Global snow and ice cover (the "cryosphere") plays a major role in global climate and hydrology through a range of complex interactions and feedbacks, the best known of which is the ice - albedo feedback. The proximity of the Earth's mean surface temperature to the triple point of water explains the fundamental importance of the cryosphere in the global climate system. Snow and ice cover undergo marked seasonal and longterm changes in extent and thickness. In the Proterozoic era,a long-lived "snowball" Earth has been proposed, while in the Pleistocene epoch, glacial and interglacial intervals alternated in a quasi-periodic manner, but with a much smaller spatial extent.

The perennial elements of the cryosphere - the major ice sheets and perennially-frozen ground - play a role in present-day regional and local climate and hydrology, but the large seasonal variations in snow cover and sea ice are of importance on continental to hemispheric scales. The characteristics of these variations, especially in the Northern Hemisphere, and evidence for recent trends in snow and sea ice extent and thickness are discussed. The relative roles of natural variability in the climate-system forcing of such trends, versus possible anthropogenic influences, cannot yet be confidentally separated. Careful monitoring and assessment of the likely causes and the possible consequences of such changes is clearly a vital task for scientists studying climate-cryosphere processes. The World Climate Research Programme recently established a new project focusing on Climate and the Cryosphere (CliC) which seeks to understand the role of the cryosphere in the climate system. Its initial plans and potential contributions to addressing the above issues will be briefly outlined.