10.3 Assumptions about future trends

10.3.1 Climate

Table 10.5 provides a snapshot of the projections on likely increase in area-averaged seasonal surface air temperature and percent change in area-averaged seasonal precipitation (with respect to the baseline period 1961 to 1990) for the seven sub-regions of Asia. The temperature projections for the 21st century, based on Fourth Assessment Report (AR4) Atmosphere-Ocean General Circulation Models (AOGCMs), and discussed in detail in Working Group I Chapter 11, suggest a significant acceleration of warming over that observed in the 20th century (Ruosteenoja et al., 2003; Christensen et al., 2007). Warming is least rapid, similar to the global mean warming, in South-East Asia, stronger over South Asia and East Asia and greatest in the continental interior of Asia (Central, West and North Asia). In general, projected warming over all sub-regions of Asia is higher during northern hemispheric winter than during summer for all time periods. The most pronounced warming is projected at high latitudes in North Asia. Recent modelling experiments suggest that the warming would be significant in Himalayan Highlands including the Tibetan Plateau and arid regions of Asia (Gao et al., 2003).

The consensus of AR4 models, as discussed in Chapter 2 and in Christensen et al. (2007) and confirmed in several studies using regional models (Lal, 2003; Rupa Kumar et al., 2003; Kwon et al., 2004; Boo et al., 2004; Japan Meteorological Agency, 2005; Kurihara et al., 2005), indicates an increase in annual precipitation in most of Asia during this century; the relative increase being largest and most consistent between models in North and East Asia. The sub-continental mean winter precipitation will very likely increase in northern Asia and the Tibetan Plateau and likely increase in West, Central, South-East and East Asia. Summer precipitation will likely increase in North, South, South-East and East Asia but decrease in West and Central Asia. The projected decrease in mean precipitation in Central Asia will be accompanied by an increase in the frequency of very dry spring, summer and autumn seasons. In South Asia, most of the AR4 models project a decrease of precipitation in December, January and February (DJF) and support earlier findings reported in Lal et al. (2001b).

An increase in occurrence of extreme weather events including heatwave and intense precipitation events is also projected in South Asia, East Asia, and South-East Asia (Emori et al., 2000; Kato et al., 2000; Sato, 2000; Lal, 2003; Rupa Kumar et al., 2003; Hasumi and Emori, 2004; Ichikawa, 2004; May, 2004b; Walsh, 2004; Japan Meteorological Agency, 2005; Kurihara et al., 2005) along with an increase in the interannual variability of daily precipitation in the Asian summer monsoon (Lal et al., 2000; May, 2004a; Giorgi and Bi, 2005). Results of regional climate model experiments for East Asia (Sato, 2000; Emori et al., 2000; Kato et al., 2000; Ichikawa, 2004; Japan Meteorological Agency, 2005; Kurihara et al., 2005) indicate that heatwave conditions over Japan are likely to be enhanced in the future (Figure 10.2). Extreme daily precipitation, including that associated with typhoon, would be further enhanced over Japan due to the increase in atmospheric moisture availability (Hasumi and Emori, 2004). The increases in annual temperature and precipitation over Japan are also projected regionally using regional climate model (Figure 10.3; Japan Meteorological Agency, 2005; Kurihara et al., 2005).

Figure 10.2. Projected number of hot days (>30°C) and days of heavy rainfall (>100 mm/day) by the high resolution general circulation model (Hasumi and Emori, 2004).

Figure 10.3. Projected change in annual mean surface air temperature (°C) and rainfall (%) during 2080 to 2100 period compared to 1981 to 2000 period simulated by a high resolution regional climate model (left: annual temperature, right: annual precipitation, Japan Meteorological Agency, 2005; Kurihara et al., 2005).

An increase of 10 to 20% in tropical cyclone intensities for a rise in sea-surface temperature of 2 to 4°C relative to the current threshold temperature is likewise projected in East Asia, South-East Asia and South Asia (Knutson and Tuleya, 2004). Amplification in storm-surge heights could result from the occurrence of stronger winds, with increase in sea-surface temperatures and low pressures associated with tropical storms resulting in an enhanced risk of coastal disasters along the coastal regions of East, South and South-East Asian countries. The impacts of an increase in cyclone intensities in any location will be determined by any shift in the cyclone tracks (Kelly and Adger, 2000).

In coastal areas of Asia, the current rate of sea-level rise is reported to be between 1 to 3 mm/yr which is marginally greater than the global average (Dyurgerov and Meier, 2000; Nerem and Mitchum, 2001; Antonov et al., 2002; Arendt et al., 2002; Rignot et al., 2003; Woodworth et al., 2004). A rate of sea-level rise of 3.1 mm/yr has been reported over the past decade compared to 1.7 to 2.4 mm/yr over the 20th century as a whole (Arendt et al., 2002; Rignot et al., 2003), which suggests that the rate of sea-level rise has accelerated relative to the long-term average.