Apart from intraseasonal and interannual variability in climate, extreme weather events such as heat waves associated with extreme temperatures, extratropical and tropical cyclones, prolonged dry spells, intense rainfall, tornadoes, snow avalanches, thunderstorms, and dust storms are known to cause adverse effects in widely separated areas of Asia. There is some evidence of increases in the intensity or frequency of some of these extreme weather events on regional scales throughout the 20th century, although data analyses are relatively poor and not comprehensive (Balling and Idso, 1990; Bouchard, 1990; Agee, 1991; Yu and Neil, 1991; Chen et al., 1992; Ostby, 1993; Bardin, 1994; Born, 1996). For example, increases in climate extremes in the western Siberia-Baikal region and eastern parts of boreal Asia have been reported in recent decades (Gruza and Rankova, 1997; Gruza et al., 1999). There also are reports of an increase in thunderstorms over the land regions of tropical Asia (Karl et al., 1995). The frequency and severity of wildfires in grasslands and rangelands in arid and semi-arid Asia have increased in recent decades (Pilifosova et al., 1996).
Some mountains in Asia have permanent glaciers that have vacated large areas during the past few decades, resulting in increases in glacial runoff. As a consequence, an increased frequency of events such as mudflows and avalanches affecting human settlements has occurred (Rai, 1999). As mountain glaciers continue to disappear, the volume of summer runoff eventually will be reduced as a result of loss of ice resources. Consequences for downstream agriculture, which relies on this water for irrigation, will be unfavorable in some places. For example, low- and mid-lying parts of central Asia are likely to change gradually into more arid, interior deserts.
Countries in temperate Asia have been frequented by many droughts in the 20th century. In China, droughts in 1972, 1978, and 1997 have been recorded as the most serious and extensive. A large number of severe floods also have occurred in China, predominately over the middle and lower basins of the Yangtze (Changjiang), Huanghe, Huaihe, and Haihe Rivers (Ji et al., 1993). Severe flooding with daily rainfall exceeding 25 cm struck during July and August 1998 in Korea. In Japan, drought disasters are significantly more frequent during years following ENSO warm events than in normal years.
Floods, droughts, and cyclones are the key natural disasters in tropical Asia. The average annual flood covers vast areas throughout the region: In Bangladesh, floods cover 3.1 Mha; the total flood-prone area in India is about 40 Mha (Mirza and Ericksen, 1996). In India, chronically drought-affected areas cover the western parts of Rajasthan and the Kutch region of Gujarat. However, drought conditions also have been reported in Bihar and Orissa States in India. In Bangladesh, about 2.7 Mha are vulnerable to drought annually; there is about 10% probability that 41-50% of the country is experiencing drought in a given year (Mirza, 1998). Drought or near-drought conditions also occur in parts of Nepal, Papua New Guinea, and Indonesia, especially during El Niño years. In India, Laos, the Philippines, and Vietnam, drought disasters are more frequent during years following ENSO events. At least half of the severe failures of the Indian summer monsoon since 1871 have occurred during El Niño years (Webster et al. 1998). In the event of enhanced anomalous warming of the eastern equatorial Pacific Ocean, such as that observed during the 1998 El Niño, a higher frequency of intense extreme events all across Asia is possible.
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