Twentieth-century variability in snow-cover conditions and approaches to detecting and monitoring changes: Status and prospects

The extent, and variability of seasonal snow cover are important parameters in the climate system. Changes in snow cover may provide an indicator of global climatic trends and are of considerable practical significance. The question of the most suitable indices of changes in snow cover conditions, in terms of their use for change detection and for monitoring applications, is discussed. The use of passive microwave-derived estimates of snow cover extent and water equivalent for continental and regional-scale mapping is illustrated. Problems in interpreting the microwave signatures, as well as difficulties in comparing such data to ground observations, are also noted. Up to now analyses have focused primarily on trends in Northern Hemisphere snow extent based on monthly averages using the NOAA weekly snow charts 1972-present, or on station data spanning 50-100 years. However, the latter are generally less readily available, or accessible. An overview is provided of current information on recent hemispheric trends and, for the former USSR, the relationship of changes in snow depth, to variations in temperature and precipitation since the late nineteenth century are described, based on newly available station records. Interpretation of these changes and comparisons with other records are presented. Model projections of changes in snow cover conditions and associated snowmelt runoff that may occur as a result of greenhouse gas-induced warming are discussed for several mountain regions. Long-term station records of snow depth variability provided a valuable context within which such modeling results can be examined.