Higher temperatures are likely to mean more rain and snow in the Baltic region, from Copenhagen to St. Petersburg and where 85 million people live. That might make the sea ever less salty and add to a polluting runoff of fertilisers from farmland.

"A tendency towards lower salinity could be expected, which is thought to have a major influence on the Baltic Sea fauna," scientists in the Baltex Assessment of Climate Change of the Baltic Sea Region said.

Many stocks of fish are already living on the edge of their ranges in the brackish Baltic Sea and lower salinity would further cut survival rates of fish larvae. Cod, sprat and herring are among Baltic Sea fish.

The Baltex study reflects a recent trend of trying to pinpoint risks of global warming for regions, rather than the entire planet. Most scientists say a build-up of heat-trapping gases in the air from burning fossil fuels is warming the world.

Decades of pollution, largely from the former Soviet Union, mean that concentrations of poisons ranging from dioxins to cadmium are far higher in the almost enclosed Baltic Sea than in more open seas or in the oceans.

The Baltic Sea is open to the North Sea only by straits between Denmark and Sweden, and it takes decades to renew its stagnant waters. The Baltic Sea is also bordered by Germany, Poland, Russia, Lithuania, Latvia, Estonia and Finland.

Von Storch said the already low salinity meant the Baltic was in many ways a 1,600 km (1,000 mile) long estuary, gathering rivers such as the Torne, Vistula and Oder, rather than a sea.

ICE-CLOGGED

Still, a projected rise in temperatures may bring benefits by making Russian and Finnish ports less clogged by ice in winter. Warmer seas could threaten some species of seal which depend on ice but help species such as porpoises.

And warmer temperatures could extend the range and growing seasons for pine and birch forests, especially in the north.

The scientists said global warming would mean more snow and rain in the region in winter, with drier summers in the south. More precipitation would bring more fresh water from rivers into the sea, formed about 10,000 years ago after the last Ice Age.

But there were many uncertainties. The Baltic gets saltier in a complex exchange when storms blow North Sea waters into the Baltic, immediately after winds in the opposite direction have driven brackish waters out.

"We have no idea as to whether these conditions will become more or less frequent" with climate change, von Storch said.

And the northern part of the Baltic will escape one widely predicted damaging impact of global warming -- rising sea levels that could swamp many coasts and low-lying Pacific islands.