The Baltic Sea last froze over completely in 1947, when an unusually severe winter covered it in ice from Poland and Germany in the south to Finland and Sweden in the north. Since then, winter ice cover has dropped sharply, and the latest record low came in 2020, when only 37,000 square kilometers of the sea, less than a tenth of its 420,000-square-kilometer surface, were frozen. The maximum winter ice extent has declined by about 30% over a little more than a century, and the harsh winters that once brought widespread ice have not occurred at all in the past 20 years.
Scientists say the sea is warming faster than the oceans and faster than other nearby seas such as the Mediterranean and Red Sea, making it a natural laboratory for studying climate change. The Baltic is shallow, almost landlocked, and linked to the Atlantic only through the Danish straits. It borders Denmark, Sweden, Germany, Poland, Lithuania, Latvia, Russia, Estonia and Finland. Its unusually low salinity, about 0.7% to 0.8% on average, and even below 0.2% in the far north, creates layered waters that limit mixing and help trap heat. In summer, thermal stratification has expanded from about three months in 1991 to five months today, and the surface has reached temperatures up to 10 degrees above the seasonal average during heat waves.
These changes damage ecosystems and human activity alike. Ringed seals need ice to breed, since females give birth in ice burrows and pups may not survive if the ice melts too early. The sea is also a major shipping route, a tourism center, and a fishing ground, while offshore wind power is expanding from 2 gigawatts in 2021 toward planned targets of 22.5 gigawatts by 2030 and nearly 47 gigawatts by 2050. But milder winters can also make navigation more dangerous because thawing and refreezing create unpredictable ice floes. Warmer water, stronger layering, and more runoff feed dead zones with little oxygen, which covered about 70,000 square kilometers, or 18% of the sea, in 2016, and have hurt commercial fish such as cod.
The article says the Baltic also shows how climate policy can work. The countries around it, together with the European Union and international bodies, have spent years cutting pollution, limiting fertilizer use, and curbing overfishing. The 1974 Helsinki Convention, updated in 1992, was the world’s first regional marine environmental treaty and led to the creation of HELCOM, which coordinates protection efforts. Researchers argue the sea demonstrates that science-based management can slow degradation and sometimes reverse it, even in heavily stressed marine systems.
Rising sea levels pose another threat, especially in the south where land is not rebounding from the last ice age. The north, including parts of Finland and Sweden, is still rising by as much as 8 to 9 millimeters a year in the Bay of Bothnia, but southern areas face stronger erosion, flooding risk, and the loss of wetlands and breeding sites. Low-lying areas near Gdansk and the Hel Peninsula are among the places at risk, and some governments are planning barriers, dams and revised coastal construction rules.
