Reconstruction of Annual Maximum Ice Extent in the Baltic Sea 1660-2005 
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Reconstruction of Annual Maximum Ice Extent in the Baltic Sea 
1660-2005 
Lars Axell and Katarina Lindquist, 
Swedish Meteorological and Hydrological Institute (SMHI), Sweden 
E-mail: Lars.Axell@smhi.se 
Abstract 
The annual maximum ice extent A max in the Baltic Sea is an important indicator of regional 
winter climate in northern Europe. Apart from modern-day operational ice charts, proxy data 
such as coastal observations, mean winter air temperature, and ice break-up dates have been 
used to reconstruct A max back to 1660. 
A consolidated time series was constructed by giving the proxy time series different weights. 
These weights were calculated as inversely proportional to the mean square errors of the time 
series during the calibration period 1961-1990. 
The consolidated time series indicates a large interannual as well as interdecadal variability. In 
spite of the recent mild winters in the Baltic Sea region, the large interannual variabilitiy of A max 
makes the calculated trend over moving 30-year windows mostly insignificant in a statistical 
sense.