Since measurements began in 1969, the North Sea has never been as warm as it was last year: an average of 11.6 degrees Celsius was recorded in 2025 according to research by Germany’s Federal Maritime and Hydrographic Agency, the BSH, which warns of the consequences of increasingly warmer seawater in the North Sea. ‘We are seeing more unstable weather and weather systems that become blocked, leading to droughts, long heat waves and water bombs,’ explains Jan Seys of the Flanders Marine Institute , the VLIZ.
North Sea becoming increasingly ‘Mediterranean’: warmest ever water temperatures measured in 2025
Colin Clapson
Published:
‘Every month of 2025 was significantly above the long-term average, with June and December being record months,’ says Tim Kruschke, head of the Marine Climate Issues department at the BSH. According to the study, there was a 55-day heatwave in the southern North Sea in 2025.
‘These results do not surprise us, as they are a continuation of what we have been seeing for years: the North Sea, the Baltic Sea and all seas worldwide are warming up,’ says Jan Seys of the Flanders Marine Institute. ‘Sea surface temperatures are on average one degree higher than in the first 2 decades of this millennium.’
More squid and lesser weever, fewer cod and shrimp
The cause of this warm water is no mystery: global warming. ‘The increasingly warmer water is affecting life in our North Sea,’ explains Seys. ‘Initially, the sea is gradually becoming “more Mediterranean” in terms of life. There are more species that prefer warmer water, such as squid, red mullet, sardines and the poisonous lesser weever.’
‘On the other hand, cold-water species are disappearing from our seas, including cod, which is leaving the southern North Sea, and North Sea shrimp also seem to be becoming less abundant.’ This became apparent in the spring, when Belgian fishermen caught very few shrimps, resulting in high prices. ‘People are used to eating shrimp all year round, but that is no longer a certainty,’ one fisherman said at the time.
In addition, climate change will cause more and more marine heatwaves. These are heatwaves like those on land, but at sea. ‘The entire system responds to this; plankton starts to behave differently, which changes the entire ecosystem. Plankton is at the base of the marine food chain,’ says Seys.
A warmer climate (and therefore a warmer North Sea) also means that there will be more storms and that they will be more intense. ‘Although we don't see this reflected in the figures for the Flemish coast just yet,’ emphasises Seys. ‘We are seeing more unstable weather and weather systems that become blocked, which can lead to droughts, long heat waves or water bombs.’
In addition, there is sea level rise. ‘Warm water expands and combined with the melting of land ice, such as glaciers and ice caps in Greenland and Antarctica, sea levels have risen by about 20 centimetres over the past century.’
The North Sea is vulnerable to warming
The warming of the North Sea is not an isolated phenomenon; on the contrary, it is a global phenomenon. According to the UN climate panel IPCC, the rate at which the oceans are warming has more than doubled since 1993. However, it is mainly our North Sea that will feel the consequences.
‘The North Sea is an enclosed sea, with an average depth of only 70 metres. As a result, not only does deep water warm up, but the North Sea also warms up faster than much deeper oceans. In that respect, we are in the “front row of global warming,” Seys warns.
By 2100, simulations predict that the temperature in the North Sea will rise by another 2.8 degrees. ‘And by 2100, in a “business as usual” scenario, we expect sea levels to rise by at least half a metre to one metre,’ Seys adds.
Scientists are now using this study to warn of the consequences of global warming. ‘Even if we were to stop greenhouse gas emissions worldwide immediately, sea levels would probably continue to rise for centuries. It is therefore important to do more to protect the climate,’ they say.