections along the Baltic Sea and North Sea coasts. Marine heatwaves can also affect climate on land, with those in the Mediterranean Sea being thought to have contributed to amplifying heatwaves and heavy precipitation events over central Europe and triggering intense extratropical cyclones.

Ocean temperatures at the surface are expected to increase further in the 21st century, between 1.3°C (with a 90% spread from 0.2 to 2.5°C, henceforth referred to as a range of SSTs) and 3.5°C (2.2 to 5.8°C) for the Baltic Sea, 1.3°C (0.9 to 2.4°C ) and 3.5°C (2.7 to 5.5°C ) for the Black Sea, 1.1°C (0.6 to 2.1ºC ) and 3.4°C (2.6 to 5.0°C ) for the Mediterranean, and 1.0°C (–0.7 to 1.8°C) and 2.4°C (1.2 to 4.6°C) for the North Sea by 2100 for the ensemble medians under SSP1-2.6 and SSP5-8.5, respectively. Marine heatwaves are also projected to increase in frequency, duration, spatial extent and maximum intensity. Such changes could have widespread effects on marine species and cause the reconfiguration of marine ecosystems.