'Incorporating' vegetation and soil schemes into atmospheric general circulation climate models

The practical importance of studying climate derives from the dependence of mankind upon the processes that occur at the atmosphere-land interface; in particular, the availability of fresh water and the growth of vegetation for food and fibre. Despite this crucial dependence, relatively little attention has yet been paid by the climate modelling community to the accurate prediction of land surface climates and we are still uncertain about both the sensitivity of the overall climate system to land surface processes and the sensitivity of land surface climates to perturbations in the overall climate (for example, the impact of doubling atmospheric carbon dioxide). The physical characteristics of vegetation, and also the soil, control the absorption of solar radiation and emission of thermal infrared radiation, the exchanges of sensible and latent heat with the atmosphere and, to a lesser extent, the exchange of momentum between the atmosphere and the surface. Most modelling groups have acknowledged similar sensitivities to gross, global-scale changes in specified surface albedo, typically the impact of a 5% increase in albedo being a reduction in precipitation of between 5% and 20% (the Charney effect e.g. Charney, 1975). More recently it has been recognized that changes in soil moisture have at least comparably significant effects (e.g. Meehl and Washington, 1988). These land-surface induced climate sensitivities have been established using highly simplistic representations of the continental surface. Canopy-plus-soil 'big-leaf' models of the land surface now exist and are being incorporated in global climate models. These big-leaf models could be the basis for the incorporation of an interactive land biosphere into global models. However their use depends upon satisfactory specification of the distribution of plants and soils and while such distributions can be obtained for the present-day they must be manufactured for all other climatic scenarios.