Changing snow and shrub conditions affect albedo with global implications

Observations suggest that shrub abundance in the Arctic is increasing owing to climate warming. We investigated the ramifications of a tundra‐to‐shrubland transition on winter energy exchange. At five sites in Alaska we suspended a 50‐m‐long cable above the vegetation and from this measured how the vegetation interacted with the snow and affected albedo. The sites defined a gradient from nearly shrub‐free tundra to a woodland with a continuous shrub canopy. Where the shrubs were small, thin‐stemmed, and supple, they were bent and buried by snow. Where they were tall, thick‐stemmed, and stiff, the shrub canopy remained exposed all winter. Where shrubs were buried, mid‐winter albedo values were high (0.85), but where they were exposed, the values were 30% lower (0.60). At these latter sites, melting began several weeks earlier but proceeded more slowly. Consequently, all sites were free of snow about the same time. Using the measurements and a solar model, we estimate that a land surface transition from shrub‐free tundra to shrubland could produce a 69 to 75% increase in absorbed solar radiation during the snow‐cover period, depending on latitude. This is two thirds the increase associated with a tundra‐to‐forest transition. When combined with measurements showing that a tundra‐to‐shrub transition would also produce a net increase in summer heating, our results suggest a positive feedback mechanism associated with a warming‐induced increase in shrubs. To our knowledge, ours is the first study to document that shrubs could alter the winter energy balance of tundra in such a substantial way.