The last ice age ended about 11,000 years ago. Since then, the Nordic waters have developed within a stable, temperate climate with four annual seasons. The climate in Scandinavia is relatively mild, partly because the Gulf Stream brings it warm water but also the prevailing winds from Western Europe and the Atlantic bringing in warm, moist air. Just as in tropical areas the water circulates from fresh to salty. The rains in the mountains fill the rivers flowing into the Baltic Sea, which in turn empties into the North Sea.
The Baltic Sea is the world's largest brackish waterbody and salinity varies greatly from 20- 25 parts per thousand (ppt) at the Danish inlets from the North Sea to only 3-4 ppt. at the top of the Bay of Bothnia. This is also a shallow sea (average depth is only 55 meters), with a low flow of water, a low water exchange and relatively few species, making it a particularly sensitive and fragile ecosystem. Calculations show that it takes about 25-30 years for the waters of the Baltic Sea to be replaced with new water; this means that pollutions affecting the Baltic will remain for a long time.