9.5.5.2 Snow and Frozen Ground

Snow cover in the NH, as measured from satellites, has declined substantially in the past 30 years, particularly from early spring through summer (Section 4.2). Trends in snow depth and cover can be driven by precipitation or temperature trends. The trends in recent decades have generally been driven by warming at lower and middle elevations. Evidence for this includes: (a) interannual variations in NH April snow-covered area are strongly correlated (r = –0.68) with April 40°N to 60°N temperature; (b) interannual variations in snow (water equivalent, depth or duration) are strongly correlated with temperature at lower- and middle-elevation sites in North America (Mote et al., 2005), Switzerland (Scherrer et al., 2004) and Australia (Nicholls, 2005); (c) trends in snow water equivalent or snow depth show strong dependence on elevation or equivalently mean winter temperature, both in western North America and Switzerland (with stronger decreases at lower, warmer elevations where a warming is more likely to affect snowfall and snowmelt); and (d) the trends in North America, Switzerland and Australia have been shown to be well explained by warming and cannot be explained by changes in precipitation. In some very cold places, increases in snow depth have been observed and have been linked to higher precipitation.

Widespread permafrost warming and degradation appear to be the result of increased summer air temperatures and changes in the depth and duration of snow cover (Section 4.7.2). The thickness of seasonally frozen ground has decreased in response to winter warming and increases in snow depth (Section 4.7.3).