Long-term change in ice on northern hemisphere lakes
Evidence of reduced ice cover on Arctic lakes and rivers comes from direct observations of the ice and from records of lake biology in sediments. Some very rapid changes have recently been observed for a group of lakes in the Canadian High Arctic. For some northern hemisphere lakes freezing dates have become later and ice break-up dates earlier over the last 150 years. Lakes where both dates have been recorded were frozen over for an average of 17 fewer days in 2004/05 than they were 150 years before. But reliable long-term data sets are available only for a small number of lakes. Data sets covering up to 100 years have been analyzed for larger groups of lakes and rivers in particular regions. They too show later freezing and earlier ice break-up, although there are regional differences. For example, in the latter half of the 20th century, river ice has been breaking up significantly earlier on western North American rivers, but not on the eastern rivers in North America. This difference is linked to a persistent low pressure system over western North America that has caused warmer winters since the mid-1970s, and is related to large-scale ocean circulation patterns such as the Pacific Decadal Oscillation. In Russia, the length of time lakes and rivers were covered with ice was between two and fourteen days less in the period 1980-2000, as compared to 1950-1979. The greatest reductions in ice-cover duration were found on lakes in Asian parts of Russia.