Even the oceans are struggling to keep their cool

On Saturday, 22 August, global ocean surface temperatures hit the highest ever recorded. In Europe, mid-August these temperatures reached around 6°C above average in the western Mediterranean Sea and above 5°C in the Atlantic off the Iberian Peninsula, according to Copernicus Marine Service data. Marine heatwaves across Europe have grown more widespread over the past decades. The share of our waters experiencing marine heatwaves has increased from 40% in the 1980s to 98% in 2025. The current situation is “very serious”, according to Zoe Jacobs, a senior researcher at the National Oceanography Centre in the United Kingdom. Jacobs is an expert in marine heatwaves in the UK and surrounding North Atlantic. “We are experiencing the longest and most intense marine heatwave that we have ever had,” she said. The past few years have seen a sharp rise in the most intense heatwaves, with sea temperatures far above the normal range for a given location and season. This development is a direct consequence of climate change. According to experts from the World Weather Attribution, an organisation researching the impact of climate change on extreme weather events, the warmest two weeks this year would have been “virtually impossible” in a preindustrial climate in three of the four European sea regions studied in their recent analysis. The oceans absorb around 89% of the Earth's excess heat, so they act as a crucial climate buffer. But as the oceans absorb more heat, they also face the consequences. Warmer oceans cause warmer air temperatures and higher humidity along coastlines, intensifying heat stress on land and feeding more moisture into the atmosphere that is later released as extreme rainfall. In the Mediterranean, high sea surface temperatures have been linked to devastating downpours, including the 2024 Valencia floods. Globally, elevated ocean heat can also intensify tropical cyclones and disrupt monsoon rainfall patterns. Below the surface, rising sea temperatures can damage entire marine species that form habitats such as corals, sponges, and macroalgae, while other cold water species risk being pushed out of their habitats. Chemically, rising sea temperatures can limit the ocean's ability to absorb carbon dioxide from the atmosphere. Warmer water is also less able to hold oxygen, a bleak outlook for Europe's already suffocating oceans. Contrary to land heatwaves, marine heatwaves happen all year round. They are defined by how far sea temperatures rise above what is considered normal for a particular place and time of year. Jacobs notes that marine environments are affected in different ways during different seasons compared to on-land heatwaves. According to Jacobs, we can't prevent the heatwaves from happening, but we can limit their intensity by trying to slow down global warming. “Reducing reliance on fossil fuels is the key overarching message,” she said.