Cryosphere

a shell of the earth, discontinuous and impermanent in configuration, in the zone of thermal interaction of the atmosphere, hydrosphere, and lithosphere. It is characterized by subfreezing or freezing temperatures at which the water contained in it in the form of vapor, in a free state, or chemically and physically bound with other components may exist in the solid phase (ice, snow, frost, and the like).

The temperature 0°C (273.15°K) defines the equilibrium between chemically pure ice and water at an atmospheric pressure of 760 mm mercury without any extraneous fields of force. Under natural conditions various impurities and solutes as well as surface forces and pressure lower the freezing point of water, and as a result the liquid phase of H₂0 is also part of the cryosphere when it is in a temporarily or stably cooled state below 0°C (salty sea and underground water, unfrozen bound water, fresh water under high pressure beneath ice sheets, supercooled drops of water in clouds and mists). The cryosphere also includes waterless rock strata and relatively dry air masses with subfreezing temperatures, where conditions for the condensation of H₂O may be created naturally or artificially, thereby also creating conditions for the formation of the solid phase of H₂O.

The cryosphere stretches from the upper layers of the earth’s crust to the lower layers of the ionosphere, interrupted by zones of varying thickness that are temporarily or permanently heated to above 0°C. The lower boundary of the cryosphere coincides with the bottom of the layer of frozen and cooled rocks. This layer has great stability and reaches its maximum depth in high latitudes, that is, in Antarctica (more than 4 km) and subarctic regions (about 1.5 km), but in the middle and low latitudes there is seasonal variability and the layer gradually disappears. At about 100 km above sea level the upper boundary of the cryosphere passes through rarefied layers of the atmosphere, above the strongly cooled mesopause that contains noctilucent clouds.

The cryosphere is characterized by occasional, short-term, seasonal, long-term, and perennial cryogenic formations. These formations include migrating systems of clouds containing atmospheric ice; short-term, seasonal, and long-term snow cover that accumulates this ice and condenses water vapors; seasonally frozen (every year or in some years) soils and rocks that contain ice in cavities and pores; and seasonal and long-term ice cover of fresh and salty bodies of water that includes ice originating in the atmosphere or at the surface of or in the water. Other formations are the seasonal and long-term ice mounds of surface and underground water; mountain glaciers and the ice sheets of polar islands and continents; and strata of frozen rock that contain underground ice of various origins (for example, constitutional, segregation, fissure, buried, or cave ice) and that do not thaw for many years, centuries, or millennia. The fact that cryogenic formations are found at certain elevations and are distributed in circumpolar fashion is related to the uneven distribution of solar radiation depending on latitude and elevation above sea level. A rough quantitative description of the main cryogenic formations is given in Table 1 (after P. A. Shumskii and A. N. Krenke, 1964, with refinements).

The dimensions of the areas of distribution of cryogenic formations give an idea of the scale of their participation in the

hydrologic cycle on earth, and the large volume of long-term accumulations of surface and underground ice testifies to the stability of the low-temperature branch of this process. The cryosphere plays an important role in all planetary climate-forming processes and, together with them, is subjected to daily, annual, and long-term fluctuations. In the cryolithozone the cryosphere gives rise to specific cryogenic and postcryogenic phenomena and corresponding relief forms. The cryosphere has some effect on the vital activity of plants and animals as well as on some types of human economic activity.

The cryosphere has presumably existed throughout all the earth’s geological history. It was most pronounced during the periods of global cooling, which were characterized by maximum development of ice sheets and areas of permafrost.

The term “cryosphere” was proposed without a precise definition of its limits by the Polish scientist A. B. Dobrowolski in 1923, although concepts of the earth’s successive stages of cooling and a special ice sheet had appeared earlier—for example, in the works of M. V. Lomonosov (1763), the French scientist J. Fourier (1820), and A. I. Voeikov (1886). In 1933, V. I. Vernadskii expanded the concept of the cryosphere and introduced the notion of the area of the earth’s cooling (to temperatures not higher than 4°C, the point of maximum water density). According to him the area occupied almost the full depth of the world ocean and thicker layers of the atmosphere and underground hydrosphere as compared to the present definition of the extent of the cryosphere. Significant contributions to the further development of ideas about the cryosphere were made by Soviet (N. I. Tolstikhin, P. A. Shumskii), French (L. Llibutry), Canadian (J. R. Mackay), English, and American (A. L. Washburn, T. L. Péwé) scientists.