Tropical Asia is physiographically diverse and ecologically rich in natural and crop-related biodiversity. The present total population of the region is about 1.6 billion, and it is projected to increase to 2.4 billion by 2025. The population is principally rural-based, although in 1995, the region included 6 of the 25 largest cities in the world. The climate in Tropical Asia is characterized by seasonal weather patterns associated with the two monsoons and the occurrence of tropical cyclones in the three core areas of cyclogenesis (the Bay of Bengal, north Pacific Ocean, and South China Sea). Climate change will add to other stresses such as rapid urbanization, industrialization, and economic development, which contribute to unsustainable exploitation of natural resources, increased pollution, land degradation, and other environmental problems.
Ecosystems: Substantial elevational shifts of ecosystems in the mountains and uplands of Tropical Asia are projected. At high elevation, weedy species can be expected to displace tree species-though the rates of vegetation change could be slow compared to the rate of climate change and constrained by increased erosion in the Greater Himalayas. Changes in the distribution and health of rainforest and drier monsoon forest will be complex. In Thailand, for instance, the area of tropical forest could increase from 45% to 80% of total forest cover, whereas in Sri Lanka, a significant increase in dry forest and a decrease in wet forest could occur. Projected increases in evapotranspiration and rainfall variability are likely to have a negative impact on the viability of freshwater wetlands, resulting in shrinkage and desiccation. Sea-level rise and increases in sea-surface temperature are the most probable major climate change-related stresses on coastal ecosystems. Coral reefs may be able to keep up with the rate of sea-level rise but suffer bleaching from higher temperatures. Landward migration of mangroves and tidal wetlands is expected to be constrained by human infrastructure and human activities.
Hydrology and Water Resources: The Himalayas have a critical role in the provision of water to continental monsoon Asia. Increased temperatures and increased seasonal variability in precipitation are expected to result in increased recession of glaciers and increasing danger from glacial lake outburst floods. A reduction in average flow of snow-fed rivers, coupled with an increase in peak flows and sediment yield, would have major impacts on hydropower generation, urban water supply, and agriculture. Availability of water from snow-fed rivers may increase in the short term but decrease in the long run. Runoff from rain-fed rivers may change in the future. A reduction in snowmelt water will put the dry-season flow of these rivers under more stress than is the case now. Increased population and increasing demand in the agricultural, industrial, and hydropower sectors will put additional stress on water resources. Pressure on the drier river basins and those subject to low seasonal flows will be most acute. Hydrological changes in island and coastal drainage basins are expected to be relatively small in comparison to those in continental Tropical Asia, apart from those associated with sea-level rise.
Food and Fiber Production: The sensitivity of major cereal and tree crops to changes in temperature, moisture, and CO2 concentration of the magnitudes projected for the region has been demonstrated in many studies. For instance, impacts on rice yield, wheat yield, and sorghum yield suggest that any increase in production associated with CO2 fertilization will be more than offset by reductions in yield from temperature or moisture changes. Although climate change impacts could result in significant changes in crop yields, production, storage, and distribution, the net effect of the changes regionwide is uncertain because of varietal differences; local differences in growing season, crop management, etc.; the lack of inclusion of possible diseases, pests, and microorganisms in crop model simulations; and the vulnerability of agricultural areas to episodic environmental hazards, including floods, droughts, and cyclones. Low-income rural populations that depend on traditional agricultural systems or on marginal lands are particularly vulnerable.
Coastal Systems: Coastal lands are particularly vulnerable; sea-level rise is the most obvious climate-related impact. Densely settled and intensively used low-lying coastal plains, islands, and deltas are especially vulnerable to coastal erosion and land loss, inundation and sea flooding, upstream movement of the saline/freshwater front, and seawater intrusion into freshwater lenses. Especially at risk are large delta regions of Bangladesh, Myanmar, Viet Nam, and Thailand, and the low-lying areas of Indonesia, the Philippines, and Malaysia. Socioeconomic impacts could be felt in major cities and ports, tourist resorts, artisinal and commercial fishing, coastal agriculture, and infrastructure development. International studies have projected the displacement of several millions of people from the region's coastal zone, assuming a 1-m rise in sea level. The costs of response measures to reduce the impact of sea-level rise in the region could be immense.
Human Health: The incidence and extent of some vector-borne diseases are expected to increase with global warming. Malaria, schistosomiasis, and dengue-which are significant causes of mortality and morbidity in Tropical Asia-are very sensitive to climate and are likely to spread into new regions on the margins of presently endemic areas as a consequence of climate change. Newly affected populations initially would experience higher fatality rates. According to one study that specifically focused on climate influences on infectious disease in presently vulnerable regions, an increase in epidemic potential of 12-27% for malaria and 31-47% for dengue and a decrease of schistosomiasis of 11-17% are anticipated under a range of GCM scenarios as a consequence of climate change. Waterborne and water-related infectious diseases, which already account for the majority of epidemic emergencies in the region, also are expected to increase when higher temperatures and higher humidity are superimposed on existing conditions and projected increases in population, urbanization, declining water quality, and other trends.
Conclusions: The potential direct effects of climate change assessed here, such as changes in water availability, crop yields, and inundation of coastal areas, all will have further indirect effects on food security and human health. The suitability of adaptation strategies to different climatic environments will vary across the diverse subregions and land uses of the region. Adaptive options include new temperature- and pest-resistant crop varieties; new technologies to reduce crop yield loss; improvements in irrigation efficiency; and integrated approaches to river basin and coastal zone management that take account of current and longer-term issues, including climate change.
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