A Quantification of Climate Feedback from Permafrost Degradation, Thermokarst-Lake Expansion, and Subsequent Methane Emission Under Climate Policy and Uncertainty

A direct consequence of climate warming in the Arctic will be the likelihood of widespread permafrost degradation. Subsequent subsidence of the landscape and hydrologic changes would then support the expansion of saturated areas such as thermokarst lakes and wetlands. These conditions over regions of carbon-rich, yedoma soils present a strong potential for increased methane emissions. In this study, we quantify the future changes in the high latitude near-surface permafrost and methane emission from thermokarst lake regions from climate projections of the 21st century. For the model simulations, we use the MIT Integrated Global System Model (IGSM) framework, which considers the full range of plausible transient climate response (TCR), emissions uncertainty with or without greenhouse gas stabilization targets, as well as a provision for uncertainty in regional climate changes. To account for this regional climate-change uncertainty, we modify the geographic shifts in precipitation, temperature and radiation conditioned by results from general circulation models (GCMs) of the Intergovernmental Panel on Climate Change (IPCC) archive. The numerical experiments with the IGSM indicate that the Arctic undergoes widespread and nearly complete degradation of the (near-surface) permafrost under a "No Policy" case. The uncertainties in TCR, emissions, and regional climate change have little effect on this end-of-century outcome, but affect the dynamic response. Under an aggressive greenhouse stabilization target and the full range of uncertainties, the IGSM simulations substantially reduce the permafrost degradation extent. Subsequent to the permafrost degradation, the simulated expanse of saturated areas can be large (up to 50%), but the uncertainties in TCR and the regional climate response have a large impact in both the dynamic and the end-of-century response. The corresponding, inferred increases in methane emission rates by the end of the century from thermokarst lakes range between 0.5-6.5 Tg-CH4/year for the "No Policy" case and 0.1-3.0 Tg-CH4/year for the stabilization projection. However, the resulting (global) atmospheric CH4 concentrations and radiative forcing from these increased thermokarst methane emissions is small, particularly when weighted against human emissions from the no-policy scenario. From the entirety of the IGSM simulations performed, we estimate that additional warming by the end of this century from the thermokarst lake methane emissions is no greater than 0.1 K. Further sensitivity simulations with the IGSM are presented to gauge the sensitivity of this temperature feedback on the uncertainty in the simulated terrestrial methane emission response.