Increases in atmospheric concentrations of GHGs from anthropogenic activities would warm the earth-atmosphere system. The radiative forcing inferred from likely future increases in GHGs and sulfate aerosols as prescribed under IS92a emission scenarios (Leggett et al., 1992; IPCC, 1995) has been used in recent numerical experiments performed with coupled atmosphere-ocean global climate models (AOGCMs). Projections of future regional climate change and also most of the impact assessment studies for Asia cited herein are based on these numerical experiments (see Chapter 3 for further details).

To develop climate change scenarios on regional scales, it is first necessary to examine if the coupled AOGCMs are able to simulate the dynamics of present-day regional climate. The multi-century control integrations of AOGCMs unforced by anthropogenic changes in atmospheric composition offer an excellent opportunity to examine the skill of individual models in simulating the present-day climate and its variability on regional scales (Giorgi and Francisco, 2000; Lal et al., 2000). A model validation exercise carried out for Asia and its subregions has indicated that each of these models shows large seasonal variations in surface air temperature over boreal Asia and only small seasonal variations over southeast tropical Asia (Figure 11-2). However, many of the AOGCMs have only limited ability for realistic portrayal of even large-scale precipitation distribution over Asia. Seasonal variations in observed precipitation over the South Asia region as a consequence of summer monsoon activity are poorly simulated by most of the models (Figure 11-3). Moreover, none of the seven models can reproduce the observed precipitation climatology over the Tibetan Plateau (Figure 11-4). Based on the pattern correlation coefficients and root mean square errors between the observed and model-simulated seasonal mean sea-level pressure, surface air temperature, and rainfall patterns over land regions of Asia and other relevant considerations in this validation exercise, the HadCM2, ECHAM4, CSIRO, and CCSR/NIES AOGCMs (developed at U.K. Hadley Climate Centre, German Climate Research Centre, Australian Commonwealth Scientific and Industrial Research Organisation, and Japanese Center for Climate System Research/National Institute for Environmental Studies, respectively) are rated to have some skill in simulating the broad features of present-day climate and its variability over Asia (Lal and Harasawa, 2000).

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