|The Regional Impacts of Climate Change|
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220.127.116.11. Trends in Climate-Related Phenomena: Floods and Droughts
For the period 1871-1984, Parthasarathy et al. (1987) identified a range of 2-30 flood years (i.e., years when precipitation is at least 26% higher than normal) in the various meteorological subdivisions in India. In the same period, the range of severe flood years (i.e., precipitation more than 51% higher than normal) was between 1 and 14. Mirza et al. (1997) analyzed peak flood discharges recorded at various stations on the Ganges, Brahmaputra, and Meghna rivers in Nepal, India, and Bangladesh and found no conclusive increasing or decreasing trends. Similarly, no trends were detected in time-series data of flooded areas in various river basins of India and Bangladesh.
Droughts also can reach devastating proportions in Tropical Asia, although the incidence is variable in time and place. In India, Parthasarathy et al. (1987) identified a range of 1-12 severe drought years (i.e., precipitation 51% less than normal) for the various meteorological subdivisions of the subcontinent during the period 1871-1984. Chronically drought-affected areas cover the western parts of Rajasthan and the Kutch region of Gujrat (SAARC, 1992). In Bangladesh, about 2.7 million ha are vulnerable to annual drought; there is about a 10% probability that 41-50% of the country is experiencing drought in a given year (GOB, 1989). Drought or near-drought conditions also can occur in parts of Nepal (Sharma, 1979) and in Papua New Guinea and Indonesia, especially during El Niņo events.
The IPCC Second Assessment Report (IPCC 1996, WG I, Section 7.2) indicates that:
Warrick et al. (IPCC 1996, WG I, Section 7.4) suggest that the observed sea-level rise has been caused largely by increases in global temperatures and related factors, including thermal expansion of the ocean and melting of glaciers and ice caps. Changes in surface water and groundwater storage, along with tectonic movements and subsidence, also may have affected local sea levels.
Major difficulties in determining regional sea-level trends for Tropical Asia relate to the limited amount of historical tide-gauge data and the region's high decadal and interannual variability. For instance, only one station in the region (Bombay) is included in the list of stations with records exceeding 75 years used by Douglas (1992) to determine global acceleration in sea level; this study showed a -0.02 mm/yr acceleration for the 109-year record (1878-1987). With reference to Bangladesh, Warrick et al. (1996) indicate that existing tide-gauge data do not yet allow an unambiguous estimate of regional and local trends in relative sea-level change and their causes.
Studies of historical rates of relative sea-level rise in the South Asia Seas region, reported by Gable and Aubrey (1990), indicate an average annual relative sea-level rise of 0.67 mm/yr. In addition, during the past half-century or so, relative sea-level changes in the region have ranged from a fall (i.e., land emergence) of 1.33 mm/yr to a rise (i.e., land submergence) of 2.27 mm/yr.
In many parts of Tropical Asia, historical sea-level records appear to show not only a eustatic component but also local anthropogenic or meteorological effects. In Bangkok, for example, extraction of water from groundwater aquifers has resulted in accelerated land subsidence (i.e., a relative rise in sea level) of around 20 mm/yr since 1960, compared with an earlier trend of about 3 mm/yr (see IPCC 1996, WG II, Section 9.3.1); in contrast, high recent rates of sea-level rise in Manila have been blamed on coastal reclamation (Spencer and Woodworth, 1993). Analysis of time-series data for 1955-1990 indicates an average sea-level rise of 1.9 mm/yr at Hondau in North Viet Nam. This finding is in broad agreement with the observed rise in global mean sea level (Granich et al., 1993). Meteorological effects generated by zonal interannual winds blowing along the Equator in the Pacific Ocean have been detected in the interannual sea-level signal that occurs along more than 8,000 km of Indian Ocean shoreline, extending from southern Java to Bombay (Clarke and Liu, 1994).
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