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75 Years of Measurement Data Show an Acceleration That No One Had Anticipated
It has long been known that the Mediterranean Sea is warming. However, a new study published in *Geophysical Research Letters* now paints a far more alarming picture: Warming and salinization are accelerating not only at the surface but also extending to depths of several thousand meters.
The process may be proceeding faster than current climate models predict - a question the researchers are now specifically investigating.
75 Years of Measurement Data from a Unique Sea
Elena Terzić and Ivica Vilibić from the Ruđer Bošković Institute in Croatia analyzed temperature and salinity measurements from research vessels and autonomous Argo buoys spanning the years 1950 to 2025 for their study. This represents a continuous dataset spanning 75 years, the likes of which exist for hardly any other marine region. “We had already observed significant changes in our previous studies, both in the deep Adriatic and at the ocean surface, and wanted to quantify the changes for the rest of the Mediterranean,” explains Terzić in a press release from the American Geophysical Union (AGU).
The upper 100 meters of the Mediterranean Sea are now about two degrees Celsius above the average for the years 1950 to 1999. Since the turn of the millennium, the Mediterranean’s sea surface has been warming at an average rate of about 0.5 degrees per decade - two to three times as fast as the global ocean average.
Furthermore, this warming is not occurring uniformly but is actually accelerating from decade to decade. This acceleration varies regionally and is most pronounced at the surface in the Adriatic Sea and the northern Ionian Sea.
If the change continues all the way to the seafloor
Particularly noteworthy: An acceleration can also be detected at depth, not just at the surface. Additionally, at depths of 1,000 to 2,000 meters, a warming rate of up to 0.3 degrees per decade is observed, and salinity is increasing at a similar rate - by about one-tenth of a gram per kilogram of water per decade.
Statistically significant changes can be detected down to depths of 3,000 to 4,000 meters, while the actual acceleration extends down to about 2,500 meters. “We were alarmed by the speed at which this is changing and how deep these significant changes extend. Warming and salinization are statistically significant down to depths of three to four thousand meters, and the acceleration itself extends down to about 2,500 meters,” said Terzić.
The Adriatic Sea as an Extreme Case
The trend is most pronounced in the Adriatic Sea. Since 2000, the deep waters of the southern Adriatic have warmed about six times faster than a typical Mediterranean basin - there, warming and salinization even extend all the way to the seafloor. The reason: winter deep-water formation in the southern Adriatic depends on the inflow of particularly salty water from the eastern Mediterranean.
This salt influx has increased significantly in recent years and makes the water dense enough to continue sinking to the bottom despite the warming. This is a counterbalancing effect that distinguishes the Adriatic from most other Mediterranean regions, where warming is increasingly stabilizing the water layers and thus hindering mixing with deeper layers.
An Early Warning System for the World’s Oceans
The researchers also view the Mediterranean as a model system for the global oceans. “The Mediterranean is small enough that we can observe how a sea reacts on timescales we can witness ourselves, and what we’re seeing is an unprecedented transformation,” Terzić said, according to phys.org. “It’s a warning about how quickly the ocean can change - and this will continue as long as our economies remain dependent on fossil fuels.” It remains unclear exactly which factors are driving this acceleration; the researchers now plan to more precisely disentangle the respective contributions of the atmosphere, evaporation, precipitation, river inflows, and water exchange with the Atlantic through the Strait of Gibraltar - in part to verify whether existing climate models accurately reflect the actual pace of these changes.
A more complex picture emerges regarding the density of Mediterranean water, which is a key driver of deep-water circulation: warmer water becomes lighter, while saltier water becomes heavier - since both effects are increasing simultaneously, the overall density has changed significantly less than temperature or salinity would on their own. However, this does not mean the all-clear has been given; rather, it shifts the problem: In most regions, the warming effect now dominates, making the water layers more stable and harder to mix - with potential consequences for nutrient cycles and oxygen supply at depth.
More information / Sources:
AGU Newsroom
Geophysical Research Letters (Terzić & Vilibić 2026)
Courthouse News Service
phys.org
Total Croatia News