Why is Vegetation Type a First-Order Climate Forcing?

As shown in Figure 1-1 in the 2005 National Research Council report, flora and fauna are climate forcings. A new paper which has just appeared (see Sturm et al. 2005: Changing snow and shrub conditions affect albedo with global implications. J. Geophys. Res., 110, G01004, doi:10.1029/2005JG000013) provides a further quantification of this effect where shrub abundance in the Arctic is increasing. The authors attribute this to a general temperature increase in the region, and suggest that the tundra to shrub conversion further enhances an increase in Arctic temperatures. This occurs because the shrubs have a lower albedo than does the tundra. This specific climate forcing was discussed in the 2005 National Research Council report (see topic 3 ), as well as in several of our papers (Eugster et al. 2000: Land-atmosphere energy exchange in Arctic tundra and boreal forest: available data and feedbacks to climate. Global Change Biology, 6, 84-115; McFadden et al. 2001: Interactions of shrubs and snow in Arctic tundra: Measurements and models. In: Soil-Vegetation-Atmosphere Transfer Schemes and Large-scale Hydrological Models, IAHS Press, Wallingford, Oxfordshire, UK,IAHS Publ. no 270, 317-325; and Liston et al. 2002: Modelled changes in arctic tundra snow, energy and moisture fluxes due to increased shrubs. Global Change Biology, 8, 17-32.)

There are remaining important issues with respect to this study. While the authors attribute tundra to shrub conversion to an increase in temperature, the conversion may also be influenced by the biogeophysical and biogeochemical effect of the observed increased atmospheric concentration of carbon dioxide over the last several decades. Shrubs may preferentially grow with respect to tundra when the carbon dioxide concentrations are higher. Indeed, the authors recognize that a range of vegetation/soil feedbacks need to be investigated before it can be definitively concluded that the tundra to shrub conversion is a general arctic (and therefore global) warming effect.

This study clearly illustrates, however, that vegetation type change even in the natural system is a first-order climate forcing and represents another global heat change effect (global warming is discussed in our August 29th blog). While the role of arctic vegetation in the carbon budget has been extensively investigated, its role in the surface energy budget has not. This new research further demonstrates why we need to look beyond the radiative forcing of carbon dioxide, methane, and the other well-mixed greenhouse gases if we are going to be able to understand global warming and climate change in general.

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