Historical Trends in Lake and River Ice Cover in Norway : signs of a changing climate

Recent studies have shown that the duration of seasonal ice cover on lakes and rivers over the Northern Hemisphere has declined over the 19th and 20th Centuries, mainly as a consequence of rising temperatures. However, lake and river ice trends have not been well documented in Norway. Quality control and homogeneity testing were performed on ice cover data from 48 Norwegian lake and river sites with long records. A total of 142 individual records of ice phenology (freeze-up, break-up and ice cover duration) were evaluated. 23 records were rejected in the quality control or in the subsequent homogeneity testing. The homogenous ice records were analyzed to examine regional and temporal changes in freeze-up, break-up as well as the total duration of ice cover. The full-length records of each site and four different time periods were analyzed: 1913 – 1942, 1954 – 1983, 1982 – 2011 and 1922 – 94. The temporal evolution of trends in ice phenology and hydro-meteorological variables over moving 30-year periods was also examined. The results of trend analyses displayed large variations in both in space and time. The records generally indicated a trend towards later freeze-up, earlier break-up and a decrease in total number of days with ice cover, though few trends were significant at the 5 % level. Freeze-up and break-up was strongly correlated with air temperature and discharge at most sites. The longest record (Mjøsa), covering the 1865 – 2013 displayed significant positive trends in freeze-up and significant negative trends ice cover duration at the 5 % level. The detected rates of change were 1.2 and –1.3 days/decade, respectively. This is similar to the findings of comparable studies from Eurasia and North America. Key words: Ice cover, ice phenology, freeze-up, break-up, homogeneity, trend, Mann-Kendall, climate change