Another Climate Feedback – The Influence Of Shrub Height In Tundra Regions On Northern High Latitude Climate”

In 2007 we published the paper

Strack, J., R.A. Pielke Sr., and G. Liston, 2007: Arctic tundra shrub invasion and soot deposition: Consequences for spring snowmelt and near-surface air temperatures. J. Geophys. Res., 112, G04S44, doi:10.1029/2006JG000297

with the abstract [highlight added]

Invasive shrubs and soot pollution both have the potential to alter the surface energy balance and timing of snow melt in the Arctic. Shrubs reduce the amount of snow lost to sublimation on the tundra during the winter leading to a deeper end-of-winter snowpack. The shrubs also enhance the absorption of energy by the snowpack during the melt season by converting incoming solar radiation to longwave radiation and sensible heat. Soot deposition lowers the albedo of the snow, allowing it to more effectively absorb incoming solar radiation and thus melt faster. This study uses the Colorado State University Regional Atmospheric Modeling System version 4.4 (CSU-RAMS 4.4), equipped with an enhanced snow model, to investigate the effects of shrub encroachment and soot deposition on the atmosphere and snowpack in the Kuparuk Basin of Alaska during the May–June melt period. The results of the simulations suggest that a complete invasion of the tundra by shrubs leads to a 2.2 C warming of 3 m air temperatures and a 108 m increase in boundary layer depth during the melt period. The snow-free date also occurred 11 d earlier despite having a larger initial snowpack. The results also show that a decrease in the snow albedo of 0.1, owing to soot pollution, caused the snow-free date to occur 5 d earlier. The soot pollution caused a 1.0 C warming of 3 m air temperatures and a 25 m average deepening of the boundary layer.

Now a new paper has appeared which further confirms our study. It is

C J W Bonfils et al, 2012: On the influence of shrub height and expansion on northern high latitude climate. Environmental Research Letters Volume 7 Number 1

with the abstract

There is a growing body of empirical evidence documenting the expansion of shrub vegetation in the circumpolar Arctic in response to climate change. Here, we conduct a series of idealized experiments with the Community Climate System Model to analyze the potential impact on boreal climate of a large-scale tundra-to-shrub conversion. The model responds to an increase in shrub abundance with substantial atmospheric heating arising from two seasonal land–atmosphere feedbacks: a decrease in surface albedo and an evapotranspiration-induced increase in atmospheric moisture content. We demonstrate that the strength and timing of these feedbacks are sensitive to shrub height and the time at which branches and leaves protrude above the snow. Taller and aerodynamically rougher shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs. These mechanisms increase, in turn, the strength of the indirect sea-ice albedo and ocean evaporation feedbacks contributing to additional regional warming. Finally, we find that an invasion of tall shrubs tends to systematically warm the soil, deepen the active layer, and destabilize the permafrost (with increased formation of taliks under a future scenario) more substantially than an invasion of short shrubs.

While the authors make the same old misstatement that the growth of the response is “in response to climate change“, when the shrubs actually are part of the climate, the Bonfils et al paper clearly exemplifies that the climate system is a more complex system than is assumed by the IPCC.   The shrubs could be increasing for a variety of reasons including nitrogen deposition from distant emissions, higher atmospheric concentrations of CO2, changes in biological stresses such as  from insects and herbivores, as well as from changes in temperatures and precipitation.  In any case, for these effects to be important to the shrubs, they must occur at the location where they grow which illustrates the need for a bottom-up view on the vulnerability of the environment to the spectrum of risks to this resource.

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