June was the warmest in large parts of Western Europe since the beginning of observations, with two heatwaves in mid and late June. Not only did the land warm up, but the western Mediterranean also reached record values for this time of year.

Heat in June 2025

At the end of June, the average sea water temperature in the area was 27°C, almost 4 degrees above the long-term average. Locally, the deviation was even greater, with temperatures above 28°C in the Gulf of Lion and the Ligurian Sea (image 1). Since the beginning of measurements, it has never been this warm there at the start of summer (image 2). The high temperature of the seawater caused less cooling at night in coastal areas and increased humidity, which increased heat stress for coastal residents. The high seawater temperatures are also detrimental to marine life, among other things due to lower oxygen levels in the water.

Impact of heatwave at sea much greater than heatwave on land

Due to climate change, the temperature of seas and oceans is rising. The temperature of the Mediterranean has already increased by 1.7°C since 1980, nearly double the average temperature increase of all seas and oceans combined (image 3). This is due to a gradual increase in temperature, but also to an increase in the number of heatwaves at sea: temporarily very high seawater temperatures. These heatwaves occur when a high-pressure area lies over the region for an extended period, causing high air temperatures, plenty of sunshine, and little wind. Heatwaves at sea last longer than those on land. Water cools down more slowly than air; it can take weeks to months for seawater warmed by a heatwave to cool down again. The impact of a heatwave at sea is therefore much greater than that of a heatwave on land.