Modelling permafrost peat plateau thaw and potential methane release

Abstract

Current Earth System Models are poorly equipped to disentangle the many interacting processes that govern the permafrost carbon feedback, resulting in a high degree of uncertainty in future projections of both thaw and fluxes. This thesis focuses on permafrost peat plateaus and palsas, and presents a pathway to including the effects of abrupt thaw of these environments on methane from the microscale to the pan-arctic in the land surface model JULES (the Joint UK Land Environment Simulator).

A two-tile approach is investigated to resolving sub-gridscale heterogeneity and microtopographic feedbacks in JULES. It is shown that including these processes improves the modelled snow depths, soil moistures and temperatures, and can increase modelled methane fluxes. This approach enables the potential distribution of discontinuous permafrost in peatlands to be modelled, and could enable the modelling of other sub-grid processes in JULES. Through testing of processes and parameters at four sites, uncertainties are quantified and the two-tile model refined.

To model rates of lateral thaw, a new method of representing thaw subsidence is developed and a 2D thermal transect is resolved from mire to palsa, driven by the output of the two-tile model. It is shown that lateral thaw rates are correlated with palsa surface temperature, and thus that lateral thaw rates are likely to increase with warming. However, the increase in lateral thaw rates is bounded by the point of total top-down subsidence of the palsa.

A simple model of peat plateau fragmentation is then constructed and investigated, leading to a function of peat plateau edge length with peat plateau fraction using a single parameter. For the first time, this function enables the modelled rate of lateral thaw to be used to model areal changes of permafrost in peat plateau complexes. The model's parameter is extracted from satellite imagery, and the associated uncertainty in areal thaw rate quantified.

Finally, these developments are brought together and applied to the pan-arctic region, and the implications of these findings on estimates of permafrost peat plateau thaw and methane release considered.