At a depth of 1,000 meters in the South Adriatic Pit, seawater temperature has risen by 0.8°C over the past decade, while salinity has increased by 0.2 PSU (Practical Salinity Units). These trends represent the fastest-recorded changes in deep Mediterranean waters and may signal a permanent shift in the region’s climate regime.

The study, led by Dr. Elena Terzić and Dr. Ivica Vilibić of the Ruđer Bošković Institute and published in Limnology and Oceanography Letters, is the result of collaboration between Croatian, Italian, and Slovenian scientists. They combined data from a variety of oceanographic platforms, including permanent moorings, research cruises, and autonomous Argo floats.

Europe’s Marine Thermostat Failing

The Adriatic Sea can be imagined as the Mediterranean’s natural “thermostat,” as it regulates temperature patterns throughout the basin via heat exchange. How does this thermostat work? In winter, cold and dense water forms in the shallow northern Adriatic, sinks to the seafloor, and flows through the Strait of Otranto into the deep Mediterranean. For centuries, this process has fueled a circulation system that ensures stability and oxygen supply to the depths of the Mediterranean.

Today, however, this natural system is changing. The traditionally cold water formed along the Croatian coast is becoming increasingly warmer and saltier before it begins its journey south. Instead of cooling the deep Mediterranean, the Adriatic now sends warmer waters that further heat the entire system.

“You can think of it as heating a house with a broken thermostat,” explains Dr. Vilibić. “Instead of keeping the temperature stable, the thermostat is now pumping more and more heat into the system.”

The main drivers of these changes are a combination of global warming and shifts in rainfall and river inflows. Rising air temperatures mean that less cold water forms at the surface, while altered precipitation and river inputs make the northern Adriatic saltier.

Acceleration Over Time

What is particularly concerning is that this trend is accelerating. While deep-water warming rates were once around 0.2°C per century, between 2012 and 2024 they jumped to 0.8°C per decade.

In the early 2010s, the layer of water with a constant temperature of 14°C was found at a depth of about 100 meters. By 2024, that imaginary line—known to scientists as an isotherm—had sunk all the way to the bottom of the South Adriatic Pit. At the same time, salinity above 38.9 PSU has become a near-permanent feature of this deepest part of the Adriatic, meaning that the deep waters are now saltier than at any point since measurements began.

“What especially worries us is the fact that the changes are accelerating,” says Dr. Vilibić. “This is not a gradual adjustment but a rapid transition to a new state that we do not yet fully understand.”

Ecological Consequences

Changes in deep waters are already affecting marine life. The Jabuka Pit, known for its rich biodiversity and protected as a no-fishing zone, is particularly vulnerable. Species adapted to cold conditions that have traditionally taken refuge in deep, cool waters now face the rapid transformation of their habitat.

“Species adapted to cold deep waters have nowhere to go,” Dr. Vilibić explains. “Unlike terrestrial animals, which can move northward as the climate warms, deep-sea species are trapped at the bottom of the ocean. At the same time, warmer waters are encouraging the arrival of tropical species from the eastern Mediterranean, transforming the entire ecosystem.”

But these changes are not just a local Adriatic phenomenon—they could affect the entire European climate system. Shifts in deep-water density influence sea-level rise along the European Mediterranean coast. Scientists estimate that current trends could contribute an additional 3.3 millimeters of sea-level rise per year.

Likewise, changes in ocean currents may alter weather patterns, the Mediterranean climate system, and coastal communities across Europe. The interdisciplinary nature of the international team enabled a comprehensive perspective on a problem that transcends national borders. The study clearly demonstrates that changes in the Adriatic are not just a regional phenomenon but a European challenge requiring a coordinated response.

Looking Ahead

Scientists warn that current climate models may be underestimating the speed of change in enclosed marine basins such as the Mediterranean. Projections for the period 2075–2100 anticipate changes that are now unfolding within just 10–20 years.

Researchers at the Ruđer Bošković Institute continue to monitor the state of the deep Adriatic through a network of permanent measurement stations and international collaborations with partners from Italy, Slovenia, and other Mediterranean countries. These measurements and studies are made possible through numerous international and national initiatives, programs, and projects, including the European research infrastructure consortia Euro-ARGO ERIC and EMSO ERIC, regional cooperation projects within the Interreg Italy–Croatia program (AdriaClimPlus), as well as national research projects (GLOMETS, C3PO) and mobility programs funded by the Croatian Science Foundation and research agencies in Slovenia and Italy.

The next step will be to develop improved climate models that can more accurately predict the pace of change in enclosed marine systems.

“Nature is warning us that something significant is happening,” concludes Dr. Vilibić. “Our task is to understand these processes and help society prepare for the changes ahead.”