Permafrost

A non-steady-state technique used to test soils in field and in laboratory has given a
possibility to study thermal conductivity of soil horizons in landscapes of the Eastern Siberia
and established variation of its values. Value of the thermal conductivity depends on water
content, organic content and density of the mineral samples. Temperature dependence was
established for frozen soil as well as for the ancient ice. Calculations based on Kudryavtsev’s
model and experimental data show a high sensitivity of the thermal mode of the active layer to
the surface disturbance. Enriched by organic material, soil horizon A has a low thermal
conductivity that differs in frozen and thawed states; it creates a negative thermal offset
decreasing the temperature of permafrost about 1.5 ÷ 2ºC and up.
Forest fires modify ground surface conditions and cause deepening of the active layer. The
subsequent disturbance of surface vegetation and organic layer changes the energy budget of
soil. Thermal conductivity of the soil horizon A increases significantly after a forest fire. Values
of the thermal conductivity of both organic (horizon A) and mineral soil are also enlarged in
surface depressions at swamp and alas sites; therefore, a process of warming of permafrost once
started could be accelerated as a result of the alteration of thermal properties of soil.
About 19% of the total land surface in the Eastern Siberia has been affected by
thermokarst. However, wide distribution of alases in the Central Yakutia is not an evidence of
its modern growth. Though thermal conductivity of upper soil horizons increase after fire,
changes of thermal mode of soil induced by fires alone are not enough to cause ice wedges
thawing. Nevertheless, climatic change followed by surface disturbance (forest fires and
clearances) causes significant transformation of soil temperature mode and thermokarst
appearance. In spite of obvious climatic warming in the area there are no noticeable changes of
surface development on undisturbed landscapes yet, and upper soils horizons still protect
permafrost.
Key words: Thermal conductivity, active layer, forest fires, thermokarst
The Simulated Current and Future Soil Thermal Regime of the Tibetan
Plateau
162
Christoph Oelke 1 , Tingjun Zhang 2 , Andrew Etringer 2
(1. Institute for Geophysics, University of M unster, Germany;
2. National Snow and Ice Data Center, University of Colorado, USA )
Abstract: The Tibetan Plateau encompasses about 7.5 % of the permafrost regions of the
Northern Hemisphere, with permafrost underlying an area of about 1.8 mio. km 2 . It is the
southernmost permafrost region of the Northern Hemisphere where low air temperatures allow
its existence at altitudes mostly between 3500m and 6000 m. The northeastern parts of the
Plateau have recently been subject to considerable attention in context with the construction of
the Qinghai-Tibet Railroad Line that leads over the Plateau for more than 950km at altitudes of
over 4000m. Melting permafrost within the 21st century in response to increasing air
temperatures and changing snow insulation represents a serious threat for constructions.
The soil thermal regime of the Tibetan Plateau is modeled by applying a one-dimensional