Snow exerts a huge influence on climate, through its high reflectivity, insulating properties, and cooling of the atmosphere, and on surface hydrology, through its effects on water resources in many parts of the world. Mean monthly snow-cover extent in the Northern Hemisphere has decreased at a rate of 1.3 per cent per decade during the last 40 years, with greatest losses in the spring and summer months. Climate models project significant decreases in snow cover by the end of this century, with reductions of 60 to 80 per cent in snow water equivalent (depth of water resulting from snow melt) in most mid-latitude regions. Increases are projected for the Canadian Arctic and Siberia. Higher temperatures and rises in snow line are projected for many mountain regions. Changes in snow cover, such as the formation of ice layers in snow due to increased frequency of snow thaw, have widespread impacts as snow is an important ecological factor. Snow-cover changes also have impacts on human well-being and economic activities, including water resources, agriculture, animal husbandry, transportation and winter recreation such as skiing.