Hydrologic controls on the permafrost carbon-climate feedback

Large-scale permafrost thaw is projected under unmitigated warming scenarios. Since permafrost soils contain enormous amounts of organic carbon, whose stability is contingent on remaining frozen, this permafrost thaw could lead to a significant release of carbon to the atmosphere, acting as a positive feedback to climate change. Significant uncertainty remains on the post-thaw carbon dynamics of permafrost-affected ecosystems. The large stocks of permafrost carbon have built up over time in part due to the saturated (and cold) soil conditions found across much of the permafrost domain. But, these conditions are likely to change as permafrost thaws. Here we focus on the hydrologic response. Prior research suggests that the hydrologic response may contain two phases, an initial wettening associated with ice melt and surface subsidence leading to more wetlands and lakes, followed by drying once the permafrost table has deepened enough to open up new channels to the groundwater system.

Simulations to year 2300 with the Community Land Model (CLM4.5BGC) suggest that, even though Earth System Models project that Arctic climate will get wetter, soil moisture conditions will become drier both at the surface and at depth in response to projected deepening of the permafrost table. Here, we examine the relative influence of these soil moisture changes (compared to the influence of warming) on soil decomposition rates by conducting an additional model experiment in which we artificially maintain the wet 1850 soil moisture conditions through throughout the simulation. We can then assess (a) how the soil moisture drying trajectory affects the rate of soil warming (answer: not much) and (b) how a different plausible soil moisture trajectory can alter the rates of soil carbon decomposition into CO₂ or CH₄. We find that he drying at depth leads to faster soil carbon decomposition, while the absence of drying leads to slower decomposition rates, but higher CH₄ emissions. Our results suggest that soil moisture and trends of soil moisture are an important part of the permafrost climate-carbon feedback picture and that improved understanding of impact of permafrost thaw on soil moisture dynamics is required to further constrain the amplitude of the permafrost climate-carbon feedback.