Mediterranean warming and salinification accelerating even thousands of meters deep

The Mediterranean Sea is warming and getting saltier at an accelerating pace, and the change reaches far below the sunlit surface, according to a new study published in Geophysical Research Letters. Physical oceanographers Elena Terzić and Ivica Vilibić of the Ruđer Bošković Institute in Croatia compiled more than seven decades of temperature and salinity measurements, gathered between 1950 and 2025 by research cruises and robotic profiling floats, and found that the rate of change itself is speeding up across virtually every region of the basin. The acceleration is strongest in the central and eastern Mediterranean, and most pronounced of all in the Adriatic Sea, where it extends all the way to the seafloor.

The finding matters because the Mediterranean is not behaving like a passive body of water that warms slowly and uniformly. Instead, the sea is undergoing a basin-wide transformation whose tempo is increasing decade by decade, a pattern that challenges the assumption of steady, linear change. The upper 100 meters of the Mediterranean have in recent years been roughly two degrees Celsius warmer than their 1950 to 1999 average. Earlier studies had detected hints that surface warming was speeding up, but no one had previously quantified whether that acceleration extended beneath the surface across the entire sea.

The numbers behind the acceleration are striking. In some regions, warming is speeding up by as much as 0.3 degrees Celsius per decade with each passing decade, and each kilogram of seawater is gaining about a tenth of a gram of salt per decade, every decade. Since 2000, Mediterranean surface waters have warmed at about half a degree Celsius per decade, two to three times faster than the global ocean surface. That represents a dramatic shift from the second half of the twentieth century, when most of the basin warmed by less than a tenth of a degree per decade. In other words, the pace of surface warming has multiplied several times over within a single generation of measurements.

What makes the study unusual is the depth to which the acceleration penetrates. The authors report that warming and salinification are statistically significant down to three or four thousand meters, and that the speed-up itself reaches to about 2,500 meters. Deep waters in most seas change slowly, insulated from atmospheric variability by layers of stable density stratification. Detecting a statistically robust acceleration at these depths means the signal of rapid surface change is being actively transported downward, most likely through the formation and sinking of dense water, rather than emerging from slow, gradual diffusion alone.

The mechanism hinges on seawater density, which governs the sea’s deep circulation. Warming makes seawater lighter, while added salt makes it heavier. Because both temperature and salinity are rising simultaneously, the density of Mediterranean water has changed far less than either property individually. Over most of the basin, warming is prevailing, so surface waters are becoming lighter and mixing less readily with the water beneath them. But in the places where dense water forms, above all the Adriatic, the added salt is still keeping surface water heavy enough to sink during cold winter conditions, carrying the new warmth and salinity into the deep interior.

The Adriatic is one of only a few sites in the Mediterranean where cold winter winds cool the surface enough to trigger deep convection. The dense water generated there spreads southward into the central and eastern Mediterranean, and it transports dissolved oxygen that sustains marine life in the deep sea. Since 2000, the deep waters of the southern Adriatic have warmed about six times faster than those of a typical Mediterranean basin. Dense water is still forming, but the water that sinks is now warmer and saltier than it used to be, which means the entire deep reservoir of the eastern Mediterranean is being progressively reset toward a new, hotter, saltier state.

Whether the Adriatic deep-water formation will continue indefinitely is one of the open questions the study raises. If warming eventually outpaces the salinity gain, surface waters could become too buoyant to sink, weakening or shutting down the ventilation that supplies oxygen to the deep sea. The new study did not measure oxygen directly, but warmer water holds less dissolved gas, and oxygen declines have already been documented in parts of the Mediterranean. The ecological implications are compounded by geography: species attempting to escape the warming have nowhere to go, because the sea is closed to the north and the deep water is warming too, leaving few thermal refuges in an increasingly enclosed basin.

The Mediterranean offers an unusually clear preview of how quickly a sea can respond to the heat trapped by greenhouse gases. The world’s oceans have absorbed more than a quarter of humanity’s carbon dioxide emissions and over 90 percent of the excess heat, buffering the climate system from far more severe atmospheric warming. Because Mediterranean waters circulate between the surface and the depths roughly ten times faster than in the open ocean, the basin compresses processes that would take centuries elsewhere into decades. The authors argue that this makes the sea an early indicator of how rapidly larger ocean basins can transform, and a demonstration that predictions assuming steady, linear change systematically underestimate the true speed of ocean response.

Terzić and Vilibić had previously documented unprecedented warming and salinification in the deep southern Adriatic, along with a telling basin-wide signature: pools of unusually salty surface water appearing across the whole Mediterranean, whereas historically such extreme salinity was confined to the sea’s far eastern end. That observation motivated the present study, which set out to quantify the changes across the rest of the basin. The consistent acceleration found everywhere they looked, including at great depth, convinced the authors that they were observing a fundamental reorganization of the sea’s heat and salt budget rather than a regional anomaly.

Several questions now drive the next phase of the research. The authors want to determine what is driving the acceleration, and how much of it comes from the atmosphere, from shifting patterns of evaporation, rainfall and river discharge, and from the Atlantic inflow through the Strait of Gibraltar, which is the Mediterranean’s principal source of replenishing water. They also intend to test whether today’s climate models capture the observed speed-up, a question that determines how much confidence can be placed in the models’ projections for the region. As Terzić notes, the Mediterranean is small enough that scientists can watch a sea respond on timescales they can witness themselves, and what they are seeing is unprecedented change, a warning of how fast the ocean can transform that will persist for as long as economies continue to rely on fossil fuels.

Subject of Research: Accelerated warming and salinification of the Mediterranean Sea from the surface to the deep ocean

Article Title: The Mediterranean is getting warmer and saltier at an ever-faster pace — even thousands of meters below the surface

Article References: The Mediterranean is getting warmer and saltier at an ever-faster pace — even thousands of meters below the surface. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Mediterranean Sea, ocean warming, salinification, deep circulation, Adriatic Sea, dense water formation, oceanography, climate change, Geophysical Research Letters, dissolved oxygen, thermohaline circulation, marine ecosystems