Heatwaves: The North Sea and the Baltic Sea were significantly too warm in summer

The North Sea and the Baltic Sea remain too warm. As recently as last year, the Federal Maritime and Hydrographic Agency (BSH) had classified the summer of 2025, with an average temperature of 15.7 degrees Celsius, as the warmest in the North Sea since records began in 1969. The summer of 2026 did not reach this figure. Nevertheless, according to a new report from the BSH, temperatures this summer were also just under one degree above the long-term average. Off the coast of Heligoland, a new record of 66 days of marine heatwaves was recorded.

​Hot summer of 2026: the North Sea and the Baltic Sea once again significantly too warm

The Water temperatures in the North Sea and the Baltic Sea From June to August 2026, temperatures were just under one degree above the average for the years 1997 to 2021. In the North Sea, the average surface temperature reached 15.6 degrees Celsius. This was the fifth-warmest summer since 1997. The Baltic Sea reached 17.2 degrees Celsius, ranking seventh among the summers recorded since 1997.

The figures are taken from the latest summer report by the Federal Maritime and Hydrographic Agency. They are slightly lower than last year’s figures, when new temperature records were set in the North Sea and the Baltic Sea.

Marine heatwaves off Heligoland

​The waters around Heligoland were among the regions of the North Sea that had warmed particularly significantly. On 1 July 2026, the surface temperature at the measuring station there was in fact almost four degrees above the average for that day since the station began operating in 2009.

But it is not just this one peak value that is remarkable: at the measuring station off Heligoland, the BSH recorded 66 days of marine heatwaves – that is, periods of at least five days with water temperatures unusually warm for the location and time of year. By way of comparison: in the spring of 2025, the BSH’s ‘Leuchtturm Kiel’ measuring station in the Baltic Sea had recorded 55 days as the previous record.

​According to forecasts, marine heatwaves are set to become more frequent in all oceans. As warmer water can dissolve less oxygen, living conditions for marine life may deteriorate. This will have particularly serious consequences for the Baltic Sea: on the seabed of this already ailing inland sea, oxygen-depleted ‘dead zones’ are also likely to become more common.

​The southern North Sea and the English Channel as hotspots

The southern North Sea was the warmest. There, and in the English Channel, temperature anomalies reached two degrees or more, with some areas recording their highest temperatures since 1997. For around a quarter of the North Sea, temperatures were among the three highest of the last 30 years.

Tim Kruschke, Head of the Marine Climate Issues Unit at the BSH, emphasised that the new record highs in the southern North Sea and the 66 days of marine heatwaves off Heligoland indicate that extreme heat events are becoming increasingly noticeable in German waters too. Although the North Sea and the Baltic Sea are not affected to the same extent as, for example, the Mediterranean or the north-east Atlantic, temperatures in many places were among the highest recorded in recent decades.

The Baltic Sea: A very warm June, followed by temperatures closer to the average

Temperatures in the Baltic Sea were not consistent. June, in particular, stood out, being almost three degrees above the long-term average and, according to the BSH, the third-warmest June since records began. In July and August, however, temperatures were generally within the average range.

The greatest positive deviations were recorded off the east coast of Sweden, along the German and Polish coasts, and in the far north of the Baltic Sea. IIn late summer, this could even be seen with the naked eye: large patches of blue-green algae were floating in the southern and western parts of the Baltic Sea, and in some cases also in harbours around Bornholm.

What warmer water means for the weather, sea levels and planning a sailing trip

Warmer seas do not necessarily lead to severe weather. However, they can contribute to “rising sea levels, because warm water expands and glaciers and ice sheets melt. This also causes water levels to rise during storm surges.”A warm summer does not, therefore, automatically lead to a stormy autumn or winter. Many factors interact to cause the formation and intensification of low-pressure systems, including air pressure distribution, high-altitude winds and the temperature difference between air masses.

Warming also supplies additional energy and moisture to the atmosphere. The European climate service Copernicus points out that high sea surface temperatures can increase evaporation and, consequently, the potential for heavy rainfall. It is not possible to determine from this how significantly this affects a single weather event. The current forecasts, radar data and warning information prior to setting sail remain crucial.

Habitats under pressure: species migration, algae and dead zones

Warmer seas are more than just a meteorological observation. The warming of the seas can have far-reaching consequences. For instance, warmer water alters habitats and food chains. Whilst some species are losing habitat at a dramatic rate, others are migrating to cooler waters. Furthermore, non-native species are spreading, which can lead to an increase in harmful algal blooms and a lack of oxygen.

This is particularly critical for the Baltic Sea: when large quantities of dead algae sink to the seabed, microorganisms consume oxygen as they break them down, until there is hardly any oxygen left dissolved in the deep water. Other substances, such as ammonium, increase in concentration, and with them come microorganisms that can carry out their metabolic processes without oxygen. In doing so, they produce hydrogen sulphide, which is toxic to many organisms – this results in ‘dead zones’ where higher life on the seabed is no longer possible.

The persistently high temperatures are also encouraging the proliferation of blue-green algae in coastal regions. Blue-green algae can produce toxins and are not entirely harmless to humans and animals: if swallowed or if there is contact with contaminated water, they can cause skin irritation, gastrointestinal problems and, in severe cases, liver or nerve damage. Sailors travelling on the Baltic Sea in summer are advised to check the latest algal bloom reports – swimming bans may be imposed at short notice.

Book recommendations

Heavy-weather sailing explains how sailing and motor yachts, as well as monohulls and multihulls, behave in heavy seas. The theory and technical section covers, amongst other things, equipment, trysails, and sea and drift anchors. In the practical section, reports on heavy-weather situations are analysed and translated into specific tactics. This ties in with the article, as reliable preparation and the correct response to the current weather conditions are more important than drawing a blanket conclusion based on warm seawater.

Weather at sea covers the basics of meteorology for cruising and racing sailors. The book covers low- and high-pressure systems, local and regional weather events, sea states, currents and meteorological trip planning. It thus complements the ‘Summer Review’ by addressing the practical question of how skippers can interpret weather information and use it to make their own route decisions.

How did you find the unusually warm water temperatures in the North Sea or the Baltic Sea this summer?