Climate change combined with pollution from farming and forestry could flip northern Europe's Baltic Sea from being a sponge for CO2 to a source of the planet-warming gas, scientists studying told AFP.
This should be a red flag, they warned, noting that other coastal marine zones around the world are trending in the same direction.
"We are at the forefront of these changes," said University of Helsinki professor Alf Norkko.
The Baltic Sea—connected to the Atlantic by the straights of Denmark, and surrounded by Germany, Poland, Finland, Sweden and the Baltic states—has warmed at twice the pace of global oceans generally.
Its relatively shallow waters are extremely sensitive to changes in the environment and climate.
AFP recently accompanied Norkko, who leads the largest marine research station in the Baltic Sea, and some of his colleagues on a research excursion to the Finnish waterfront town of Hanko.
Slender terns dart above the lush marsh-like landscape surrounding the over 120-year-old field station, a common sight along Finland's 1,100-kilometer (680-mile) coastline, which is dotted by more than 81,000 islands.
Measurements conducted since 1926 show that average sea temperature have spiked by two degrees Celsius over the last 30 years.
"The Baltic Sea is basically a small bathtub compared to the global oceans," said doctoral researcher Norman Gobeler, an expert on marine heat waves.
"We are seeing the first effects of the temperature increase."
Linking marine ecosystems to climate change
Scientists warn the Baltic Sea could become a source of planet-warming carbon dioxide gas.