Long-term changes in lake ice cover in Finland*

The freeze-up and break-up records of almost ninety lakes, and ice thickness of about thirty lakes, were analysed in order to identify long-term changes in the ice regime in Finland. The longest time series of break-up and freeze-up of ice in lakes are available from the early 19th century, while the earliest ice thickness measurements started in the 1910s. The analysis showed that there is a significant change towards earlier ice break-up in Finland except in the very north from the late 19th century to the present time. There is also a significant trend towards later freeze-up and thus also towards a shorter ice cover duration for the longest time series. However, for most lakes, for which data are not available prior to 1900, there are no significant trends. The ice thickness seems to have increased over the last 40 years, although there are significant trends only in half of the investigated lakes and significant decrease in the maximum ice thickness was found in four lakes in southern Finland. The increased ice thickness is most likely due to heavy snow on the ice and production of snow ice.