Compound effects of permafrost thaw, vegetation dynamics, and increasing CO2 alter carbon budget of the permafrost zone

Under changing climatic conditions, the Arctic is undergoing massive changes. Due to Arctic amplification, the Arctic is warming faster than any other region on Earth. The combination of warming and CO₂ fertilization leads to increases in productivity and changes in vegetation composition, with shrubs invading the Tundra, and trees also shifting northwards. At the same time, permafrost thaws, adding previously frozen carbon deposits to the active carbon cycle. The net carbon balance resulting from all of these coupled processes is less clear than one might think and requires an integrated modelling approach.

We use ICON-Land, the land surface model of the ICON Earth System Model, to investigate changes in the carbon cycle of the permafrost region. We have extended the soil carbon model YASSO by introducing a vertical dimension in order to consider carbon storages in deeper frozen soil layers. Furthermore, we are considering Arctic-specific shrub PFTs in our dynamic vegetation scheme in order to represent the changes in vegetation composition expected in a changing climate, thus allowing a complete assessment of carbon cycle changes.

We initialise the soil C pools for the preindustrial climate state from the Northern Circumpolar Soil Carbon Database to insure initial C pool sizes close to measurements. We then determine changes in vegetation composition and soil C storage in transient model experiments following historical and future climate changes under RCPs 2.6, 4.5, and 8.5. Based on these experiments, we quantify the greenhouse gas balance under future climatic conditions. While the permafrost soils lose carbon in all scenarios, productivity increases, especially if the vegetation can adapt to the changed climatic conditions, leading to lower carbon release.