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Agricultural adaptation to climatic variability is an evolving process. Planned interventions through research, extension, or pricing or marketing policies can have inadvertent detrimental impacts for poor farmers. Agricultural productivity in tropical Asia is sensitive not only to temperature increases but also to changes in the nature and characteristics of monsoons. An increase in leaf surface temperatures would have significant effects on crop metabolism and yields, and it may make crops more sensitive to moisture stress (Riha et al., 1996). Cropping systems may have to change to include growing suitable cultivars (to counteract compression of crop development), increasing crop intensities (i.e., the number of successive crops produced per unit area per year), or planting different types of crops (Sinha et al., 1998). Farmers will have to adapt to changing hydrological regimes by changing crops. For example, farmers in Pakistan may grow more sugarcane if additional water becomes available, and they may grow less rice if water supplies dwindle. The yield ceiling must be raised and the yield gap narrowed while maintaining sustainable production and a friendly environment. Development of new varieties with higher yield potential and stability is complementary to bridging the yield gap. Efficient production of a socially optimal level of agricultural output in this region ultimately may depend on biotechnological applications, but only if these applications prove to be environmentally sustainable.
Groundwater is the main source of freshwater in many parts of tropical Asia, particularly in semi-arid regions. Water resources already are limited in terms of supply and demand in this region. The aquifers in most countries have been depleted by high withdrawal and low recharge rates, and significant drawdown problems exist. Even with increases in precipitation, surface runoff may diminish in some river basins under projected climate change scenarios because of greater evaporation in a warmer atmosphere (higher hydrological elasticity). Increased runoff in some river basins can cause deleterious effects such as greater flooding, waterlogging, and salinity. More than 25% of the irrigated land in the Indus basin already is affected by waterlogging and salinization (Hillel, 1991). Freshwater availability in the coastal regions is likely to undergo substantial changes as a result of a series of chain effects. Improvements in runoff management and irrigation technology (e.g., river runoff control by reservoirs, water transfers, and land conservation practices) will be crucial. Increasing efforts should be directed toward rainwater harvesting and other water-conserving practices to slow the decline in water levels in aquifers. Recycling of wastewater should be encouraged in drought-prone countries in tropical Asia. However, major water development decisions to augment water supplies may have greater relative hydrological, environmental, and social impacts than climate change per se in the shorter term. Climate change will affect the benefits to be accrued by future water development projects and therefore should be taken into consideration in the context of water resource policies and planning.
With rapid development of the economy in several countries in tropical Asia during recent decades, the patterns of land use and land cover have been modified significantly; a sequence of transitions and conversions is discernable. As a result of different natural and socioeconomic conditions, the speed and scale of land-use change is very diverse in different parts of Asia. This process has contributed to significant losses in total forest cover; changes in standing biomass and the soil carbon budget; extinction of mammals, birds, and vascular plants; soil degradation; and threats to food security. Excessive human and livestock population pressure in association with inappropriate agricultural extension activities also lend an explanation for widespread land degradation in tropical Asia. Salinization and acidification of soil in low-lying coastal areas would adversely affect cropland in addition to land losses from permanent inundation of deltas from anticipated sea-level rise.
Many of the major rivers originating in mountains and highlands are charged with sediments, depending on the types of land uses in the watersheds (e.g., from forestry and agroforestry to open agriculture). Marginalization of production areas in highlands will continue to increase soil losses, land slips, and slides in the region. It has been suggested that upland micro-watersheds can be hydroecologically sustainable only if good forest cover and dense forests are maintained (Rai, 1999). Under changing climate conditions, the pressure toward small- and large-scale transfers of land for agricultural and urban uses may grow.
Sea-level rise poses the greatest threat and challenge for sustainable adaptation within south and southeast Asia. The sea level already is rising in many locations, primarily as a result of geological processes and anthropogenic manipulations. Projected sea-level rise along the Asian coastlines represents an increase of three to four times over present rates (Chansang, 1993; Midun and Lee, 1995; Mimura and Harasawa, 2000). The potential impacts of accelerated sea-level rise include inundation of low-lying deltas and estuaries, retreat of shorelines, and changes in the water table (Wong, 1992; Sivardhana, 1993). Episodic flooding from high storm surges would penetrate much further inland. Salinization and acidification of soil in low-lying coastal areas will adversely affect agricultural production, in addition to land losses from permanent inundation of deltas from anticipated sea-level rise. The impacts will vary from region to region because of local factors such as land subsidence, susceptibility to coastal erosion or sedimentation, varying tidal ranges, and cyclonicity (Bird, 1993). The Ganges-Brahmaputra in Bangladesh, the Irrawaddy in Myanmar, the Choo Phraya in Thailand, and the Mekong and Song Hong in Vietnam are among the key low-lying river deltas in tropical Asia that are most vulnerable to sea-level rise. Local-level social and institutional adaptations in sensitive regions and development and promotion of risk management can potentially prevent accelerated impacts to these deltas through protection of the ecology of the region from further human interventions so they can sustain themselves at least in a short time horizon.
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