Climate change and river runoff in Scandinavia, approaches and challenges

Two main approaches dominate the studies of a possible temperature rise impact on river runoff: analyses of historical hydrological and climatological observation series and modelling of the river flow under an assumption of a predetermined temperature rise. The historical climatic and hydrological series provide a perfect tool to depict a variety of environmental situations, liable to correspond to a certain temperature change, i.e. produce synthetic climatic and hydrologic pictures on the realistic data available for the period with such climatic change. An application of this approach in Scandinavia has shown that already a small rise in the mean annual temperature of ~ 1°C, can cause notable changes in the river flow regime. Application of this approach is, however, often limited by the scarcity of long climatological and hydrological series, representing different environments. Another limitation is a poor knowledge about the influence of low frequency components (trends) of several decades or more, which might have a significant influence on the development of both climatological and hydrological series.The modelling approach can produce series of any desired length. However, the results of model studies heavily depend on the accuracy and relevance of the used model and climate scenarios. The modelling approach in the impact studies of climate change has two types of uncertainties. The first one concerns uncertainty in the climate model simulations, especially on a regional scale. Validation of the results of these models at present faces some serious obstacles, especially what concerns relevant hydrological “ground truth”. The second type concerns hydrological models used for forecasting of effects of climate change on river runoff. The paper offers a review of some important findings for a regionalized Scandinavian data set and discusses a number of essential problems and challenges connected to the studies of hydrological impacts of a possible climate change.