By Washington Post
The most extreme heat event on the planet right now is happening not on land but at sea.
A prolonged, record-smashing marine heat wave is scorching the Mediterranean, where water temperatures have hit levels unprecedented for the early summer. The Mediterranean鈥檚 average temperature is currently 26 degrees Celsius (78.8 degrees Fahrenheit), compared with a long-term average of 23 degrees Celsius (73.4 degrees Fahrenheit) at this time of year.
On a gradient map showing ocean temperature anomalies, the Mediterranean is a deep, blazing red.
In certain parts of the sea, particularly the western basin around Spain, France and Italy, temperatures are more than 7 degrees Celsius (12.6 degrees Fahrenheit) above the average.
鈥淔or large water bodies that鈥檚 ludicrous,鈥 Jeff Berardelli, a U.S. meteorologist, said on social media.
For most people, the event might not seem as obvious as the brutal heat wave hitting the region on land. Europe this week baked under an intense heat dome that led the Eiffel Tower to close its summit. But elevated marine temperatures can have extensive and dangerous consequences.
They can raise temperatures and humidity in coastal communities. They can provide extra moisture to the atmosphere, fueling stronger storms. And they can upend ecosystems, bleaching coral and triggering mass mortality events of certain species 鈥 such as sea grasses and sponges 鈥 that cannot escape to cooler waters.
鈥淭his particular event is a very concerning one,鈥 said Karina von Schuckmann, a senior adviser on ocean science for policy at Mercator Ocean International. She described the ocean as the 鈥渟entinel鈥 for planetary warming because it absorbs most of the excess energy generated linked to greenhouse gas emissions.
The Mediterranean as a hotspot
Marine heat waves are marked by long-lasting, expansive areas of well-above-average ocean temperatures, and are increasing in frequency and intensity amid a warming climate.
The Mediterranean 鈥 semi-enclosed and in one of the world鈥檚 fastest-warming regions 鈥 has become particularly prone to heat waves. The U.N. Intergovernmental Panel on Climate Change (IPCC) has described the Mediterranean as a 鈥渉otspot鈥 for global climate risks. A 2023 study said that Mediterranean marine heat waves have grown in intensity, duration and frequency, 鈥渆specially in the last decade.鈥
The Mediterranean typically hits its warmest point deep in the summer, around Aug. 18, topping out on average at 26.1 Celsius (79 Fahrenheit). Its all-time warmest temperature came last year, on Aug. 13, hitting 28.5 (83 Fahrenheit). But last year, in early July, the Mediterranean was 24 Celsius (75.2 Fahrenheit) 鈥 2 degrees Celsius (3.6 Fahrenheit) cooler than it is now.
A buoy near the island of Mallorca, Spain, reported a water temperature of 31 degrees Celsius (87.8 degrees Fahrenheit) this week, while water offshore of Venice and Sanremo, Italy, was around 30 degrees Celsius (86 degrees Fahrenheit), according to satellite estimates.
A severe (Level 3 out of 5) marine heat wave is affecting ocean areas between Spain and Sardegna, currently qualifying as one of the most intense on the planet. In recent days, the heat wave intensified to extreme (Level 4 out of 5) levels in the Alboran Sea, south of Spain, as well as in the Gulf of Lion, south of France.
What drives heat in the oceans
For a marine heat wave to form and intensify, several atmospheric and oceanic drivers need to converge.
During June, a powerful high-pressure system, spanning the lower and middle atmosphere, has been anchored over southern Europe and the Mediterranean Sea.
High-pressure systems cause air to sink, compress and warm. This particular dome of high pressure drew unusually warm air from North Africa into Europe, further increasing the level of heat it could produce.
Underneath the system, winds were light, and sunshine was plentiful 鈥 preventing cooler, subsurface seawater from churning to the surface.
Long-term warming of planet and its oceans, including a record-breaking year globally in 2024, is raising the bar on the potential intensity and duration of marine heat waves.
The convergence of natural and human-influenced factors is probably intensifying the heat wave in the Mediterranean, a region that saw consecutive years of record-breaking ocean warmth in 2023 and 2024.
A significant change in weather patterns, such as stronger winds and cooler air temperatures, is needed for this marine heat wave to ease. With another serious heat wave possible in mid-July, the Mediterranean heat is likely to last at least a few more weeks.