|The Regional Impacts of Climate Change|
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Few studies have been done on the possible impact of climate change on fisheries in Tropical Asia, despite the fact that commercial and subsistence marine and freshwater fisheries and aquaculture are important for the food security and the economies of many countries in the region. In the Philippines, for instance, fisheries account for about 4% of gross national product (GNP). The fisheries sector employs nearly a million people-about 26% of whom are engaged in aquaculture operations, 6% in commercial fishing, and 68% in marine and freshwater municipal fishing (i.e., artisinal, small-scale, or traditional fisheries) (Lim et al., 1995). Overexploitation of inshore and inland fisheries in most countries of south and southeast Asia threatens fishery resources as well as the livelihoods of the fishermen.
Section 16.2 of the Working Group II contribution to the SAR (IPCC, 1996) identified climate change impacts on several types of fisheries and considered health and infrastructural issues. For marine fisheries, slight changes in environmental variables-such as temperature, salinity, wind speed and direction, ocean currents, strength of upwelling, and predators-can sharply alter the abundance of fish populations (Glantz and Feingold, 1992). Rises in sea level also may cause saline water fronts to penetrate further inland, which could increase the habitat of brackish-water fisheries. Coastal inundation also could damage the aquaculture industry.
The effect on inland fisheries may be felt strongly because large sections of the region's population consume freshwater fish. Fisheries at higher elevations may be particularly affected by lower oxygen availability resulting from increases in temperature. In the plains, the timing and amount of precipitation may affect the migration of fish species from the river to the floodplains for spawning, dispersal, and growth. If the magnitude and extent of floods increase, more flood-control projects would further deplete floodplain fisheries. Current annual losses of open-water fish in Bangladesh from flood-control and drainage projects are estimated to be 65 kg/ha (Mirza and Ericksen, 1996).
Scenarios of climate change in the mountainous regions of the world are highly uncertain; they are poorly resolved even in the highest-resolution GCMs (IPCC 1996, WG II, Section 5.2). A few impact studies have been carried out in the mountain regions of Tropical Asia, where topography and elevation also have an important influence on the regional climate.
In Nepal, Bhutan, and northern India, for instance, mountains provide food, fuel, and fresh water-which are needed for human survival and are fundamental resources for tourism and economic development. Tourism contributes about 24% of Nepal's foreign-exchange earnings (HMGN, 1992). Seasonal variations in the water resources of Nepal's Himalaya region are very high: The maximum-to-minimum discharge ratios for the Karnali, Sapt Gandaki, and Sapt Kosi rivers are 120:1, 180:1, and 105:1, respectively. This variation is due mainly to heavy and concentrated rainfall during a short time period; steep topography, which encourages higher surface runoff; and the high rate of deforestation in the watersheds (Uprety, 1988).
At high elevations in the Himalayas, an increase in temperature could result in faster recession of glaciers and an increase in the number and extent of glacial lakes-many of which, according to Watanabe et al. (1994), have formed in the past several decades. The rapid growth of such lakes could exacerbate the danger from glacial lake outburst floods (GLOFs), with potentially disastrous effects. GLOFs result from the failure of moraine dams when excessive hydrostatic pressure is exerted on dam walls as a result of increased melting and increased water depth in lakes. In a warmer world, GLOFs could be expected to increase in number and severity throughout the Himalayas of India, Nepal, and Bhutan. Recent examples, which may be suggestive of increasingly frequent occurrences in the future, include the catastrophic outburst of two glacial lakes in the Lunana area, northern Bhutan, in October 1994 (which resulted in the deaths of 21 persons and damage to villages, washed away bridges, and filled water with debris and large logs) and the 1985 outburst flood in Khumbu Himal, Nepal (Watanabe and Rothacher, 1996).
Devastating floods occur in the central Himalaya of Nepal during the monsoon months of June through September. Monsoon floods differ from GLOFs and floods caused by breaching of landslide dams: They are of longer duration and less sudden in onset. The incessant rainfall of August 1987 submerged parts of central and eastern Terai in waters of as much as 1 m (BNJST, 1989). Damage from floods takes several forms, including the destruction of footbridges that often provide the only link between remote mountain villages; demolition of irrigation diversions; mass-wasting by undercutting of steep, stream-adjacent slopes; and damage to floodplain agricultural land by erosion and sedimentation (Marston et al., 1996).
The effects of climate change on soil erosion and sedimentation in mountain regions of Tropical Asia may be indirect but could be significant. An erosion rate in the range of 1-43 tons/ha-with an average of 18 tons/ha-was calculated in three small experimental plots in central Nepal. Over the 3-year period of the experiment, 40-96% of the annual losses occurred during two storms (Carver and Nakarmi, 1995). Uprety (1988) found a statistically significant correlation between monsoon precipitation in the Karnali and Spata Kosi watersheds in Nepal and sediment load. In a single storm in 1993, Sthapti (1995) reported a sediment load into the reservoir of the Kulekhani hydropower project in Nepal that was about 35 times higher than the design value. Increased severity and frequency of monsoonal storms and flooding in the Himalayas, which are expected outcomes of climate change, may significantly alter the area's erosion, river discharge, and sediment dynamics. Eventually, this may affect existing hydropower reservoirs, as well as those planned for construction in the Himalayas. Part of the generated sediment may be deposited on agricultural lands or in irrigation canals and streams, which will contribute to a deterioration in crop production and in the quality of agricultural lands.
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