The 16 countries of Tropical Asia range in size from about 61,000 ha (Singapore) to 300 million ha (India). The region 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 the population is projected to increase to 2.4 billion by 2025; although this population is principally rural, in 1995, the region included 6 of the 25 largest cities in the world. Exploitation of natural resources associated with rapid urbanization, industrialization, and economic development has led to increasing pollution, land degradation, and other environmental problems. Climate change represents a further stress. Over the long period of human occupation in the region, human use systems have developed some resilience to a range of environmental stresses. However, it is uncertain whether such resilience can continue in the face of rapid socioeconomic development, increasing population, and projected changes in climate.
Climate Characteristics and Trends
Climate in Tropical Asia is characterized by seasonal weather patterns associated with the two monsoons and the occurrence of tropical cyclones in the two core areas of cyclogenesis (the northern Indian Ocean and the northWestern Pacific Ocean). Over the past 100 years, mean surface temperatures across the region have increased in the range of 0.3-0.8�C. No long-term trend in mean rainfall has been discernible over that period, although many countries have shown a decreasing trend in the past three decades. Similarly, no identifiable change in the number, frequency, or intensity of tropical cyclones has been observed in the region over the past 100 years; however, substantial decadal-scale variations have occurred.
Substantial elevational shifts of ecosystems in the mountains and uplands of Tropical Asia are projected. At high elevations, weedy species can be expected to displace tree species, although the rates of vegetation change could be slow 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; 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 dessication. 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 may 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 play a critical role in the provision of water to continental monsoon Asia. Increased temperature and increased seasonal variability in precipitation are expected to result in accelerated recession of glaciers and increasing danger from glacial lake outburst floods. A reduction in flow of snow-fed rivers, accompanied by increases in peak flows and sediment yields, 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 term. Runoff from rain-fed rivers may change in the future, although a reduction in snowmelt water would result in a decrease in dry-season flow of these rivers. Larger populations and increasing demands in the agricultural, industrial, and hydropower sectors will put additional stress on water resources. Pressure will be most acute on drier river basins and those subject to low seasonal flows. Hydrological changes in island and coastal drainage basins are expected to be small, apart from those associated with sea-level rise.
The sensitivity of major cereal and tree crops to changes in temperature, moisture, and carbon dioxide (CO2) concentration of the magnitudes projected for the region has been demonstrated in many studies. For instance, projected impacts on rice, wheat, and sorghum yields suggest that any increases in production associated with CO2 fertilization will be more than offset by reductions in yield resulting from temperature and/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 and local differences in growing season, crop management, and so forth; noninclusion 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.
Sea-level rise is the most obvious climate-related impact in coastal areas. 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 the large deltaic 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, ports, and tourist resorts; artisinal and commercial fisheries; coastal agriculture; and infrastructure development. International studies have projected the displacement of several million people from the region's coastal zone in the event of 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.
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 case fatality rates. In presently vulnerable regions, increases in epidemic potential of 12-27% for malaria and 31-47% for dengue are anticipated, along with an 11-17% decrease for schistosomiasis. 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 rates, water quality declines, and other factors.
Adaptation and Integration
Strategies for adapting to different climatic conditions will be quite diverse. For example, responses to impacts on agriculture will vary from region to region, depending on the local agroclimatic setting as well as the magnitude of climate change. New temperature- and pest-resistant crop varieties may be introduced, and new technologies may be developed to reduce crop yield losses. Countries in Tropical Asia could improve irrigation efficiency from current levels, to reduce total water requirements. Integrated approaches to river basin management, which already are used in a number of countries in the region, could be adapted regionwide. Such approaches could increase the effectiveness of adapting to the often-complex potential impacts of climate change that generally transcend political boundaries and encompass upstream and downstream areas. Similarly integrated approaches to coastal zone management can include current and longer-term issues, including climate change and sea-level rise.
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