4.2 Changes in Snow Cover
The high albedo of snow (0.8 to 0.9 for fresh snow) has an important influence on the surface energy budget and on Earth’s radiative balance (e.g., Groisman et al., 1994). Snow albedo, and hence the strength of the feedback, depends on a number of factors such as the depth and age of a snow cover, vegetation height, the amount of incoming solar radiation and cloud cover. The albedo of snow may be decreasing because of anthropogenic soot (Hansen and Nazarenko, 2004; see Section 2.5.4 for details).
In addition to the direct snow-albedo feedback, snow may influence climate through indirect feedbacks (i.e., those in which there are more than two causal steps), such as to summer soil moisture. Indirect feedbacks to atmospheric circulation may involve two types of circulation, monsoonal (e.g., Lo and Clark, 2001) and annular (e.g., Saito and Cohen, 2003; see Section 3.6.4), although there are large uncertainties in the physical mechanisms involved (Bamzai, 2003; Robock et al., 2003).
In this section, observations of snow cover extent are updated from IPCC (2001). In addition, several new topics are covered: changes in snow depth and snow water equivalent; relationships of snow to temperature and precipitation; and observations and estimates of changes in snow in the Southern Hemisphere (SH). Changes in the fraction of precipitation falling as snow or other frozen forms are covered in Section 18.104.22.168. This section covers only snow on land; snow on various forms of ice is covered in subsequent sections.