Assessing the Contributions of Thermokarst and Thermal Erosion in Permafrost Feedbacks to Climate

Most studies quantifying the effects of permafrost thaw on carbon cycling focus on gradual top-down thawing of permafrost. However, the thaw of ice-rich permafrost may trigger more abrupt changes via thermokarst and thermal erosion. While the slow gradual deepening of the seasonally thawed layer affects permafrost carbon at regional scales, thermokarst and thermal erosion can potentially affect large volumes of soil at local scales. These processes expose previously-frozen carbon to microbial processes but also alter hydrology. The fate of permafrost carbon in these situations depends on whether material is exposed to aerobic or anaerobic conditions, which will determine the overall rate of carbon mineralization as well as the balance between carbon dioxide and methane emissions. We are synthesizing process-level studies to parameterize a book-keeping model to track the effects of thermokarst disturbance on changes in carbon storage and in carbon dioxide and methane emissions. The goal of this modelling is to enable a comparison of changes in carbon storage and greenhouse gas exchanges between those resulting from deepened active layer versus those due to thermokarst and thermal erosion. The book-keeping model is based on state-and-transition conceptual models for generalized abrupt thaw trajectories in the northern permafrost region. Our initial results show that cumulative carbon release associated with wetland thermokarst are equivalent to up to 55% of estimated carbon emissions stemming from top-down thaw, suggesting that these localized, abrupt thaw processes are important to consider in assessments of permafrost feedbacks to climate.