188.8.131.52 Changes in Snowfall
Winter precipitation has increased at high latitudes, although uncertainties exist because of changes in undercatch, especially as snow changes to rain. Snow cover changes are discussed in Section 4.2. Annual precipitation for the circumpolar region north of 50°N has increased during the past 50 years (not shown) by approximately 4% but this increase has not been homogeneous in time and space (Groisman et al., 2003, 2005). Statistically significant increases were documented over Fennoscandia, coastal regions of northern North America (Groisman et al., 2005), most of Canada (particularly northern regions) up until at least 1995 when the analysis ended (Stone et al., 2000), the permafrost-free zone of Russia (Groisman and Rankova, 2001) and the entire Great Russian Plain (Groisman et al., 2005, 2007). However, there were no discernible changes in summer and annual precipitation totals over northern Eurasia east of the Ural Mountains (Gruza et al., 1999; Sun and Groisman, 2000; Groisman et al., 2005, 2007). Rainfall (liquid precipitation) has increased during the past 50 years over western portions of North America and Eurasia north of 50°N by about 6%. Rising temperatures have generally resulted in rain rather than snow in locations and seasons where climatological average (1961–1990) temperatures were close to 0°C. The liquid precipitation season has become longer by up to three weeks in some regions of the boreal high latitudes over the last 50 years (Cayan et al., 2001; Groisman et al., 2001; Easterling, 2002; Groisman et al., 2005, 2007) owing, in particular, to an earlier onset of spring. Therefore, in some regions (southern Canada and western Russia), snow has provided a declining fraction of total annual precipitation (Groisman et al., 2003, 2005, 2007). In other regions, particularly north of 55°N, the fraction of annual precipitation falling as snow in winter has changed little.
Berger et al. (2002) found a trend towards fewer snowfall events during winter across the lower Missouri River Basin from 1948 to 2002, but little or no trend in snowfall occurrences within the plains region to the south. In New England, there has been a decrease in the proportion of precipitation occurring as snow at many locations, caused predominantly by a decrease in snowfall, with a lesser contribution from increased rainfall (Huntington et al., 2004). By contrast, Burnett et al. (2003) found large increases in lake-effect snowfall since 1951 for locations near the North American Great Lakes, consistent with the observed decrease in ice cover for most of the Great Lakes since the early 1980s (Assell et al., 2003). In addition to snow data, Burnett et al. (2003) used lake sediment reconstructions for locations south of Lake Ontario to indicate that these increases have been ongoing since the beginning of the 20th century. Ellis and Johnson (2004) found that the increases in snowfall across the regions to the lee of Lakes Erie and Ontario are due to increases in the frequency of snowfall at the expense of rainfall events, an increase in the intensity of snowfall events, and to a lesser extent an increase in the water equivalent of the snow. In Canada, the frequency of heavy snowfall events has decreased since the 1970s in the south and increased in the north (Zhang et al., 2001a).