|The Regional Impacts of Climate Change|
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126.96.36.199.2. Freshwater wetlands
Freshwater wetlands or peatlands are accumulations of decomposed organic matter that have an ability to store water, reduce floodwater peaks, and provide water during dry periods. More than 10% (43 million ha) of global peatlands are located in the humid tropical zone of Asia; by far the largest area of peatlands is in Indonesia, especially the Kalimantan region, followed by Malaysia and Papua New Guinea (Rieley et al., 1995). Tropical peatlands require high rainfall input and are sensitive to drought and changes in land use; impacts on drainage, for example, compromise the sustained utilization of the peatland resource. Projected increases in evapotranspiration and increased rainfall variability are likely to have negative impacts on the viability of peatlands, resulting in dessication and shrinkage.
Changes in high-elevation ecosystems can be expected as a consequence of global warming. For instance, the present distribution of species is projected to shift to higher elevations, although the rates of vegetation change are expected to be slow, and colonization success would be constrained by increased erosion and overland flows in the highly dissected and steep terrain of the Greater Himalayas. Weedy species with a wide ecological tolerance will have an advantage over others. High-elevation tree species-such as Abies, Acer, and Betula-prevail in cold climates because of their adaptations to chilling winters. Temperature increases would result in competition between such species and new arrivals. The sensitivity of plants to climatic factors and, in particular, water stress in the summit region of Mt. Kinabalu, Borneo-the highest mountain in southeast Asia-recently has been demonstrated (Kitayama, 1996).
In addition to the biodiversity of the region's natural terrestrial ecosystems, Tropical Asia has a unique crop-related biodiversity. In particular, the Indochina-Indonesian and Hindustani regions have been identified as centers of diversity of a great many crops and other economically important plants that originate in these regions. Tropical Asia abounds in diverse cereal, tree, spice, and fruit species, which are endowed with unique genetic properties. Thousands of accessions of rice, cucurbits, legumes, mangoes, citrus, and other species and varieties have been collected and conserved. Maintenance of this diversity to allow further evolution to occur in response to climate change may be important for future generations, particularly when the effects of increasing human populations also are taken into account. Moreover, because of their interdependence and shared evolutionary history, the plants, animals, and microbes in the region strongly influence each other. For example, a change in insect population and diversity could influence the evolution of plant biodiversity, and vice-versa. These subjects-which are the basis of ecological sustenance, crop and animal improvement, and ultimately economic growth-remain unexplored.
The Indo-Pacific borderlands contain the greatest shallow-water and intertidal biodiversity on Earth. Coral reefs have the highest biodiversity of any marine ecosystem; coral reef macrobiota represent about 4-5% of the described global biota. Coral reef diversity and the majority of the region's coral genera are centered around the archipelagoes of the Philippines and Indonesia. Similar rich concentrations of species in the region occur in mangroves and tropical sea grasses (Woodroffe, 1990; Mukai, 1993).
The effects of climate change on coral reefs have been reviewed by Wilkinson and Buddemeier (1994). Brown et al. (1994) have shown that coral reefs are particularly sensitive to prolonged increases in seawater temperature and increased irradiance. Oxidation stress during exposure to elevated temperatures also has been shown to cause coral bleaching (Lesser, 1997). In Tropical Asia, projected increases in surface air temperature could lead to increases in seawater temperature, posing a major threat to the health of coral reefs. In Indonesia, where severe bleaching took place as a result of seawater warming during an ENSO event in 1983, coral reefs have failed to show continued recovery beyond the initial recovery noted in 1988 (Brown and Suharsono, 1990).
Mangroves constitute a significant part of tropical coastal biodiversity. They are found throughout Tropical Asia, where they occupy more than 75,000 km2, or 40% of the world's total area of mangroves. Indonesia alone, with 42,530 km2, accounts for 23% of the world total; India (with 6,700 km2), Malaysia (with 6,424 km2), and Bangladesh (with 5,767 km2) each have over 3% of the world total (Spalding, 1997). Over the past few decades, mangrove forests in Tropical Asia have declined considerably as a consequence of human activities. The Philippines and Thailand have experienced reductions of 60% and 55%, respectively, over 25 years. Between 1980 and 1990, the area of mangroves declined by more than 37% in Viet Nam and more than 12% in Malaysia. These figures suggest a loss of nearly 7,500 km2 of mangrove forests in these four countries alone-representing more than 4% of the current global total.
Mangroves may be affected by climate change-related increases in temperature and sea-level rise. Although the temperature effect on growth and species diversity is not known, sea-level rise may pose a serious threat to these ecosystems. In Bangladesh, for instance, there is a threat to species in the three distinct ecological zones that make up the Sundarbans-the largest continuous mangrove area in the world. If the saline water front moves further inland, Heritiera fomes (the dominant species in the landward freshwater zone) could be threatened. Species in the other two ecological zones (Excoecaria agallocha in the moderately saltwater zone and Ceriops decandra in the saltwater zone) also could suffer. These changes could result in economic impacts: Direct employment supported by the Sundarbans is estimated to be in the range of 500,000-600,000 people for at least half of the year (ESCAP, 1987), and a large number of these people-who are directly employed in the industries that use raw materials from the Sundarbans (e.g., woodcutting; collection of thatching materials, honey, beeswax, and shells; fishing)-may lose their sources of income. Sea-level rise also may threaten a wide range of mammals, birds, amphibians, reptiles, and crustaceans living in the Sundarbans. A large part of the coastal area that now is protected by the Sunderbans will be vulnerable to cyclonic storms and surges. In Thailand, the potential impact of sea-level rise on the mangrove community is a serious issue (Aksorakaoe and Paphavasit, 1993).
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