Estimating future permafrost carbon-climate feedbacks using a coupled Earth System Model

Permafrost soils located in high latitudes contain about 1500 petagrams of carbon. The strong and rapid warming of the Arctic climate threatens this important carbon stock. Permafrost thaw exposes previously frozen organic matter to decomposition by microorganisms, resulting in CO2 and CH4 emissions into the atmosphere that contribute to strengthening the initial warming. On the other hand, rising atmospheric CO2 concentration increases vegetation primary productivity in a feedback known as fertilization effect. In addition, the melting of permafrost may also likely provide more nitrogen in the soil that could stimulate plant growth. The balance between these competing processes is thought to be the primary driver of future permafrost carbon stocks. However, both the amplitude and timing of future net carbon emissions of permafrost areas remain highly uncertain. Reducing the uncertainty on net carbon balance in high latitudes would help improve the accuracy of carbon budget, and thus would impact political and social decisions towards the net zero target.

Up to then, the impact of different future climate scenarios (RCP, SSP or similar) on permafrost have been largely explored with offline Land Surface Model simulations but feedbacks with the atmosphere and ocean cannot be represented in such configurations. Using the IPSL Earth System Model that couples the atmosphere, the ocean and continental surfaces (i.e., ORCHIDEE model), the future evolution of climate-carbon feedbacks in permafrost regions is assessed. In particular, the feedback between climate change and the carbon cycle and the fertilization effect are analyzed with the C4MIP formalism (γ and β parameters). A particular focus is put on the role of the nitrogen cycle through its interactions with the carbon cycle. In addition, the impact of soil insulation by soil organic carbon and surface mosses on thermal transfers is also analyzed in the context of coupled land - atmosphere simulations. The importance of surface insulation i) to maintain realistic surface air temperature, especially during spring time, avoiding large surface - atmosphere feedbacks with unrealistic cooling in the arctic and ii) to sustain large permafrost extent (close to observations) is highlighted.