Multiple marine heat waves lead to new record temperatures in parts of North Sea

Water temperatures in the North Sea and the Baltic Sea this summer were markedly above the long-term average, with parts of the North Sea registering the highest temperatures ever recorded, figures released by the Maritime and Hydrographic Agency (BSH) have shown. Amid recurring heatwaves, the average temperature in the North Sea during the summer was 15.6 degrees Celsius, one degree above the average from 1997 to 2021 and the fifth-warmest season since 1997, when modern measurement methods began combining satellite data with actual measurements from fixed stations and ships.

In the southern parts of the North Sea and in the English Channel, temperatures were two degrees above the long-term average, and partially the highest in decades. BSH said that the higher temperatures occurred especially during the first month of summer, with June on average being 1.5 degrees warmer than during the reference period, while August was slightly below average. 

Average water temperatures in the Baltic Sea reached 17.2 degrees during the summer, also almost one degree above the long-term average. In June, temperatures were even three degrees higher than usual, making it the third-warmest June since the beginning of measurements. 

The waters around the German North Sea Island Heligoland were among the warmest parts of the sea, as the BSH registered 66 days with marine heat waves. According to the agency, marine heat waves that last longer than five days exert immense pressure on ecosystems and are becoming more likely as the oceans are heating due to global warming. On 1 July, the water surface temperature near the island that lies about 50 kilometres off the German coast was almost four degrees above the average for this day since measurements began in 2009. 

“Especially the record temperatures in the southern North Sea and the marine heatwaves around Heligoland show that extreme weather events are starting to make themselves felt in our region,” said BSH climate expert Tim Kruschke. The agency said rising sea temperatures are changing marine habitats, making some species disappear from a warming region while other, formerly non-endemic species settle in new areas. 

At the same time, warmer water increases the likelihood of algae blooming and reduces oxygen levels. Higher temperatures also let seawater expand, making storm tides more dangerous, whereas the ocean’s capability to absorb carbon dioxide decreases, thus reinforcing climate change. 

Extreme heat and drought have impacted Germany and many other countries in Europe over the summer 2026, leading to thousands of heat-related deaths, low river levels, harvest failures and other damages to ecosystems and the economy.