11.2.3. Projections of Future Climates: Regional Scenarios

11.2.3.1. Background Information

The first detailed climate scenario for south and southeast Asia was developed by the Climate Impact Group (1992), as part of the Asian Development Bank's 1994 regional study on global environmental issues (including climate change); the methodology is outlined by Whetton (1994). Analyses were carried out using data obtained in experiments with four general circulation models (GCMs): the Canadian Climate Centre model (CCCJ1), the United Kingdom Meteorological Office model (UKMOH), the Geophysical Fluid Dynamics model (GFDLA), and the Australian CSIRO9 model. All of the experiments used an atmospheric model coupled to a simplified ocean model and were run to equilibrium conditions for present levels of greenhouse gases (GHGs) and for doubled CO₂ levels.

Comparisons of the control simulation of the GCMs with present-day climatology included results for the region's surface temperature, mean sea-level pressure, and precipitation. The broad-scale observed patterns are well simulated, including the summer and winter monsoons. Results described by Suppiah (1994), based on the aforementioned models, indicated strengthening of the monsoon circulation and an increase in wet-season rainfall under enhanced greenhouse conditions-that is, an increase in summer rainfall in the southWest monsoon region and an increase in winter rainfall in the northeast monsoon region.

11.2.3.2. Temperature and Rainfall

Temperature and rainfall scenarios are based on simulated changes averaged over two broad seasons: the southWest monsoon and the northeast monsoon. Temperature scenarios for Tropical Asia reported by Whetton (1994) and the Climate Impact Group (1992) suggest that temperature would increase throughout most of the region, although the amount of warming is projected to be less than the global average. Moreover, results presented in Table 11-3 indicate that there may be differences within the region, depending on proximity to the sea. Thus, warming is projected to be least in the islands and coastal areas throughout Indonesia, the Philippines, and coastal south Asia and Indo-China and greatest in inland continental areas of south Asia and Indo-China-except from June to August in south Asia, where reduced warming could occur.

In terms of rainfall, the models considered by Whetton (1994) suggested an April-to-September maximum over south Asia and the Indo-China peninsula and a minimum over Indonesia and areas near Australia. Projections of regionally averaged changes in rainfall for the years 2010 and 2070 are given in Table 11-4.

Other simulations of changes in rainfall indicated a tendency for an increase in wet-season rainfall in both monsoon regions, with changes ranging from -5% to +18% (Climate Impact Group, 1992). More consistent and much larger rainfall increases are projected for the south Asia subregion wet season, with values ranging from +17% to +59%. Changes in dry-season rainfall are less consistent and are estimated only as broad-scale regional rainfall changes; local-scale changes could be much greater.

Systematic increases in average rainfall intensity are a common feature in simulated daily rainfall experiments, along with associated increases in the projected frequency of heavy rainfall events. Whetton et al. (1994) conclude that there is reason for higher confidence in increasing rainfall intensity in south and southeast Asia under enhanced greenhouse conditions than in increases or decreases in total rainfall in particular regions.

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