Exploring new climate feedbacks in the Arctic

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The Arctic is warming at over twice the planetary rate because of specific feedbacks such as the snow-albedo feedback. Discovering and quantifying other feedbacks is essential for reliable projections of Arctic and planetary climate. The thawing of permafrost and the resulting atmospheric release of permafrost carbon as CO2 and CH4 has not been included in the latest IPCC climate projections, illustrating our insufficient understanding of Arctic climate feedbacks. I will study feedbacks relevant to the evolution of the permafrost thermal regime and to its carbon budget. Warming favors shrubs growth on tundra, leading to enhanced permafrost warming because of the extra thermal insulation provided by deeper snow in shrubs. This effect must be quantified. We first observed that the snow thermal conductivity was lower in shrubs, enhancing their permafrost warming potential. Alternatively, shrubs, by absorbing solar radiation in the fall, can lead to snow melt and the formation of melt-freeze crusts of high thermal conductivity, possibly reversing the shrub warming effect. These complex snow-shrubs-permafrost interactions will be studied by: (1) observing the correlation between meteorology, shrub properties and snow physical properties; (2) measuring the impact of shrubs on the winter surface energy budget. This requires measuring albedo but also vertical irradiance profiles as solar radiation penetrates >10 cm depth in the snow; (3) performing 3-D radiation transfer simulations using the DART model to understand and predict shrub impact on the 3-D radiation field, energy deposition in the snowpack and on snow metamorphism; (4) quantify soil shading in summer by measuring radiation transfer, therefore gaining a year-round understanding of shrub impact on the radiation budget of the surface and on the permafrost thermal regime; (5) implementing findings in a land surface and snow physics model.***Shrub growth also leads to carbon sequestration by enhancing biomass and litter buildup, possibly mitigating the microbial respiration of thawed old carbon. I will study the evolution of carbon stocks of soils near Umiujaq, Nunavik, where lichen tundra is being invaded by dwarf birch shrubs. Soils there contain almost no old carbon, facilitating the quantification of new carbon. Biomass and litter will be quantified, and shrub age obtained by dendrochronology. The structure of the organic matter in litter will be analyzed to understand its degradation kinetics and quantify mid- to long-term storage potential. ***Lastly, biologists and I will collaborate on the interactions between snow, climate and the population dynamics of lemmings. We will test the hypothesis that rain on snow events in the fall, by leading to a hard basal snow layer, impede the ability of lemmings to feed and reproduce in winter, thus preventing a population irruption.