According to the German Hydrographic and Maritime Transport Office (BSH), the temperature of the North Sea in 2025 was the hottest ever recorded. Compared to the long-term average from 1997 to 2021, it was 0.9°C warmer.
The BSH examines the surface temperatures of the North Sea and the Baltic Sea weekly, combining satellite data with measurements from stations and ships.
“In 2025, the North Sea reached an average temperature of 11.6 degrees Celsius, which is extraordinary, and the highest value recorded in the data series since 1969,” the BSH stated. The Baltic Sea was the second warmest. The warming is structural, not accidental.
A consequence of climate change
In the annual average, the surface temperature in all areas of the North Sea in 2025 was at least 0.5°C above the long-term average, in many regions even more than 1°C above it. It was hot in the northern and central parts of the North Sea and in the transition area to the Baltic Sea.
The analyses of the three-dimensional model data show that heating occurs not only on the surface but throughout the entire water column. The seas store enormous amounts of heat – a clear consequence of the advancing climate change,” explains Dr. Helen Morrison, co-head of the Department of Operational Modelling at BSH.
Record-breaking temperatures
The Baltic Sea also experienced record-breaking temperatures in 2025. The annual average of the surface temperature was 9.7°C, an increase of 1.1°C compared to the long-term average from 1997 to 2021. This made 2025 the second-warmest year for the Baltic Sea since the beginning of the BSH data series in 1990. Only 2020 was warmer.
The surface temperatures in almost all areas were at least 0.5°C above the long-term average. In the southwestern Baltic Sea, including the German waters and the Gulf of Finland, the temperatures were more than 1°C above the long-term average.
Rising sea levels
The BSH and the Danish Meteorological Institute (DMI) also create climate simulations that predict further significant warming unless greenhouse gas emissions drop dramatically. By 2100, the simulations suggest a temperature increase of +2.8°C in the North Sea and +3.0°C in the Baltic Sea relative to the period from 1971 to 2000.
With warming, the sea level also rises due to the melting of glaciers and ice sheets, among other factors. Global sea levels could rise by 0.63 to 1.01 meters by 2100, compared to the reference period from 1995 to 2014 – unless greenhouse gas emissions drop dramatically.
Fauna and flora
“But even if we stop emitting greenhouse gases worldwide immediately, sea levels are expected to continue to rise for centuries,” warns BSH President Helge Heegewaldt. “This is dangerous for our coast. It is therefore important to do more to protect the climate. We can thus gain time to better prepare for the consequences of climate change and to protect our coasts better and, thus, also the local population.”
All data show that the seas are warming in the long term – the Baltic Sea even faster than the North Sea. The warmer the waters, the more often marine heat waves occur, putting fauna and flora in the sea under pressure.
