Remote Sensing of the Cryosphere

Seasonal Snow Cover

Of the seasonal changes that occur on Earth's land surface, perhaps the most profound is the accumulation and melt of the seasonal snow cover. During winter, snow covers 45 million km2, about 30% of the land surface, affecting climate, weather, and the water balance. Its high albedo reduces the absorbed solar radiation, its low thermal diffusivity insulates the ground, and its large enthalpy of fusion (335,000 J/kg) affects the heat and moisture fluxes. Therefore, snow cover is a huge influence on the hydrologic cycle during the winter and spring for much of Earth's land area. Near many mountain ranges, the seasonal snow cover is the major source of runoff, filling rivers and recharging aquifers that over a billion people, one-sixth of the world's population, depend on for their water resources and that are at risk in a warming climate (Barnett et al. 2005). Snow affects large-scale atmospheric circulation. Early-season snow cover variability in the northern hemisphere, for example, leads to altered circulation patterns, which in turn have implications for climate predictability (Cohen and Entekhabi 1999). For example, winters with greater than normal Eurasian snow cover are usually followed by a weaker Asian monsoon (Liu and Yanai 2002).

Mountain Glaciers and Ice Caps

Occurring in mountain valleys and in high mountain basins at all latitudes, glaciers derive from accumulation of seasonal snow that does not fully melt during the summer. Years of accumulation allow the overlying snow to compress the lower layers until they close off the pore spaces between the snow grains and become glacial ice. As a plastic, the ice flows slowly downhill under the force [Page 398]of gravity. At the lower elevations, snowfall is less and melt rate is greater, so there the accumulation is less than the melt. The difference is made up from the excess ice flowing from the higher elevations, and the integration of accumulation and melt is called the mass balance of the glacier.

Currently, most glaciers are shrinking, and this evidence of widespread negative mass balance is one of the more powerful indicators of worldwide climate change. The loss of mass – the transition of ice to water – is a major contributor to sea-level rise in this century, along with thermal expansion of the warming ocean (Meier 1984, Gregory and Oerlemans 1998).

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