Record Water Temperatures Recorded in the North and Baltic Seas

Experts are raising concerns over unusually high water temperatures in the North and Baltic Seas, with some areas reporting values up to two degrees Celsius above the long-term average. According to the Federal Maritime and Hydrographic Agency (BSH) in Hamburg, the average surface temperatures for March, April, and May have reached unprecedented levels this spring.

The North Sea experienced its warmest spring since records began in 1997, with average temperatures soaring to 8.7 degrees Celsius. This marks a significant increase of 0.9 degrees compared to the long-term average from 1997 to 2021. Notably, nearly the entire North Sea recorded surface temperatures at least 0.5 degrees above the norm, with marked spikes observed in certain regions.

The most significant deviations were noted in the eastern North Sea, particularly off the coasts of Norway and Denmark, where temperatures exceeded the long-term mean by up to 2 degrees. In the German section of the North Sea, surface temperatures were also notably elevated--ranging from 0.8 to 1.5 degrees above average--making it the fourth to sixth warmest spring in the same records.

In the Baltic Sea, the average temperature for spring reached 5 degrees Celsius, which is one degree higher than the long-term average and ranks as the sixth warmest spring on record since 1997. There were significant regional disparities, with the northernmost parts being slightly cooler than average, while southern areas experienced much warmer conditions.

In the southwestern Baltic Sea, which includes German waters and the Danish islands, temperatures were more than 2 degrees above the long-term mean, marking it as the warmest spring on record since 1997. In Kiel, a marine heatwave persisted for 55 days--a record duration since 1989--for water depths of half a meter. During such heatwaves, temperatures must remain among the highest 10 percent for the respective season over a 30-year period. From March 28 to May 21, temperatures averaged 2.6 degrees above the norm from 1991 to 2020, peaking at 4.3 degrees higher than the average.

The BSH employs a combination of satellite data and measurements from stations and ships to analyze surface temperatures weekly in both the North and Baltic Seas. The spring average for 2025 was derived from weekly averages from March to May and compared with the spring averages from 1997 to 2021.

Kerstin Jochumsen, head of the BSH Marine Science Department, commented on the findings, indicating that the data shows a consistent trend of warming in both seas. This warming is attributed to climate change and is increasingly altering marine environments.