Dynamics of Greenhouse Gas Release in Thaw Lake Terrain

Geomorphic feedbacks to changing climate are nonlinearities that can dominate melting of permafrost (IPCC, 2001). In permafrost lowland regions underlain by ice-rich permafrost, comprising about 5 percent of Earth's land surface, the principal means of permafrost melting and therefore release of greenhouse gases via decomposition of organic soils might be thaw lake expansion by coupled processes of heat transfer, thaw- derived subsidence, and gravity-driven mass movement. We describe a three-dimensional cellular model for thaw lake landscape evolution in which surface elevation, depth to permafrost, temperature gradient, ice content, undecomposed organic soil content and water depth are tracked at each cell. These quantities change through abstractions of thaw lake dynamics, derived in part from coupled two-dimensional thermal-geomorphic simulations of lake expansion (West and Plug, 2006). Modeled lakes initiate, expand, partly or fully drain, and permafrost and ground ice re-form (depending on climate) in modeled landscapes at rates consistent with measurements and paleo-environmental reconstructions. At least two levels of self-organization and associated emergent behavior occur in the model: 1) The timescale over which modeled lakes evolve far exceeds that of heat conduction, subsidence and erosion processes; 2) landscapes react to climate perturbations more slowly than lakes, because slowly-evolving morphology and ground-ice distribution (imposed by previous thermokarst) govern the response of the landscape to subsequent climate forcing. Rates of greenhouse gas emissions from selected regions of Northern Alaska and NW Canada are projected over timescales of 100 to 10,000 y for a range of climate scenarios. Owing to emergent behaviour, the bulk rate at which permafrost melts and greenhouse gases are released depends on past history extending to millenia, which itself depends both on mean trends and extremes in climate. Supported by Natural Science and Engineering Research Council and the Andrew W. Mellon Foundation.