The Baltic Sea is one of the largest brackish water bodies in the world. It is an especially sensitive sea, because it is a shallow and semi-enclosed body of water that receives a considerable load of pollutants from the surrounding countries. The pollution of the sea has become one of the most important common environmental issues for countries in Northern Europe. Today's environmental problems are, however, the collective result of political decisions made in the past. Hence there is an evident need to understand the current environmental changes of the Baltic Sea from historical perspectives.