 "People walk at Trocadero plaza near Eiffel Tower during a heat wave, July 2, 2025 in Paris. AP")
Europe is reeling from searing temperatures as a marine heatwave in the Mediterranean Sea, along with a heat dome, has sent mercury soaring. Spain and England recorded their hottest June since records began. Germany experienced its hottest day of the year on Wednesday (July 2), with temperatures set to touch 40 degrees Celsius.
France has issued a red alert, the highest extreme heat warning, for 16 départements, including Paris. The top of the Eiffel Tower remained closed on Wednesday, with staff urging people to take precautions amid the ongoing heatwave.
Hot temperatures have also scorched the United Kingdom, which is reporting its second heatwave of the summer. Spain and France have also reported casualties as temperatures shoot up.
But what is the marine heatwave that has led to red alerts in parts of Europe? We will explain.
What is a marine heatwave?
Marine heatwaves (MHWs) are prolonged periods of abnormally high ocean temperatures. As the planet warms, the oceans are absorbing excess heat trapped in the atmosphere from greenhouse gases.
A marine heatwave occurs when the surface temperature of the sea rises to 3 or 4 degrees Celsius above the average temperature for at least five days.
According to the US National Oceanic and Atmospheric Administration (NOAA), these heatwaves can last for weeks, months or even years.
The frequency of the phenomenon has nearly doubled since the 1980s.
According to the 2021 report by the International Union for Conservation of Nature (IUCN), marine heatwaves have surged by 50 per cent over the past 10 years and have become more intense.“MHWs can last for weeks or even years. They can affect small areas of coastline or span multiple oceans. MHWs have been recorded in surface and deep waters, across all latitudes, and in all types of marine ecosystems,” the report said.
What’s driving the surge in marine heatwaves?
Human activities are causing an increase in the frequency of marine heatwaves. Burning fossil fuels like oil, coal and natural gas releases greenhouse gases into the atmosphere. These gases trap heat, leading to global warming and climate change.
A recent study published in Proceedings of the National Academy of Sciences found that climate change is to blame for the majority of marine heatwaves in recent decades.
With global temperatures soaring 1.3 degrees Celsius above pre-industrial levels, the ocean has absorbed 90 per cent of the excess heat. The ripple effects are also being seen on land.
Impact of marine heatwaves
Marine heatwaves are hurting marine life. Ocean warming has affected coral reefs, which bleach and die when surface temperatures are too high.
About 84 per cent of reefs globally witnessed bleaching-level heat stress between January 2023 and March 2025, New York Times (NYT) reported, citing a recent report.
As several marine animals rely on coral reefs for survival, any damage to corals could also threaten their existence.
Excess warming of the ocean also impacts weather patterns, making hurricanes likely to be more severe and destructive.
Marine heatwaves are expected to continue to surge due to climate change. Moreover, “predicted increases in the frequency, duration and severity of marine heatwaves over the next century have the potential to disrupt marine life, from individual animals to whole ecosystems, as well as the important services that they provide, such as fisheries and aquaculture,” as per a University of Plymouth article.
Marine heatwave in Mediterranean
The Mediterranean Sea has been regularly warming since the 1980s, with a significant increase over the past decade.
The Mediterranean marine heatwaves have become more noticeable in recent summers. The ocean heat is extending to land, triggering a spike in temperatures. It has also led to deadly floods and destructive fires.
Sea surface temperatures in the Mediterranean have touched their highest level for June. Amid a marine heatwave, water temperatures in the Mediterranean Sea have risen nine degrees above average for this time of year.
The Mediterranean is warming faster than the global average. If global warming remains unchecked, the region could change permanently beyond recognition.
With inputs from agencies