Permafrost in a Simple Carbon-Climate Model

Permanently frozen soil, or permafrost, currently stores around a fourth of global soil carbon, and a warming climate makes this carbon increasingly vulnerable to decomposition and release into the atmosphere in the form of greenhouse gases. The resulting climate feedback can be estimated using Earth system models (ESMs), but the high complexity and computational cost of these models make it challenging to use them for estimating uncertainty, exploring novel scenarios, and coupling with other models. We have added a representation of permafrost to the simple, open-source global carbon/climate model Hector and calibrated the results to be consistent with both historical data and 21 ^st century ESM projections of permafrost thaw. We include permafrost as a separate land carbon pool that becomes available for decomposition into both methane and carbon dioxide once thawed. The thaw rate is controlled by biome-specific air temperature increases from a pre-industrial baseline.

When calibrated to previous ESM results, we found that by 2100 thawed permafrost emissions increased Hector's atmospheric CO ₂ concentration by 10-15% and the atmospheric CH ₄ concentration by 10-20%, depending on the future scenario. This resulted in around 0.5°C of warming over the 21 ^st century. The fraction of thawed permafrost carbon available for decomposition was the most significant parameter controlling the long-term temperature response of the model.

Hector is simple and computationally efficient, running almost instantly, so it can be cheaply run over a wide range of parameter values to explore the climate impacts of uncertainties in the permafrost carbon feedback. In the future, Hector's permafrost module can be easily coupled with integrated assessment models to explore the effects of climate policies, estimate the economic consequences of warming from this feedback, and improve evaluation of climate and energy policy using such models.