The summer of 2026 brought exceptionally warm conditions to Germany’s coastal waters. According to a statement from the Federal Maritime and Hydrographic Agency (BSH), the North Sea reached an average surface temperature of 15.6 degrees Celsius, which was just under one degree above the long-term average for the period 1997 to 2021. This means that 2026 ranks as the fifth-warmest summer since records began in 1997. Only the years 2025, 2003, 2014 and 2018 recorded even higher temperatures. The southern North Sea and the English Channel stood out in particular: here, temperatures exceeded the long-term average by two degrees Celsius or more. Some areas set new record highs since 1997. For around a quarter of the North Sea, temperatures were among the three highest of the last 30 years. The Baltic Sea was also significantly warmer than usual, reaching an average surface temperature of 17.2 degrees Celsius, which is just under one degree above the long-term average, according to the BSH.
According to the data, temperature trends varied across the summer. In June, temperatures in the North Sea were more than 1.5 degrees Celsius above the long-term average. July, too, remained significantly too warm. August, on the other hand, was actually slightly cooler than usual. A similar pattern was observed in the Baltic Sea, with even more extreme fluctuations: at almost three degrees Celsius above the long-term average, June was the third-warmest June since records began. In July and August, however, temperatures settled at more average levels. According to the BSH, these temporal differences illustrate that marine heatwaves do not occur evenly throughout the summer, but can be concentrated in specific periods.
Marine heatwaves off Heligoland
The waters around Heligoland were among the regions of the North Sea that had warmed up particularly sharply, as the agency emphasises. In 2026, the BSH had already recorded 66 days of marine heatwaves at the Heligoland-RM monitoring station. On 1 July 2026, the surface temperature there was almost four degrees Celsius above the climatological average for that day since the station began operating in 2009. Marine heatwaves are defined as extreme events lasting at least five days with unusually high water temperatures for the time of year and the specific location. The warmer the oceans become, the more frequently such marine heatwaves occur. These extreme events place considerable strain on marine flora and fauna and can have long-term effects on marine ecosystems.
In 2026, the Baltic Sea experienced its seventh-warmest summer since 1997. Temperatures in the Baltic Sea were above the long-term average almost everywhere, although the deviations varied from region to region. Particularly high temperatures were recorded off the east coast of Sweden, along the German and Polish coasts, and in the far north. In these areas, temperatures rose by 1.5 degrees Celsius or more above the long-term average. The spatial distribution of the temperature anomalies shows that not all areas of the Baltic Sea are affected equally by the warming. Coastal waters often react more quickly and more strongly to atmospheric influences than the open sea.
Far-reaching consequences for marine ecosystems
“In particular, the new record high temperatures in the southern North Sea and the 66 days of marine heatwaves off Heligoland show that extreme heat events are also having a significant impact on our local seas,” emphasises Dr Tim Kruschke, Head of the Marine Climate Issues Unit at the BSH. Warmer seas have far-reaching consequences for the ocean and for people. They alter habitats and food webs: for some species, their habitat is shrinking drastically; others are migrating to cooler waters, whilst non-native species are spreading. Harmful algal blooms and oxygen depletion are on the rise. Sea levels are rising because warm water expands and glaciers and ice sheets are melting. This also causes water levels to rise during storm surges. Furthermore, warmer seas store less carbon dioxide, meaning more remains in the atmosphere and exacerbates climate change.
The BSH monitors the surface temperatures of the North Sea and the Baltic Sea on a weekly basis, combining satellite data with measurements from stations and ships. Using the weekly averages for June, July and August, it calculated the summer average for 2026 and compared it with the summer average from 1997 to 2021. The BSH carries out these analyses as part of the DAS core service ‘Climate and Water’. Together with other federal authorities, it thus supports the German Strategy for Adaptation to Climate Change, providing advice to various stakeholders in politics and society. Not only is the average temperature rising, but extreme events such as marine heatwaves are also becoming more frequent and lasting longer. The BSH will present further findings on marine extremes at the Extreme Weather Congress on 23 and 24 September 2026 in Hamburg.
Further information is available at bsh.de