Using a state-of-the-art Earth system model, we show that increasing shrub area leads to climatic warming – through albedo and evapotranspiration feedbacks – which, in turn, counteracts local soil cooling due to shading from the shrubs. This results in overall soil warming and enhanced permafrost vulnerability to future climate change.

A recent study in (Global Change Biology) demonstrated, through field-manipulation experiments, that shading of the ground by shrubs keeps the permafrost beneath comparatively cool. This lead the authors to hypothesize that "expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw". But shrubs also alter the albedo of the land surface, with their dark stems protruding above the snowpack and their comparatively darker leaves yielding lower surface albedos. Enhanced absorption of solar energy also potentially causes atmospheric and soil warming, and possibly counteracts the shrub shading effect.

Through experiments using the Community Earth System Model (CESM), we investigated how shrub-induced climate feedbacks affect permafrost vulnerability, publishing our results in Environmental Research Letters.

We found, in agreement with the field manipulation study, that beneath shrubs ground temperatures were cooler and active layer thicknesses – ALT, the depth to which soil thaws each summer – were shallower (around 11 cm, on average) compared with grassy tundra. However, in a hypothetical pan-Arctic +20% shrub area simulation, the Arctic climate warmed considerably, driven by surface albedo changes as well as increased atmospheric moisture content related to enhanced shrub transpiration. This climatic warming heated the soil and deepened the ALT by around 10 cm. So, when climate feedbacks are considered, our results indicate that shrub area expansion may actually increase rather than decrease permafrost vulnerability.

Next, we plan to study how changes in shrub abundance might alter the Arctic carbon balance. As shrub abundance increases, will carbon sequestration in growing shrubs or carbon emissions from decomposing freshly-thawed soil organic matter dominate?