Across the Mediterranean and at depths of thousands of meters, the sea is becoming warmer and saltier at an increasingly faster rate. The acceleration is greater in the central and eastern Mediterranean and particularly pronounced in the Adriatic, where it reaches all the way to the seafloor. This was the finding of a study published in the American Geophysical Union’s journal Geophysical Research Letters.

In recent years, the upper 100 meters of the Mediterranean have been approximately two degrees Celsius warmer than the average for the 1950–1999 period. Previous studies had identified indications that surface warming was accelerating, but it had not been measured whether this change was also accelerating below the surface and whether this was occurring throughout the Mediterranean.

Researchers from Croatia’s Ruđer Bošković Institute had recently recorded an unprecedented rise in temperature and salinity in the deep waters of the southern Adriatic, as well as indications of changes across the Mediterranean: areas of unusually salty surface water throughout the Mediterranean, which historically had been observed only in its easternmost part.

To further investigate these indications, the same researchers compiled temperature and salinity measurements from research expeditions across the Mediterranean for the period from 1950 to 2025.

They found that temperature and salinity are increasing almost everywhere, and that these increases are accelerating. In some areas, temperatures are rising by as much as 0.3 degrees Celsius per decade, while the water is becoming saltier by approximately one-tenth of a gram per kilogram of water every decade.

Differences in seawater density are what drive the deep circulation of the oceans. Warming makes seawater lighter, while the addition of salt makes it heavier. Because both changes are increasing, the density of Mediterranean waters has changed far less than either their temperature or salinity.

The researchers also found that since 2000, the deep waters of the southern Adriatic have warmed approximately six times faster than those of a typical Mediterranean basin. As long as the increase in salinity outweighs the effect of warming, the water continues to sink, now carrying this heat into the depths of the Mediterranean.

The new study did not measure oxygen levels, but warmer water can hold less oxygen, while declines in oxygen levels have already been recorded in parts of the Mediterranean. At the same time, marine species seeking to escape rising temperatures have fewer and fewer refuges available to them, as deep waters are also warming.

The oceans have absorbed more than a quarter of the carbon dioxide emissions produced by human activity and more than 90% of the excess heat trapped by greenhouse gases. In doing so, they keep Earth’s climate milder than it would otherwise be.

The Mediterranean shows how quickly a sea can respond to this additional burden. Its waters circulate between the depths and the surface approximately ten times faster than those of the open oceans. For this reason, according to the researchers, the Mediterranean provides an early indication of how quickly larger ocean basins could also change as a result of global warming.