11.1.2.2. Trends and Variability in Key Climate Variables

11.1.2.2.1. Surface air temperature

Climate differs widely within Asia. In boreal Asia, the climate generally is humid and cool; it is classified as continental type. Permafrost covers as much as 90% of boreal Asia. An average annual mean increase in surface air temperature of about 2.9°C in the past 100 years has been observed in boreal Asia. Warming was more pronounced during the period 1951-1995 in all of boreal Asia region except for the coast of the Arctic Ocean and Chukotka (Rankova, 1998). The mean surface air temperature increase is most pronounced in the winter, at a rate of approximately 4.4°C over the past 100 years (Gruza et al., 1997). Summer temperatures in central Siberia have exhibited decreasing trends, however.

The climate of arid and semi-arid Asia is the warm temperate type, with hot and wet or dry summers. The highest value of DTR (on the order of 20°C) is experienced in this region. A mean maximum temperature of >45°C in July is not uncommon in some parts of arid Asia. In most of the Middle East, the long time series of surface air temperature shows a warming trend. An increasing tendency in spatially averaged seasonal and annual air temperatures has been observed in Kazakhstan over the past century. Maximal warming has occurred in spring. The mean annual surface temperature has risen by about 1.3°C during 1894-1997 (Pilifosova et al., 1997); in the arid regions of China, air temperature has obviously increased since the 1970s (Chen, 1995). In Pakistan, annual mean surface temperature has a consistent rising trend since the beginning of 20th century (Chaudhari, 1994).

In temperate Asia, the average annual mean surface temperature in Mongolia has increased by approximately 0.7°C over the past 50 years; consequently, noticeable changes have taken place in the length of the cold and warm seasons (Khuldorj et al., 1998). Surface temperature in northeast China has increased in winter but decreased in summer since 1905 (Ren and Zhou, 1994; Ren, 1998); observations also reveal a 1-2°C decrease in temperature in some parts of southeastern China. In Japan, the surface air temperature has shown a warming trend during the past century (Yoshino, 1998a; Japan Meteorological Agency, 1999).

Tropical Asia has a unique climatological distinction because of the pervasive influence of the monsoon. In tropical Asia, climate uniformity is differentiated by three factors: latitude, relief, and continentality. The entire tropical Asia region stretches over 38° in latitude, so the differences resulting from this factor are pronounced. In spite of some differences, the climates of countries have one factor in common: The Asian monsoon modulates them all to a large extent. For countries near the Equator, only small seasonal variations occur, although most countries in this region experience clearly marked cold and warm seasons. The spatial range of temperature in this region is significantly large during winter. Extreme temperatures of over 45°C occur over the northwest part of the region during May-June. Several countries in this region have reported increasing surface temperature trends in recent decades. In Vietnam, annual mean surface temperature has increased over the period 1895-1996, with mean warming estimated at 0.32°C over the past 3 decades. Annual mean surface air temperature anomalies over Sri Lanka during the period 1869-1993 suggest a conspicuous and gradually increasing trend of about 0.30°C per 100 years (Rupakumar and Patil, 1996). The warming trend over India has been reported to be about 0.57°C per 100 years (Rupakumar et al., 1994).

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