cycle and through changes in surface albedo owing to land surface and land cover changes, natural or anthropogenic.
Simple model representations of vegetation in general circulation models (GCMs) suggest that the main influence of vegetation in greenhouse warming, and the main issue in comparing preindustrial (natural) vegetation to industrial (cultivated) vegetation, takes the form of albedo. Most notably, dark forests show minimal winter albedo influence in response to snow cover, whereas the loss of such forests exposes large areas that suddenly influence albedo considerably during summer and even more so during winter in the presence of snow cover. Hansen et al. (1997) show this land-use shift to be the major contribution of vegetation changes to a doubled CO2-induced warming, with vegetation changes accounting for approximately 25 percent of the CO2 direct influence. Changes in land surface characteristics can lead to similar effects, realized mostly, again, through albedo. Vegetation and land surfaces also affect climate through the hydrological cycle, as discussed in the section on the hydrological cycle below.
At the present time, our knowledge is poor regarding the effects of different vegetation covers in climate change. We must improve even our most basic understanding of these effects.
What are the effects of human activity and climate change on ecosystem structure and function? From paleoclimatic records we know that natural vegetation responds to climatic change: individual species respond according to their climatic tolerances, and ecosystem compositions change as a result. Competitive and trophic interactions among species are thereby altered, redefining where organisms can survive and reproduce. The responses of organisms to climatic change will be greatly influenced by human land-use patterns and other anthropogenic influences. Associated with ecosystem structural changes are changes in the biogeochemical cycling of carbon and nutrients, in ways that remain difficult to anticipate. Finally, the distribution of disease-carrying organisms will change as part of ecosystem restructuring and redistribution.
What are the relative contributions of the different processes by which vegetation and soils store or lose carbon? Vegetation and soils store three times as much carbon as the atmosphere or upper ocean, yet large uncertainties remain about the quantitative contributions of various processes. Forest regrowth in the northern hemisphere, related to changing