11.1.3.3. Precipitation

Figure 11-6: Projected changes in area-averaged annual (a) and seasonal [(b) DJF and (c) JJA)] mean surface air temperature and precipitation over land regions of Asia for the 2050s and 2080s as obtained in select AOGCMs.

In general, all AOGCMs simulate an enhanced hydrological cycle and an increase in annual mean rainfall over most of Asia (Giorgi and Francisco, 2000). An area-averaged annual mean increase in precipitation of 3±1% in the 2020s, 7±2% in the 2050s, and 11±3% in the 2080s over the land regions of Asia is projected as a result of future increases in the atmospheric concentration of GHGs. Under the combined influence of GHGs and sulfate aerosols, the projected increase in precipitation is limited to 2±1% in the decade 2020s, 3±1% in the 2050s, and 7±3% in the 2080s. Figure 11-6 depicts projected changes in precipitation relative to changes in surface air temperature, averaged for land regions of Asia for each of the four skilled AOGCMs on an annual mean basis as well as during winter and summer for the 2050s and 2080s. The increase in precipitation is maximum during NH winter for both the time periods (Lal and Harasawa, 2000b). Clearly, intermodel differences in projections of precipitation are relatively large particularly during the winter even when they are averaged for the entire Asian continent—suggesting low confidence in projections of future precipitation in current AOGCMs.

The increase in annual mean precipitation is projected to be highest in boreal Asia. During the winter, boreal Asia and the Tibetan Plateau have the most pronounced increase in precipitation (Table 11-2). Over central Asia, an increase in winter precipitation and a decrease in summer precipitation are projected. Because the rainfall over this region is already low, severe water stress conditions—leading to expansion of deserts—are quite possible with a rise in surface air temperature here. The area-averaged annual mean and winter precipitation is projected to increase in temperate Asia. The models show high uncertainty in projections of future winter and summer precipitation over south Asia (with or without direct aerosol forcings). The effect of sulfate aerosols on Indian summer monsoon precipitation is to dampen the strength of the monsoon compared to that seen with GHGs only (Lal et al., 1995a; Mitchell et al., 1995; Cubasch et al., 1996; Roeckner et al., 1999). The overall effect of the combined forcing is at least partly dependent on the land/sea distribution of aerosol forcing and on whether the indirect effect is included along with the direct effect. To date, the effect of aerosol forcing (direct and indirect) on the variability of the monsoon has not been investigated.

Recent observations suggest that there is no appreciable long-term variation in the total number of tropical cyclones observed in the north Indian, southwest Indian, and southwest Pacific Oceans east of 160°E (Neumann, 1993; Lander and Guard, 1998). For the northwest subtropical Pacific basin, Chan and Shi (1996) found that the frequency of typhoons and the total number of tropical storms and typhoons has been more variable since about 1980. Several studies since the SAR have considered likely changes in tropical cyclones (Henderson-Sellers et al., 1998; Knutson et al., 1998; Krishnamurti et al., 1998; Royer et al., 1998). Some of these studies suggest an increase in tropical storm intensities with carbon dioxide (CO₂)-induced warming.

Some of the most pronounced year-to-year variability in climate features in many parts of Asia has been linked to ENSO. Since the SAR, analysis of several new AOGCM results indicates that as global temperatures increase, the Pacific climate will tend to resemble a more El Niño-like state (Mitchell et al., 1995; Meehl and Washington, 1996; Knutson and Manabe, 1998; Boer et al., 1999; Timmermann et al., 1999). Collins (1999) finds an increased frequency of ENSO events and a shift in their seasonal cycle in a warmer atmosphere: The maximum occurs between August and October rather than around January as currently observed. Meehl and Washington (1996) indicate that future seasonal precipitation extremes associated with a given ENSO event are likely to be more intense in the tropical Indian Ocean region; anomalously wet areas could become wetter, and anomalously dry areas could become drier during future ENSO events.

Several recent studies (Kitoh et al., 1997; Lal et al., 2000) have confirmed earlier results (Kattenberg et al., 1996) indicating an increase in interannual variability of daily precipitation in the Asian summer monsoon with increased GHGs. Lal et al. (2000) also report an increase in intraseasonal precipitation variability and suggest that intraseasonal and interannual increases are associated with increased intraseasonal convective activity during the summer. The intensity of extreme rainfall events is projected to be higher in a warmer atmosphere, suggesting a decrease in return period for extreme precipitation events and the possibility of more frequent flash floods in parts of India, Nepal, and Bangladesh (Lal et al., 2000). However, Lal et al. (1995b) found no significant change in the number and intensity of monsoon depressions (which are largely responsible for the observed interannual variability of rainfall in the central plains of India) in the Bay of Bengal in a warmer climate. Because much of tropical Asia is intrinsically linked with the annual monsoon cycle, a better understanding of the future behavior of the monsoon and its variability is warranted for economic planning, disaster mitigation, and development of adaptation strategies to cope with climate variability and climate change.

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