ALBERT

Description: Permafrost regions have been identified to host a soil organic carbon (C) pool of global importance, storing more than 1500 PgC. A large portion of this C pool is currently frozen in deep soils and permafrost deposits. Permafrost thaw hence may result in mobilization of large amounts of C as greenhouse gases, dissolved organic C, or particulate organic matter, with substantial impacts on C cycling and C pool distribution. Understanding potential consequences and feedbacks of permafrost degradation therefore requires better quantification of processes and landforms related to thaw. While many predictive land surface models so far consider a gradual increase in the average active layer thickness across the permafrost domain, rapid shifts in landscape topography and surface hydrology caused by thaw of ice-rich permafrost are much more difficult to project. Field studies of thermokarst and thermo-erosion indicate highly complex and rapid landscape-ecosystem feedbacks. Contrary to top-down permafrost thaw that may affect any permafrost type at the surface, both thermokarst and thermo-erosion are considered pulse disturbances that are closely linked to presence of near-surface ice-rich permafrost, are active on short sub-annual to decadal time scales, and may affect C stores tens of meters deep. Here we present a comprehensive review synthesizing measured and modeled rates of thermokarst and thermo-erosion processes from the scientific literature and own observations across the northern Hemisphere permafrost regions. The goal of our synthesis is (1) to provide an overview on the range of thermokarst and thermo-erosion rates that may be used for parameterization of thermokarst and thermo-erosion in ecosystem and landscape models; and (2) to assess simple back-of-the-envelope scenarios of the magnitude of C thaw due to thermokarst and thermo-erosion versus projected active layer thickening. Example scenarios considering thermokarst lake expansion and talik growth indicate that rapid thaw processes have a high possibility to contribute substantially to permafrost C mobilization over the coming century.