Knowledge of the development of permafrost has been arrived at gradually. In the 1930s M.I. Sumgin substantiated the theory of the degradation of permafrost (with warming, thawing and northward retreat). Having compared the severe climate of glacial epochs with the warmer present one, Sumgin came to the conclusion that the permafrost formed simultaneously with the ice sheets and had subsequently thawed retreating northward, i.e. it had degraded. He presented data indicating displacement of the southern limit of permafrost northward. However such researchers as S.G. Parkhomenko, P.I. Koloskov, P.N. Kapterev, D.V. Redozubov and others pointed out the fact of new permafrost formation, permafrost temperature decrease and its increase in thickness in other regions of the country. Contrary to Sumgin they followed the alternative theory - that of aggradation, i.e. they reasoned that the process of permafrost advance is taking place at the present time. According to their views the permafrost is a result of recent heat exchange (the last 3-5 thousand years) in the system 'atmosphere-lithosphere'. These two points of view were antagonistic for 10-15 years and only V.A. Kudryavtsev's basic works (1953-1963) clarifying the basis for the present theory of permafrost development gave the proper interpretation of these points of view.
Numerous works by Kudryavtsev from 1954 on, showed that the thermal state of the permafrost depends on heat exchange through the Earth's surface, between the atmosphere and the upper layers of the lithosphere, the character of which is defined by:
1) the composition and properties of the soil and rock materials and the processes operating in them;
2) the amount of direct and transformed solar heat arriving at the Earth's surface;
3) the specific features of the Earth's surface absorbing radiant and thermal energy;
4) the amount of heat arriving at the surface from the Earth's interior.
The character and conditions of heat exchange are extremely dynamic. Permafrost strata, their formation, development, transition to the thawed state and all their characteristics (distribution, occurrence, composition, structure and temperature regime) are changing in response to change in the combined natural conditions affecting the course of heat exchange between the atmosphere and the upper layers of the lithosphere. Thus, the dynamics and evolution of the Earth's cryosphere are closely connected with the palaeogeographical and geological evolution of its regions within continents and oceans.
Periodic changes of ground heat exchange through the Earth's surface are responsible for the dynamics of the temperature pattern of the litho-sphere's upper layers. In the case of temperature transition through 0°C the perennial freezing of the subsurface begins. Climatic fluctuations being various in their amplitude and duration propagate differently through the upper layers of the lithosphere. It is known that daily temperature fluctuations extend down for a few tens of centimetres, yearly ones up to 15-25 m, 30-40 year ones to 40-70 m; 300 year ones to 100-150 m and so on. The duration of the temperature fluctuation varies within wide limits from days to a thousand and a hundred thousand years. Therefore the depth of propagation of the heat waves through the ground and thus the thickness of the negative temperature zones will depend on the duration of the fluctuations. Upper boundary conditions for the formation of permafrost and its dynamics in the context of thermal physics are expressed in terms of the following parameters: mean perennial temperature on the permafrost surface t£feran, amplitude of the temperature fluctuations Aper on this surface, as well as the period rper of the many years temperature fluctuations.
Temperature behaviour on the permafrost surface for the many-years period is represented by a rather complicated summary curve, which is a combination of temperature fluctuations of shorter periods. To show the complexity of the temperature conditions on the surface one can superpose (as a rough example) only three various-period temperature fluctuations (Fig. 12.1), although the actual temperature curves for the surface are found to be much more complicated. At the same time, as is shown in Fig. 12.1 (curve I V), both increase and decrease of the surface temperature, dictated by the fluctuations of the shorter periods, can be observed on the background of a general temperature increase resulting from a 300-year fluctuation.
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