The Federal Maritime and Hydrographic Agency (BSH) once again recorded extremely high water temperatures in the North Sea and Baltic Sea in 2025, as well as a significant increase in heat content. "In 2025, the North Sea reached an average temperature of 11.6 °C, the highest value in the BSH data series since 1969. Compared to the long-term average from 1997 to 2021, it was 0.9 °C warmer. Every single month was well above the long-term average, with June and December setting records," says Dr Tim Kruschke, Head of the Marine Climate Division at the BSH.
The annual average 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 was particularly warm in the northern and central parts of the North Sea and in the transition area to the Baltic Sea.
North Sea: Heat record also at depth
In 2025, the North Sea stored more heat than ever before at 1.46 million petajoules, around 35,000 petajoules more than in the previous year. This difference corresponds to around three times Germany's primary energy consumption in 2024, with the heat content reaching new records from April to September in particular. "The analyses of the three-dimensional model data show that the warming is not only taking place at the surface, but throughout the entire water column. The oceans store enormous amounts of heat - a clear consequence of advancing climate change," explains Dr Helen Morrison, Co-Head of the Operational Modelling Unit at the BSH.
The ocean models operated at the BSH not only provide forecasts on water levels, currents and salinity, but also on temperature - from the surface to the seabed. This allows the heat content of the North Sea to be calculated over the entire water column.
BSH study: Second warmest year since measurements began in the Baltic Sea
The Baltic Sea also experienced record-breaking temperatures in 2025. The annual average surface temperature was 9.7 °C, an increase of 1.1 °C compared to the long-term average from 1997 to 2021, making 2025 the second warmest year for the Baltic Sea since the BSH data series began in 1990. Only 2020 was warmer. Surface temperatures were at least 0.5 °C above the long-term average in almost all areas. The south-western Baltic Sea, including the German waters, and the Gulf of Finland were particularly warm. Here, temperatures were more than 1 °C above the long-term average in many areas.
At the Kiel lighthouse, the BSH observed a 55-day marine heatwave in the Kiel Fjord in spring 2025. Longer than ever before. Temperatures were on average 2.6 °C above the long-term average - at peak times even 4.3 °C above. The warmer the seas, the more frequently such marine heatwaves occur. These extreme events lasting at least five days with unusually high water temperatures for the time and place put marine flora and fauna under pressure. The BSH analyses the surface temperatures of the North Sea and Baltic Sea on a weekly basis, combining satellite data with measurements from stations and ships. This data proves that the seas are warming in the long term - the Baltic Sea even faster than the North Sea. In addition, the BSH and the Danish Meteorological Institute (DMI) have produced climate simulations that predict further significant warming unless greenhouse gas emissions are drastically reduced (RCP8.5 scenario).
By 2100, the simulations suggest a temperature change of +2.8 °C in the North Sea and +3.0 °C in the Baltic Sea compared to the period from 1971 to 2000
The BSH prepares these and other analyses as part of the DAS basic service "Climate and Water". Together with other federal authorities, it supports the German Strategy for Adaptation to Climate Change (DAS) in order to advise various stakeholders from politics and society. Sea levels are also rising as a result of global warming. This is due to the expansion of water at higher temperatures and the melting of glaciers and ice sheets. Sea level measurements prove this: In Cuxhaven, the sea level has risen by over 25 centimetres since 1900, in Warnemünde by just under 20 centimetres.
Global sea levels could rise by 0.63 to 1.01 metres by 2100 compared to the reference period from 1995 to 2014, according to climate projections by the Intergovernmental Panel on Climate Change (IPCC), unless greenhouse gas emissions fall drastically (SSP5-8.5 scenario). Storm surges will also reach higher water levels as a result (The storm surge of the century 2023). "Even if we immediately stop emitting greenhouse gases worldwide, sea levels will probably continue to rise for centuries. This is particularly dangerous for our coasts. It is therefore important to do more to protect the climate. We can gain time to better prepare for the consequences of climate change and to better protect our coasts and thus also the local population," emphasises BSH President Helge Heegewaldt.
