This chapter does not describe changes in the major ice sheets as this is dealt with in detail in Chapter 11.

2.2.5.1 Snow cover, including snowfall

Satellite records indicate that the Northern Hemisphere annual snow-cover extent (SCE) has decreased by about 10% since 1966 largely due to decreases in spring and summer since the mid-1980s over both the Eurasian and American continents (Figure 2.13a; Robinson, 1997, 1999). Winter and autumn SCE show no statistically significant change. Reduction in snow cover during the mid- to late 1980s was strongly related to temperature increases in snow covered areas (Figure 2.13b). There is a highly significant interannual (+0.6) and multi-decadal correlation between increases in the Northern Hemisphere spring land temperature and a reduction in the Northern Hemisphere spring snow cover since data have been available (1966). Snow cover extent has decreased about 10% since 1966. The improvements in the quantity and quality of the visible satellite imagery used to produce the operational snow-cover product cannot account for the observed changes in snow cover.

Longer regional time-series based on station records and reconstructions suggest that Northern Hemisphere spring and summer SCEs in the past decade have been at their lowest values in the past 100 years. In the other seasons, it is likely that extents in the latter portion of the 20th century exceeded those of earlier years (Brown, 2000).

Reconstructions for North America suggest that while there has been a general decrease in spring SCE since 1915, it is likely that winter SCE has increased (Brown and Goodison, 1996; Frei et al., 1999; Hughes and Robinson, 1996; Hughes et al., 1996). Similar to the results in North America, in Eurasia April SCE has significantly decreased; but lack of data has prevented an analysis of winter trends (Brown, 2000). Over Canada, there has been a general decrease in snow depth since 1946, especially during spring, in agreement with decreases in SCE (Brown and Braaten, 1998). Winter depths have declined over European Russia since 1900 (Meshcherskaya et al., 1995), but have increased elsewhere over Russia in the past few decades (Fallot et al., 1997). The common thread between studies that have examined seasonality is an overall reduction in spring snow cover in the latter half of the 20th century.

There have been relatively few studies of snowfall trends across the globe. Statistically significant increases in seasonal snowfall have been observed over the central USA in the 20th century (Hughes and Robinson, 1996). In recent decades, snowfall has also been heavier to the lee of the North American Great Lakes than earlier in the century (Leathers and Ellis, 1996). These findings are in line with observations from Canada and the former Soviet Union, reflecting a trend towards increased precipitation over the mid-latitude lands in the Northern Hemisphere (Groisman and Easterling, 1994; Brown and Goodison, 1996; Ye et al., 1998).

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