The Mediterranean Sea has been warming and turning saltier for decades. New measurements show that change is now speeding up. And the speed-up reaches thousands of feet down, not only at the surface.

Elena Terzić, a physical oceanographer at Croatia’s Ruđer Bošković Institute, led the work with her colleague Ivica Vilibić.

The team pulled together 75 years of temperature and salinity readings, gathered by research ships and robotic floats scattered across the whole sea, from 1950 to 2025.

“What is really surprising is that we see significant changes in every region, reaching great depths,” Terzić said.

Surface warming was already clear

In recent years, the Mediterranean’s upper 330 feet (100 meters) have run about 3.6 degrees Fahrenheit (2 degrees Celsius) warmer than they were between 1950 and 1999, on average.

The sea has also lived through severe marine heatwaves in recent years, and scientists already suspected the underlying surface warming was speeding up.

No one had measured whether that speed-up reached farther down, across the whole basin.

Terzić and Vilibić had already found unusually fast warming in the deep waters of the southern Adriatic.

They had also spotted patches of unusually salty surface water spreading across the sea that used to show up only at its far eastern end.

Those earlier finds pushed the pair to check the rest of the Mediterranean too.

The pace itself is now climbing

The answer turned out to be yes, by a wide margin. In some parts of the sea, warming is speeding up by close to 0.3 degrees Celsius per decade per decade.

Salt levels are rising the same way, climbing by about a tenth of a gram for every 2.2 pounds of seawater each decade.

Since 2000, the surface has warmed by roughly 0.9 degrees Fahrenheit (0.5 degrees Celsius) per decade, two to three times faster than the world’s ocean surface as a whole.

That is a sharp jump from the second half of the last century, when most of the basin warmed by less than 0.2 degrees Fahrenheit (0.1 degrees Celsius) per decade.

The changes are not confined to a thin surface layer, either. The signal is strong enough to rule out chance down to roughly 9,800 to 13,100 feet (three to four thousand meters).

The speed-up itself keeps going to about 8,200 feet (2,500 meters).

Warm water floats, salty water sinks

What happens next comes down to density, and density depends on both heat and salt. Warm water is lighter. Salty water is heavier.

Because both are rising at once, their opposing effects on density can partly offset each other, even as the water grows warmer and saltier.

Across most of the basin, warmth is winning that contest. The surface is growing lighter, so it mixes less easily with the water below it.

But in the few places where dense water actually forms and sinks, the extra salt is still winning instead. The Adriatic is the clearest example, and its water keeps sinking anyway.

The Adriatic still pumps water downward

Cold winter winds chill the Adriatic’s surface water enough to make it sink.

That sunken water then spreads into the central and eastern Mediterranean, carrying oxygen down into the deep sea. It is one of the few places in the whole basin where this still happens.

But the water making that trip has changed. Since 2000, the deep waters of the southern Adriatic have warmed about six times faster than a typical patch of the Mediterranean.

Salt is still winning out over heat for now, which is why the water still sinks at all.

“Dense water is still forming, but we have indications that the way it forms is changing, and the water that sinks is now warmer and saltier than it used to be,” Terzić said.

Warm water holds less dissolved oxygen, and oxygen levels have already been dropping in other parts of the Mediterranean, though this study did not measure oxygen directly.

Similar heat-driven die-offs have already hit other marine life in the region.

Species trying to escape the heat have fewer places left to go. The sea is closed off to the north, and now even its deep water is warming.

Mediterranean changes offer a global warning

The world’s oceans have absorbed more than a quarter of humanity’s carbon dioxide. They have also absorbed over 90% of the extra heat trapped by greenhouse gases, one reason ocean heat keeps setting records.

The Mediterranean does the same job, just faster: its water cycles between surface and depths about ten times faster than in the open ocean.

That speed makes the Mediterranean a useful warning sign. Its rapid transformation could offer clues about how quickly larger ocean basins might change.

It also suggests that a steady, straight-line forecast can undersell how fast things actually move.

Searching for the cause

No one yet knows how much of the speed-up comes from the atmosphere. Nor is it clear how much comes from shifting evaporation, rain and river patterns, or from Atlantic water flowing in through the Strait of Gibraltar.

Terzić and Vilibić want to work that out next.

They also want to check whether today’s climate models can even reproduce the speed-up, which will help determine how much confidence researchers can place in projections for the region.

“The Mediterranean is small enough that we can watch how a sea responds, on timescales we can witness ourselves, and what we see is unprecedented change,” Terzić said.

“It is a warning of how fast the ocean can change, and it will continue for as long as our economies keep relying on fossil fuels.”

The full study was published in the journal Geophysical Research Letters.

NOTE – This article was originally published in Earth and can be viewed here