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Rice is central to nutrition in Asia. In 1997, rice provided about 700 kilocalories per person per day or more for approximately 2.9 billion people, most of whom live in developing countries of Asia and Africa. During the 1990s, rice production and productivity in Asia grew at a much slower rate than did population. Yield deceleration of rice (the annual growth rate declined from 2.8% in the 1980s to 1.1% in the 1990s) in Asia has been attributed to water scarcity, indiscriminate addition and inefficient use of inputs such as inorganic fertilizers and pesticides, and policy issues and the reliance on a narrower genetic material base with impacts on variability (Hazell, 1985; Matson et al., 1997; Naylor et al., 1997). Several other factors also have contributed to productivity stagnation and the decline of rice (lower output/input ratio) in the intensive cropping system (two to three rice crops per year). Key factors currently contributing to the yield gap in different countries of Asia include biophysical, technical/management, socioeconomic, institutional/policy, technology transfer, and adoption/linkage problems.

Urbanization in Asia has accentuated increased demand for fresh vegetables; this demand is to be met by new production areas combined with more intensified horticulture crop management to raise the productivity per unit of land and water. In most cases, urban and peri-urban agriculture initiatives with uncontrolled use of agrochemicals are a high-risk activity. Adequate steps need to be taken at regional and local levels to safeguard specialized and diversified urban production systems (vegetables, fruits, and root crops) through sustainable intensification of natural resource use and strengthening of decision support systems. Increased productivity and sustained production of food grains and legumes, industrial crops (oil, gum and resins, beverage, fiber, medicines, aromatic plants), and horticultural crops through crop diversification is critical for food and nutritional security in Asia.

Even minor deviations outside the "normal" weather range seriously impair the efficiency of externally applied inputs and food production. Moisture stress from prolonged dry spells or thermal stress resulting from heat-wave conditions significantly affect the agricultural productivity when they occur in critical life stages of the crop (Rounsevell et al., 1999). As reported in IPCC (1998), stress on water availability in Asia is likely to be exacerbated by climate change. Several studies aimed at understanding the nature and magnitude of gains or losses in yield of particular crops at selected sites in Asia under elevated CO₂ conditions and associated climatic change have been reported in the literature (e.g., Lou and Lin, 1999). These studies suggest that, in general, areas in mid- and high latitudes will experience increases in crop yield, whereas yields in areas in the lower latitudes generally will decrease (see also Chapter 5). Climatic variability and change will seriously endanger sustained agricultural production in Asia in coming decades. The scheduling of the cropping season as well as the duration of the growing period of the crop also would be affected.

In general, increased CO₂ levels and a longer frost-free growing season are expected to enhance agricultural productivity in north Asia. The area under wheat cultivation is likely to expand in the north and west. The increase in surface temperature also may increase the growing season in temperate Asia, thereby prolonging the grain-filling period, which may result in higher yields (Rosenzweig and Hillel, 1998). In Japan, for example, simulation studies and field experiments indicate that enhanced CO₂ levels in a warmer atmosphere will substantially increase rice yields and yield stability in northern and north-central Japan (Horie et al, 1995a). In south central and southwestern Japan, however, rice yields are expected to decline by at least 30% because of spikelet sterility and shorter rice growing duration (Matsui and Horie, 1992). Climate change should be advantageous to wheat yield in northeast China. Because of an increase in respiration in a warmer atmosphere demanding more water availability, rice yield in China is expected to decline (Wang, 1996a). In central and north China, higher temperatures during teaseling and drawing stages and low soil moisture could result in reduced wheat yield. Increases in precipitation should be favorable for pests, diseases, and weeds in the south (Wang, 1996b; Dai, 1997). In tropical Asia, although wheat crops are likely to be sensitive to an increase in maximum temperature, rice crops would be vulnerable to an increase in minimum temperature. The adverse impacts of likely water shortage on wheat productivity in India could be minimized to a certain extent under elevated CO₂ levels; these impacts, however, would be largely maintained for rice crops, resulting in a net decline in rice yields (Aggarwal and Sinha, 1993; Rao and Sinha, 1994; Lal et al., 1998d). Acute water shortage conditions combined with thermal stress should adversely affect wheat and, more severely, rice productivity in India even under the positive effects of elevated CO₂ in the future.

Key findings on the impacts of an increase in surface temperature and elevated CO₂ on rice production in Asia—based on a study carried out for Bangladesh, China, India, Indonesia, Japan, Malaysia, Myanmar, the Philippines, South Korea, and Thailand under the Simulation and System Analysis for Rice Production Project at the International Rice Research Institute—are summarized in Table 11-5. Two process-based crop simulation models—the ORYZA1 model (Kropff et al., 1995) and the SIMRIW model (Horie et al., 1995b)—suggest that the positive effects of enhanced photosynthesis resulting from doubling of CO₂ are more than offset by increases in temperature greater than 2°C (Matthews et al., 1995a).

Table 11-5: Model-simulated mean change (%) in potential yields of rice in Asia under fixed increments of air temperature and ambient CO2 level (Matthews et al., 1995b).
Model Used and Ambient CO2 Levels	Percent Change in Mean Potential Rice Yield in Asia resulting from Surface Air Temperature Increment of
	0°C	+1°C	+2°C	+4°C
ORYZA1 Model
340 ppm	0.00	-7.25	-14.18	-31.00
1.5xCO2	23.31	12.29	5.60	-15.66
2xCO2	36.39	26.42	16.76	-6.99
SIMRIW Model
340 ppm	0.00	-4.58	-9.81	-26.15
1.5xCO2	12.99	7.81	1.89	-16.58
2xCO2	23.92	18.23	11.74	-8.54

More than 10,000 different species of insect pest are found in the tropics, 90% of which are active in the humid tropics. The occurrence, development, and spread of crop diseases depend on integrated effects of pathogen, host, and environmental conditions. The survival rate of pathogens in winter or summer could vary with an increase in surface temperature (Patterson et al., 1999). Higher temperatures in winter will not only result in higher pathogen survival rates but also lead to extension of cropping area, which could provide more host plants for pathogens. Thus, the overall impact of climate change is likely to be an enlargement of the source, population, and size of pathogenic bacteria. Damage from diseases may be more serious because heat-stress conditions will weaken the disease-resistance of host plants and provide pathogenic bacteria with more favorable growth conditions. The growth, reproduction, and spread of disease bacteria also depend on air humidity; some diseases—such as wheat scab, rice blast, and sheath and culm blight of rice—will be more widespread in temperate and tropical regions of Asia if the climate becomes warmer and wetter.

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