Heatwaves: The North Sea and the Baltic Sea set to be too warm again in the summer of 2026

The summer of 2026 has left a clear mark on Germany’s coastal waters. According to the latest data from the Federal Office for Maritime Transport and Hydrography From June to August, the average surface temperature in the North Sea was 15.6 degrees Celsius. This was just under one degree higher than the average for the years 1997 to 2021 and made it the fifth-warmest summer since records began. In the Baltic Sea, the average temperature reached 17.2 degrees, which was also just under one degree above the long-term average. This makes it the seventh warmest summer there since 1997.

66-day marine heatwave off Heligoland

The situation around Heligoland was particularly striking. At the BSH Heligoland-RM station, 66 days of marine heatwaves had already been recorded by 2026. These are defined as periods of at least five days during which the water temperature is unusually high for the location and time of year. On 1 July, the surface temperature there was almost four degrees above the climatological daily average since the station began operating in 2009.

Regionally, too, the measurements showed marked differences: in the southern North Sea and the English Channel, temperatures were in some places more than two degrees above the long-term average. In the Baltic Sea, areas off the Swedish coast and along the German and Polish coasts were particularly warm.

How this affects recreational boaters

For skippers, warm water is not, at first glance, purely a matter of comfort. High water temperatures can, amongst other things, encourage algal blooms, alter the growth of fouling on the hull and may increase the strain on the local flora and fauna. Particularly in calm, shallow bays and harbour areas, it is worth keeping an eye on water quality and local advice.

The extent to which cyanobacteria spread depends not only on light and nutrient inputs, but also on water temperature, wind and currents. Where visible blooms are present, motorboat users and swimmers should follow the advice issued by the authorities. BOOTE has already explained what to do in the event of a Algal bloom on the German Baltic Sea coast Please note this.

Weather forecasting is also becoming more important. Warm seawater evaporates more readily and can provide additional energy to weather systems. This does not automatically mean more storms, but can, under the right conditions, lead to heavy rain, thunderstorms and strong gusts. In its analysis, BOOTE examines why weather patterns along the coast are changing and why prolonged periods of light or strong winds are becoming increasingly noticeable. Unpredictable weather on the Baltic Sea coast classified.

When preparing for a cruise, it is therefore essential not only to check the weather forecast once before setting sail, but also to update it whilst underway, particularly in hot weather, when there is a risk of thunderstorms or when weather fronts are approaching. For longer voyages, reliable information on the sailing area is also helpful, such as the Distance tables and port guides for the Baltic Sea.

Further articles on this topic

• Unpredictable weather on the Baltic Sea coast explains the wind and weather trends observed along the coast.
• Blue-green algae now also found on the German Baltic coast shows what water sports enthusiasts should look out for when they see algae build-up.
• Vibrio bacteria in the Baltic Sea classifies the health risks associated with high water temperatures.
• Planning a sailing trip on the Baltic Sea: distance tables and ports helps you practise longer shots in the shooting ground.

Book recommendation

Weather at sea explains the basics of high- and low-pressure systems, regional weather events, sea states, currents and meteorological trip planning. Particularly against the backdrop of ever-warmer seas, the book helps readers not only to access weather information but also to better understand its significance for route planning, departure times and safety margins on board. The numerous diagrams make the interrelationships easy to understand, even without prior knowledge of meteorology.

What was your experience of the summer of 2026 on the North Sea or the Baltic Sea? Please let us know in the comments whether you noticed unusually warm water, algae, fouling or changing weather conditions during your sailing trip.