The average surface temperature of the North Sea in 2025 was 11.6 degrees Celsius – the highest ever recorded in the BSH data series. The Baltic Sea also recorded one of the warmest years on record. This is according to information presented by the BSH at the Extreme Weather Congress in Hamburg on 23 and 24 September. According to the data, the warming was not limited to the surface: the heat content of the North Sea also reached a record high. This measure describes how much heat the sea stores, from the surface water right down to the seabed.
The monthly temperature series from the BSH make it clear that this is not a short-term anomaly. The North Sea and the Baltic Sea have been warming over long periods of time. The consequences range from changes to habitats to strain on coastal infrastructure.
Summer 2026: Many days of heatwave off Heligoland
And it wasn’t just 2025 that was special. These high figures continued into 2026. Following an exceptionally warm spring, temperatures in the North Sea and the Baltic Sea remained well above the long-term average throughout the summer. The Helgoland-RM station was particularly notable: by early September, the BSH had recorded 66 days of marine heatwaves there.
A marine heatwave is defined as a period of at least five days during which the water temperature at a particular location is unusually high for the time of year. Such events highlight what annual and summer averages alone do not reflect: even if an average value deviates only slightly, individual sea areas can be severely affected over a prolonged period.
The Summer review for the North Sea and the Baltic Sea highlights these regional differences. The additional heat can have consequences for the marine environment. Habitats and food chains are changing, and heat-loving species may spread. Furthermore, the risks of algal blooms and oxygen depletion are increasing.
The severe icy winter of 2025/26 does not contradict this. Ice formation is determined primarily by the prevailing weather conditions, such as periods of frost and easterly winds. Long-term measurement series are crucial for climatic classification, not individual cold winters.
Rising sea levels increase the starting elevation
In addition to global warming, rising sea levels are of particular concern to coastal areas. This rise can already be measured at the Cuxhaven and Warnemünde gauges. As a result, storm surges now build up against a higher mean sea level. Under comparable wind and tidal conditions, higher water levels may therefore be reached in future.
The extent to which sea levels will rise by the end of the century depends on the climate scenarios. However, when it comes to extreme water levels, it is not only the average value that matters. Wind direction, storm intensity, tides and regional characteristics also determine how critical a situation becomes along the coast or in a harbour.
Extreme water levels could become more frequent
Climate simulations carried out by the BSH and the University of Hamburg suggest that extreme water levels currently observed in the German Bight could occur three to five times more frequently by the year 2100. This does not mean that all storm surges will, across the board, become five times more frequent. Rather, the projection describes how often certain high water levels could be reached in future.
For sailors and harbour operators, this does not result in a fixed figure for an individual berth. The simulations do not provide a specific forecast for individual marinas or recreational boat harbours. However, they do highlight that weather and water level information, the assessment of berths and trip planning become even more important in onshore winds.
More information You can find information on the Extreme Weather Conference at here.