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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
cloud droplet number are available yet. Therefore, and because of their universality, the physically based approaches described formerly should be used in future studies of aerosol-cloud interactions.
Since the first IPCC assessment, great improvements have been made in the description of cloud microphysics for large-scale clouds. Whereas early studies diagnosed cloud amount based on relative humidity, most models now predict cloud condensate in large-scale clouds. The degree of sophistication varies from just predicting the sum of cloud water and ice (Rasch and Kristjánsson, 1998) to predicting cloud water, cloud ice, snow, and rain as separate species (Fowler et al., 1996). Because the aerosol indirect effect is based on the change in cloud droplet number concentration, some models predict cloud droplet number concentrations using one of the above-described physically based aerosol activation schemes as a source term for cloud droplets (Ghan et al., 1997; Lohmann et al., 1999). There is currently a great discrepancy in models between the sophisticated treatment of cloud microphysics in large-scale clouds and their very rudimentary treatment in convective clouds. Furthermore, there is a mismatch between aerosol activation and cloud formation in most climate models because cloud formation relies on a saturation adjustment scheme whereas aerosol activation relies on a subgrid-scale vertical velocity. Part of this problem will be solved within the next decade when climate models can be run at higher spatial resolution and with smaller time steps.
Including Land Surface Models
Changes in land use pose a nonnegligible climate forcing as well. Climate models are just beginning to include detailed land surface models that are coupled to the simulation of the atmosphere. Also, carbon-cycle feedbacks have been shown to be very important in predicting climate change over the next century (e.g., Schimel et al., 2001; Jones et al., 2003). One important question is whether the terrestrial carbon cycle becomes a net source of carbon dioxide during the next century. To address this issue, vegetation-meteorology-biogeochemical cycle interactions need to be included in climate models.
Diabatic Forcing Heterogeneity
A variety of heterogeneous diabatic forcings have been shown to alter the climate both in the region where this forcing occurs and at large distances through teleconnections. These forcings include land-cover change and vegetation dynamics, soil moisture, ocean color, and aerosols (e.g., Chung and Ramanathan, 2003; Shell et al., 2003; Claussen et al., 2004). On the regional scale, there is general agreement on the importance of these