Europe is experiencing one of the most remarkable climate events of recent years. While record-breaking air temperatures have captured most headlines, another development is unfolding over the surrounding seas, and it may prove equally important.
Satellite observations from the Copernicus Marine Service reveal exceptionally high sea surface temperature anomalies across much of Europe’s surrounding waters.
The western Mediterranean, parts of the North Sea, the Baltic Sea, and sections of the northeast Atlantic are several degrees warmer than the long-term average, with some regions exceeding 5-6°C above normal.
These marine heatwaves are not simply a consequence of the atmospheric heatwave – they are increasingly becoming an active driver of future extreme weather.
The Ocean Remembers
Unlike land, which cools relatively quickly after sunset or after a cold front passes, the ocean stores enormous quantities of heat.
More than 90% of the excess heat trapped by greenhouse gases has been absorbed by the world’s oceans, making them Earth’s largest heat reservoir.
When sea surface temperatures remain exceptionally warm, they continue releasing heat and moisture into the atmosphere long after the peak of a heatwave has passed.
This stored energy can reduce nighttime cooling, increase humidity, suppress relief from heat, and provide additional energy for storms and heavy rainfall. In other words, warm seas can help sustain and even amplify future heatwaves.
Why the North Sea and Baltic Sea Matter
The North Sea and Baltic Sea deserve particular attention. Unlike the deep Atlantic Ocean, both are relatively shallow. The average depth of the North Sea is around 95 metres, while the Baltic Sea averages only about 55 metres.
Because there is much less water available to distribute incoming solar energy, these seas warm much faster during prolonged sunny weather.
Once established, marine heatwaves can persist for weeks because atmospheric circulation often remains stable under blocking high-pressure systems.
This creates a powerful positive feedback:
- Firstly, persistent high pressure heats the sea;
- Then, the warmer sea releases more heat and moisture;
- The atmosphere cools less efficiently at night;
- And finally, the warm air mass becomes even harder to remove.
Although this feedback is weaker than in tropical oceans, it is increasingly important for Europe as marine heatwaves become more frequent.
The Baltic Sea: A Climate Hotspot
The Baltic Sea has emerged as one of the fastest-warming marine regions on Earth.
Several factors contribute to this, including shallow waters, limited exchange with the Atlantic, low salinity, and long summer daylight at northern latitudes.
Because of these characteristics, even modest atmospheric heatwaves can rapidly elevate sea surface temperatures.
During recent years, the Baltic has repeatedly experienced marine heatwaves that would have been considered exceptional only a few decades ago.
The World Meteorological Organization has highlighted marine heatwaves around Ireland, the United Kingdom and the Baltic Sea as part of Europe’s increasingly unusual climate conditions.
The Mediterranean: Europe’s Heat Engine
While the Baltic warms quickly, the Mediterranean stores vastly more heat. The current marine heatwave has produced anomalies exceeding 5°C in parts of the western Mediterranean, particularly near southern France and western Italy.
A warmer Mediterranean can fuel heavier rainfall, intensify thunderstorms, increase humidity during heatwaves, prolong extremely warm nights, and increase evaporation that later feeds torrential rain.
Scientists increasingly describe the Mediterranean as a climate change hotspot because warming there exceeds the global ocean average.
Marine Heatwaves Are Becoming More Common
Marine heatwaves were once relatively rare in European waters. Today they are becoming increasingly frequent, longer-lasting and more intense.
Copernicus observations show that sea surface temperatures across the world’s oceans remain close to or at record levels, reflecting both long-term climate change and additional natural variability such as the developing El Niño.
This combination raises the probability that future European summers will begin with oceans already primed to reinforce atmospheric heat extremes.
A Dangerous Feedback Loop
Climate scientists often describe interactions between the atmosphere and the ocean as a coupled system rather than two separate components.
When both are unusually warm, they can reinforce one another. Heatwaves warm the sea. Warm seas then increase atmospheric heat and humidity.
That additional energy makes subsequent heatwaves more persistent. This feedback does not guarantee that every future summer will be hotter than the previous one.
Weather variability will always play an important role. However, the baseline from which heatwaves develop is steadily rising.
Looking Ahead
The striking Copernicus satellite imagery is more than just a visualization of warm water.
It may represent an early warning.
If shallow seas such as the North Sea and Baltic Sea continue warming during future summers, they could increasingly act as regional heat reservoirs that reduce nighttime cooling and prolong heatwaves over northern Europe.
Meanwhile, the Mediterranean’s growing heat content may continue to enhance humidity, heavy rainfall events and marine ecosystem stress.
Europe is therefore facing not only hotter land temperatures, but warmer surrounding seas capable of reinforcing the very extremes that heated them in the first place.
The ocean is no longer merely responding to climate change. It is becoming an increasingly active participant in Europe’s future heatwaves.
References:
- Copernicus Climate Change Service (C3S) – Sea surface temperature records and European heatwaves.
- Copernicus Marine Service – Sea Surface Temperature Anomaly Maps (June 2026).
- World Meteorological Organization (WMO) – Marine heatwaves and global ocean warming.
- ECMWF / Copernicus Climate Bulletins – European climate monitoring.
