| Summary: | Climate has always attracted considerable interest, and climate observations have been made in various ways for most of human history. Regional climate and how it varies is of particular interest, as it sets the scene for our everyday life. This thesis analyses the past climate of the Baltic Sea Basin and relates ice coverage and river runoff to changes in atmospheric circulation. The regional climate of the Baltic Sea Basin has been analysed using relevant climatic time series for the past 100–500 years. The time series used in the thesis describe parameters such as station-based and gridded air temperature, sea level, ice cover extent, river ice break-up dates, and river runoff. To describe the atmospheric circulation over the area, gridded sea level pressure data have been used to construct time series describing the occurrence of high and low-pressure systems as well as westerly and northerly winds. The definition of climate was analysed and a proper climate averaging time was found to be 15 years, corresponding to a loss of variability of 90 %. The analysis used annual averages and revealed positive trends in high-pressure activity and air temperature, possibly indicating a north-ward shift of the low-pressure tracks. The winter climate of the past five centuries was examined through a comprehensive analysis of the longest time series, describing winter severity, available for the Baltic Sea Basin. The covariation of several climatic variables was examined using new statistical techniques. Over the last 500 years, 15 time periods stood out, giving a climatic imprint with respect to winter severity, circulation patterns, and interannual variability. Both warm and cold periods were identified in the past Baltic Sea climate; their onset was probably caused by perturbations of the system, although correspondences with solar and volcanic activity can be identified for certain of them. On the interannual timescale, describing year-to-year variability, warm periods are associated with less variability while cold ...
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