LONG-TERM OBSERVATIONS FOR MONITORING OF THE CRYOSPHERE

Variations of the cryosphere over decadal-to-century timescales are as sessed by a survey of data on sea ice, snow cover, glaciers and ice sh eets, permafrost and lake ice. The recent variations are generally con sistent across the different cryospheric variables, especially when pl aced into the context of variations of temperature and precipitation. The recent warming over northern land areas has been accompanied by a decrease of snow cover, particularly during spring; the retreat of mou ntain glaciers is, in an aggregate sense, compatible with the observed warming; permafrost extent and lake ice duration show similar variati ons in areas for which data are available. Corresponding trends are no t apparent, however, in data for some regions such as eastern Canada, nor in hemispheric sea ice data, especially for winter. The data also suggest an increase of snowfall over high latitudes, including the Ant arctic ice sheet. Estimates of both the climatic and the statistical s ignificance of the recent variations are hampered by data inhomogeneit ies, the shortness of the records of many variables and the absence of central archives for data on several variables. The potential of moni toring by satellite remote sensing has been realized with several vari ables (extent of sea ice, snow cover). Other cryospheric variables (sn ow depth, ice sheet elevation, lake ice, mountain glaciers) may be ame nable to routine monitoring by satellites pending advances in instrume ntation, modifications of satellite orbit, and further developments in signal detection algorithms. The survey of recent variations leads to recommendations concerning the use of historical data, in situ measur ements, and remote sensing applications in the monitoring of the cryos phere.