aerosol and cloud optical depth cause a reduction of solar radiation at the surface. For the surface energy balance to reach a new equilibrium state, the latent and sensible fluxes have to adjust. Because evaporation has to equal precipitation on the global scale, a reduction in the latent heat flux leads to a reduction in precipitation. As shown in model simulations by Liepert et al. (2004), despite an increase in greenhouse gases, increases in optical depth due to the direct and indirect anthropogenic aerosol effects can cause a reduction in evaporation and precipitation. This mechanism is consistent with observations of decreased evaporation from open pans of water over the last 50 years. Roderick and Farquhar (2002) characterized steadily decreasing pan evaporation due to decreases in solar irradiance resulting from increasing cloud coverage and aerosol concentration. Increasing aerosol optical depth associated with scattering aerosols alone in otherwise clear skies has been shown to produce a larger fraction of diffuse radiation at the surface, which results in greater carbon assimilation into vegetation (and therefore greater transpiration) without a substantial reduction in the total surface solar radiation (Niyogi et al., 2004).

LAND-COVER AND LAND-USE CHANGES

Land-use changes include irrigation, urbanization, deforestation, desertification, reforestation, grazing of domestic animals, and dryland farming. Each of these alterations in landscape produces significant changes in radiative forcing (e.g., Pitman, 2003; Kabat et al., 2004). Global maps of land-cover changes over the past 300 years are shown in Figure 2-3. In addition, changes in tropical forests have been reported by O’Brien (2001). There are historical land use datasets (e.g., Ramankutty and Foley, 1999) as well as satellite-based land-cover datasets. Satellite products include the DISCover dataset developed under the auspices of the International Geosphere-Biosphere Programme (Loveland et al., 2000) and land-cover datasets based on MODIS/Terra data (Strahler et al., 1999).

Land-use and land-cover changes can affect the Earth’s radiative bal-



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