Investigation of vegetation-snow-permafrost-climate interactions for improved predictions of the permafrost thermal regime

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The thawing of permafrost caused by global warming could release tens of Pg of carbon in the form of CO2 and CH4 to the atmosphere. This could constitute a major positive feedback in a warming climate. Furthermore, permafrost thawing has a significant impact on Northern communities and on the development of the Arctic. Quantifying the rate of permafrost evolution and mitigating its effect is therefore an issue of scientific, societal and economic interest.Today, accurate predictions of permafrost thawing are not possible because processes that determine the permafrost thermal regime are insufficiently understood. In particular, land surface models (LSMs) neglect or oversimplify many key processes, such as snow and top soil physical properties. This is especially the case for the thermal conductivity of the snow, top soil and vegetation layers, and how these variables depend on the type of vegetation. Since snow acts as an insulating layer that limits permafrost winter cooling, effect of the snowpack on the surface energy budget must be quantified accurately. We propose to set up snow instrumental stations in the subarctic and Arctic to monitor continuously snow and top soil physical properties, in particular their thermal conductivity. We will use novel systems that we have tested in the Alps for that purpose. We will also study the radiative budget of the snow surface, focussing on light scattering, by measuring the specific surface area of surface snow.We have selected sites ranging from the boreal forest to high arctic tundra. The data obtained will serve to produce novel parameterizations in a coupled snow-soil model, which will be coupled to LSMs to predict the thermal evolution of permafrost under warming scenarios. Collaborations with experts in polar vegetation and permafrost, in particular those involved in the NSERC-funded ADAPT project have been established. Collaborations with modelling groups (Meteo France and the Canadian Centre for Climate Modelling) have been initiated and we will also act as a link between both these groups.