Changing snow and shrub conditions affect albedo with global implications

Abstract

[1] 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.Citation: Sturm, M., T. Douglas, C. Racine, and G. E. Liston (2005), Changing snow and shrub conditions affect albedo with global implications,

Characterizing snowmelt variability in a land‐surface‐hydrologic model

“…Another important process that contributes to the snowmelt variability in Arctic basins is the difference in melt rates introduced by the shrub canopy in shrub tundra areas compared to open tundra areas that have only a short vegetation cover (Liston et al, 2002;Marsh et al, 2003;Sturm et al, 2005). Figure 9 illustrates the difference in SWE between the simulated shrub tundra and simulated open tundra areas for the duration of the model runs.…”

Soil temperature response to 21st century global warming: the role of and some implications for peat carbon in thawing permafrost soils in North America

“…Heat transport in snow is largely dependent on the microstructure of the snow (pore and grain distribution and size) which is difficult to quantify (Sturm et al, 1997), particularly for continental-scale simulations. The thermal conductivity of snow is therefore often calculated using empirical functions that are based on measurable properties of snow.…”

“…Anticipated changes in the vegetation as a result of warming air temperatures towards shrubbier vegetation could potentially provide more shading, change snowpack and alter heat flux into the soil as well (Sturm et al, 2005).…”

“…Soil temperatures are not only influenced by air temperatures but also by the presence or absence of an insulating layer of snow on the surface (Sturm et al, 1997). The response of soil temperatures to changes in air temperature strongly depends on the temporal distribution, depth, and density of the snowpack during the winter (Lawrence and Slater, 2010).…”

Two years with extreme and little snowfall: effects on energy partitioning and surface energy exchange in a high-Arctic tundra ecosystem

“…This pattern was reflected in the average air temperature of the two snowmelt periods which showed only small differences between 2013 and 2014 although the snowmelt period in 2014 was much longer compared to 2013. The combination of snow layer thickness, snow physical properties such as density or grain size (Warren, 1982), the fraction of exposed dark underlying surface and type of vegetation (Sturm and Douglas, 2005) controls the snow and soil surface albedo, with direct impact on R net . Our results showed that low and partly absent snow cover in the winter 2012/2013 resulted in a short snowmelt period dominated by low albedo of the snow-soil surface and increased average R net .…”