| Summary: | A multi-proxy data set from a sediment core from the Central Gotland Basin shows an extreme peak in many variables in a depth of 70-80 cm. Dating suggests an age of 700-790 varve years (before 1950) which fits into the Medieval Warm Period (MWP) from 1100-1250 AD. The variables are used as regional climate proxies for the Baltic Sea to reconstruct the biological variability during this period, to specify the necessary climate condition for the biological response, and to reconstruct in detail the climate during the MWP in the Baltic Sea area. The reconstruction of the climate conditions from sediment proxies and other observations indicates two regime shifts during the MWP from cold winters and cool summers ~1150 to cold winters and hot summers for approximately 30 to 40 years followed by a period of warm winters and hot summers ~1190. Using various climate data sets from the 20th century, composites of high and low North Atlantic Oscillation (NAO) indices are constructed for winter and summer situations to understand the regional influence and response of the Baltic Sea to longer lasting regime shifts in climate.
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