HR: 09:45h
AN: C31A-08    [Abstracts]
TI: Why could Permafrost be sometimes so persistent?
AU: * Romanovsky, V E
EM: ffver@uaf.edu
AF: Geophysical Institute, University of Alaska Fairbanks, Geophysical Institute POB 750109, Fairbanks, AK 99775-7320, United States
AB: Recent changes in climate make permafrost warmer and less stable in many locations throughout the Arctic and sub-Arctic. At some of these locations, permafrost already started to thaw. However, most of the permafrost in the Northern Hemisphere is still generally stable and the threshold of the widespread permafrost thawing is not crossed yet. Moreover, from paleo-environmental studies, it is known that permafrost survived much warmer- than-now climatic conditions that existed during the early and middle Holocene in the areas where permafrost now is just one degree Centigrade or less from the melting point of ice. This paradox has not been explained yet, though it is very important to do so, especially in the view of better understanding the future trajectories of permafrost evolution under conditions of possible further climate warming. One of the possible explanations of this phenomenon is related to the fact that some amount of ice in fine- grained material, such as frozen silt or clay, starts to melt long before the soil temperature reaches the 0 C threshold. During a warming period, a zone of distributed heat sinks appears in the upper permafrost layer. As permafrost temperature moves closer to 0 C, this zone starts to spread to the deeper permafrost, eventually occupying the entire permafrost layer. As a result, a significant part of the heat flux entering thawing permafrost from its top boundary (permafrost table) penetrates through the phase boundary between the permafrost and the bottom of the growing talik and dissipates within the entire permafrost body. This heat is spent on partial melting of the constituent ice within the permafrost. Portioning of the amount of heat consumed by the thawing permafrost at the permafrost table and within the entire permafrost body depends on the thermal conditions at the ground surface and on the soil characteristics. The percentage of the heat flux that dissipates inside the permafrost layer is especially significant when long-term heat flux into the upper permafrost is small and when the temperature interval where distributed phase change occurs is large. Because this percentage could be as large as 80% or more, "formal" thawing of permafrost (downward movement of the permafrost table) could be delayed and/or slowed down very significantly. Large amount of excess ice in the upper permafrost would make the permafrost system even more inertial. All these effects together can delay and/or slow down the complete permafrost disappearance for centuries. In this presentation I will show measured permafrost temperature data that can support this hypothesis. Results of some relevant numerical experiments will be also presented.
DE: 0702 Permafrost (0475)
DE: 0764 Energy balance
DE: 0768 Thermal regime
DE: 0774 Dynamics
DE: 0798 Modeling
SC: Cryosphere [C]
MN: 2007 Fall Meeting