The Baltic Sea catchment basin covers about 2.1 million km² or 17% of the European continent, and encompasses the territories of 14 countries (clockwise around the Baltic Sea: Denmark, Norway, Sweden, Finland, Russian Federation, Estonia, Latvia, Lithuania, Belarus, Ukraine, Poland, Slovakia, Czech Republic, Germany), with a total population of 85 million.

Climate conditions vary substantially from sub-arctic regimes in northern Scandinavia to moderately humid temperate zones in southern Poland, thus showing high spatial and temporal variability. This makes the region unique among European water basins, creating specific demands on models and scientific concepts.

The Baltic Sea itself is a unique brackish marginal sea with complex hydrography and strongly variable sea-ice conditions. The basin‘s net annual water discharge to the Atlantic Ocean is comparable to major river systems such as the Mississippi and Mackenzie Rivers. Recent floods and devastating storms hitting the basin have increased the public and political awareness of the risks that climate change may imply on Northern Europe.

The Baltic Sea and its rivers are not only a resource for fishery, hydropower and transportation; the region is also an area of increasing importance for tourism, leisure and water sports. In view of recent harmful incidents (e.g. traffic accidents in the Baltic Sea, and disastrous floods like in Sweden and Poland), research dedicated to the entire Baltic Sea basin for the assessment, mitigation of and adaptation to the risk is urgently needed. The application of coupled regional atmosphere/ ocean/ land models developed by the BALTEX science community that are nested into global coupled atmosphere/ocean/land models allow more detailed projections of climate change in the Baltic Sea basin for various scenarios of human activities at the global scale. This input is needed for mitigation measures like the enforcements of the Kyoto Protocol but also for adaptation to possible climate change.

Some countries in the eastern Baltic Sea basin have experienced considerable political, industrial and socio-economic changes in recent years. These rapid changes form a new challenge for the sustainable development of the Baltic Sea region and create growing demands for policy-relevant scientific information. In 2004, Poland, Lithuania, Latvia and Estonia became members of the European Union (EU), and therefore eight EU member states now encircle the Baltic Sea, thus increasing the EU research interest in the BALTEX region.