20.86 degrees Celsius: global sea surface temperature has reached a record high for this time of year, according to the European climate service Copernicus. This is bad news, as warmer seawater can lead to more extreme weather events, and marine life underwater is being severely affected – just as we are on land. The Belgian North Sea is also much warmer than usual.
On 21 June, a temperature of 20.86 degrees Celsius was recorded. This broke the previous record – 20.83 degrees Celsius – which had stood since records began. We’re only talking about tenths or even hundredths of a degree, but for such a vast body of water, the differences are inevitably small. However, every fraction of a degree counts.
“We’re talking about global averages here. If you look specifically at the Mediterranean, we’re already talking about local temperature variations of 6 to 8 degrees,” explains Jan Seys of the Flanders Marine Institute (VLIZ). “In the North Sea, too, we’re seeing differences of 3 degrees compared with 30 years ago.”
The measurements relate to the oceans outside the polar regions. The overall average is calculated on the basis of in-situ observations and satellite data.
The new record has been influenced by a temporary weather phenomenon, El Niño, which always pushes up temperatures. But there have been many El Niños in the past, and yet we are still facing a record high. This is because, generally speaking, oceans are simply getting warmer and warmer due to global warming. It is precisely this that is causing scientists concern.
Extra energy for storms
After all, a warmer ocean has major consequences. It provides extra energy for storms, and increased evaporation raises the likelihood of extreme rainfall. Moreover, the atmosphere simply remains warm for longer, which makes it warmer on land.
The consequences underwater we cannot see with the naked eye, but ecosystems are being completely disrupted. Certain species cannot cope with the heat and die, whilst others try to migrate. This can disrupt the entire food chain, and thus also have major consequences for fisheries.
‘We are already seeing plankton dying off almost en masse in the summer. Yet they form a crucial link in the ecosystem,’ says Seys. ‘All other marine life depends on that plankton. As a result, we are already seeing shifts in fish stocks: cod is disappearing from our waters, whilst we are seeing more squid.’
What’s more, there is also the issue of ocean acidification. “Because there is more carbon dioxide (CO₂) in the atmosphere, the ocean is absorbing more of it. This is literally acidifying the system. Half of all underwater life – everything that needs calcium carbonate to build a skeleton or shell – is consequently facing problems.”
A 144-day marine heatwave
Whilst the report focuses on global average sea temperatures, there have already been quite a few marine heatwaves in recent times. We speak of a marine heatwave when sea water temperatures deviate from the averages for at least five days.
There is currently one in the western Mediterranean that experts have labelled ‘extreme’. According to the Spanish research service CEAM, temperatures are 8 degrees above the average for this time of year.
In 2024, the Valencia region was hit by severe flooding that claimed more than 200 lives and caused extensive damage. At the time a natural phenomenon was exacerbated by the warm sea water.
The water off our own North Sea coast is also exceptionally warm, says Jan Seys. “Along our coast, we’ve been experiencing a single, persistent marine heatwave since February. 144 days, to be precise.”
“Between 25 and 27 June, we recorded absolute record highs. The temperature was a full 3.1 degrees higher than normal (compared with the 1982–2011 period). That makes these the warmest three days in 26 years.”
The report that has been presented focuses on an average value for the world’s oceans. But there are regional differences. “The southern North Sea, the Baltic Sea and the Mediterranean are warming up noticeably more,” says Seys. “And that is not good news.”
This warming affects not only marine life, but also people on land. “If the sea is warmer, it is less able to cool the land in summer. The sea’s natural ‘air-conditioning effect’ is lost, causing temperatures on land to rise as well. This leads us into a vicious circle.”
Scientists are sometimes in the dark about this, Seys notes. “For the first time, we’re seeing scientists genuinely baffled by phenomena that even their most advanced computer models couldn’t have predicted. That’s a cause for concern for all of us.”