sheet, there is major concern about the possibility of a West Antarctic Ice Sheet collapse. The West Antarctic Ice Sheet is grounded on bedrock below sea level, and its collapse has the potential of raising the world sea level over a few hundred years. Most of the discharge of the West Antarctic Ice Sheet is via the five major ice streams (faster flowing ice) entering the Ross Ice Shelf, the Rutford Ice Stream entering Filchner–Ronne Ice Shelf of the Weddell Sea, and the Thwaites Glacier and Pine Island Glacier entering the Amundsen Ice Shelf. Opinions differ as to the present mass balance of these systems (Bentley 1983, 1985), principally because of the limited data. The West Antarctic Ice Sheet is stable so long as the Ross Ice Shelf and Filchner-Ronne Ice Shelf are constrained by drag along their lateral boundaries and pinned by local grounding of ice rises. The Ice Ages In glacial times, the cryosphere considerably increased its size to cover a considerable part of the lands of northern Eurasia and America, lowering the sea level by more than a hundred meters and creating large ice shelves that connected the entire north of the planet. Also thanks to this (and continental drift), the first hominids, which arose in Africa, were able to reach all parts of the world, which would then separate with the return to normal seas and oceans. Science "Cryospheric sciences" is an umbrella term for the study of the cryosphere (not unlike atmospheric sciences, encompassing meteorology, climatology, and aeronomy). As an interdisciplinary Earth science, many disciplines contribute to it, most notably geology, hydrology, and meteorology and climatology; in this sense, it is comparable to glaciology. See also Climate system Cryobiology Effects of climate change on the cryosphere Glaciology International Glaciological Society (IGS) International Association of Cryospheric Sciences (IACS) Polar regions of Earth Physical impacts of climate change Retreat of glaciers since 1850 References Further reading Brown, R. D., and P. Cote, 1992: Inter annual variability in land fast ice thickness in the Canadian High Arctic, 1950–89. Arctic, 45, 273–284. Chahine, M. T., 1992: The hydrological cycle and its influence on climate. Nature, 359, 373–380. Flato, G. M., and R. D. Brown, 1996: Variability and climate sensitivity of landfast Arctic sea ice. J. Geophys. Res., 101(C10), 25,767–25,777. Groisman, P. Ya, T. R. Karl, and R. W. Knight, 1994b: Changes of snow cover, temperature and radiative heat balance