Can effects from global warming be seen in Swedish snow statistics?

Abstract: This study is a result from a major investigation about the snow conditions in Sweden since the beginning of the twentieth century. For this purpose, data were analysed with respect to the maximum snow depth and the number of days with snow cover every year from some more than forty selected stations. These stations were then divided into different regions and means were calculated for each series. The data are presented in the shape of different histograms in the four following categories; the whole period in request (1900-2003), the latest 43 years (1961-2003), consecutive mean values for every decade and time series with the highest frequented fluctuations equalized. To be able to detect any trends in the plotted time series two statistical methods, simple linear regression and Mann-Kendall’s test, were applied. The calculations belonging to these tests are showed in tables. To be able to answer the question if the global warming can be related to the latest 3-4 decades predominantly warm winters in the southern part of Sweden I have been studying correlations in snow data with respect to the northern hemispheres mean temperature for the winter season. Corresponding estimates of the correlation coefficients have also been made with respect to the Swedish winter mean temperature. The response of the tests shows that it has not been such dramatic change in the snow conditions in the long run. The magnitude of the slope for the adjusted regression lines implies that the maximum snow depth and the number of days with snow cover in average have been on a fairly constant level during the latest hundred years. When it comes to the maximum snow depth one can distinguish a tendency for a small rise in Götaland and northern Norrland. This is also the only cases which are statistical significant for the period in request (1905-2003). For the shorter period 1961-2003 however, the number of days with snow cover has decreased quite substantially in the southern part of Sweden corresponding to a decrease about 40% in Götaland and 20% in Svealand. The test based on simple linear regression gives significant results in both cases while Mann-Kendall only establishes the trend for Götaland. A closer view of the maximum snow depth for the shorter period (1961-2003) does not give the same response but there is at least evidence for a significant decrease in Svealand in the test with simple linear regression. It corresponds to a decrease of about 30% since 1960. One cannot immediately relate the changes in the Swedish snow climate to the global warming. Estimated values of the correlation coefficient do not even give significant results for the period 1961-2003 despite of the fact that the global mean temperature has raised quite considerably since 1970. The corresponding calculations for the Swedish winter mean temperature show that it plays a very important roll if the precipitation in Götaland and Svealand is coming as rain or snow while it does not matter at all in northern Norrland.