Thermokarst as a short-term permafrost disturbance, Central Yakutia

The widespread occurrence of alas depressions in Central Yakutia is not necessarily evidence of modem thermokarst activity. Typically, a near-surface 'shielding layer', formed as the result of deep thaw in exceptionally warm years, protects underlying icy sediment from thaw. In spite of current climatic warming, there is no noticeable increase in regional thermokarst in Central Yakutia. Periodic forest fires significantly increase soil thermal conductivity and progressive soil salinization is occurring; however, these are insufficient to cause thaw of the underlying icy sediments. Instead, active thermokarst should be regarded as a short-term catastrophic process. Use of the Kudryavtsev algorithm indicates that the depth of thaw beneath a water body in Central Yakutia can reach 8-10 m within 50 years, and 20+ m within 300 years. Field observations show that current thermokarst has a tendency for fast (5-10+ cm/year) subsidence. Copyright (C) 2004 John Wiley Sons, Ltd.