Permafrost distribution in this region has changed substantially during warming periods of the Quaternary Period. The southern limit of lowland permafrost moved northward at a rate of 60 km/�C. In the alpine areas of Tibet, the boundary of lower-elevation permafrost changed by 160 m/�C; on the northern slope of the Himalayas, this sensitivity is about 80 m/�C (IPCC 1996, WG II, Section 7.2.3).
The length of many mountain glaciers in Pamirs, Tian Shan, and Altay has decreased by up to 4 km during the past two centuries. Fluctuations of 224 glaciers in central Asia from the 1950s to the 1980s can be summarized as retreating (73%), advancing (15%), and stable (12%). The mean equilibrium-line altitude-at which snow accumulation is equal to snow ablation for a glacier-is estimated as being 50-80 m lower today than during the Little Ice Age in the 18th century and the first half of the 19th century (IPCC 1996, WG II, Box 5-1 and Section 7.2.2).
Over the past century, the average annual temperature in Temperate Asia has
increased by more than 1�C (Figure 10-2). This increase has been most evident
since the 1970s. It is a reflection primarily of the warming of the winter season
since that time, although temperatures in all the other seasons also show a
slight increase (see Annex A). Decadal time scale variability is evident from
long-term variations of annual and seasonal temperatures. In terms of precipitation,
large decadal variability seems to have masked a smaller positive trend (see
Figure 10-2 and Annex
|Figure 10-2: Trends in average precipitation (top) and temperature (bottom) in the Temperate Asia region, 1901-96 (see Annex A).|
Subregionally, over the past 100 years, there has been a 2-4�C temperature increase in eastern and northeastern Temperate Asia and a 1-2�C temperature decrease in the eastern half of south China, except for the coastal area. These trends also are reflected in corresponding seasonal temperature distributions, except that the summer temperature in central Siberia exhibits a negative trend. Over the same period, there has been a 20-50% increase in annual precipitation in east Siberia; a 10-20% increase across the Korean peninsula, northeast China, the Huaihe River Basin, and the Yellow River Basin; and a 10-20% decrease in Japan and the southern half of east China, including Taiwan. The increasing trend of annual precipitation in northeast China is manifested mainly in spring and summer rainfalls, whereas in south China the winter precipitation shows a certain degree of positive trend, in contrast to the trend in annual precipitation (see Annex A).
The temperature trend in east China (east of 105�E) differs from the global warming tendency. The peak warming period occurred in the 1940s. Since then, there has been a general cooling trend in this area (particularly in the southWestern area)-in contrast to the linear warming trend in Mongolia and the northern part of China. Similarly, the mean temperature in south China has decreased by about 0.8�C from the 1950s to the 1980s (Ding and Dai, 1994; Yatagai and Yasunari, 1994).
Rainfall over Mongolia concentrates mainly in the summer. Annual precipitation in this country is 100-400 mm over the steppe and less than 150 mm in the southern Gobi. In the Gobi area, summer rainfall decreased over the period 1970-1990; in particular, the number of days with relatively heavy rainfall (greater than 3 mm/day) dropped significantly (Mijiddorj et al., 1992).
The east Asian monsoon greatly influences temporal and spatial variations in the hydrological cycle over many parts of the region. For example, the summer monsoon accounts for 70% of the total annual runoff in China; for northern China, it often concentrates in a few storms during the flood season. The characteristics of the hydrological cycle are significantly different north and south of the Huaihe River. South of the river, the ratio of evaporation to precipitation is 0.51, whereas to the north it is 0.75. The evaporation-to-precipitation ratio is higher than 0.8 for the Yellow River and the Hailuan River (DH-MWR, 1987).
At a time scale longer than 100 years, summer monsoons generally are stronger during (globally) warmer periods, leading to wetter conditions in northern China. On the other hand, drier conditions prevail over most of the monsoon-affected area during (globally) colder periods. At the 10-100 year time scale, however, such a global-regional relationship is not obvious (Yan and Marienicole, 1995).
Tropical cyclones (typhoons) are important not only because they cause disasters along the coasts south of 40�N but also because they are beneficial carriers of water resources to inland areas. The frequency, path, and intensity of typhoons vary greatly from year to year, with clear differences between ENSO and non-ENSO years (Nishimori and Yoshino, 1990). It is difficult to extrapolate impacts on ENSO behavior under global warming, however.
Because of the rising population, rapid industrialization, and increasing level of air pollution, heat-island effects are clearly evident in recent urban climate records of this region. The difference between average city temperatures during two periods-1961-90 and 1931-60-correlates closely with the urban population. For cities in Japan with a population of 6-7 million, the temperature difference is about 0.8�C; for Beijing, it is only about 0.4�C (Japan Meteorological Agency, 1996). Cities located in the northern part of South Korea show significantly higher warming trends than those in the southern part, especially in the anomalies of monthly mean minimum temperature. Thus, even if the global warming rate is lowered in the future, warming trends of the urban climate due to heat-island effects should be considered.
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