Executive Summary

Introduction

Temperate Asia is composed of three regions: monsoon Asia, excluding its tropical subregion; the inner arid/semi-arid regions; and Siberia. The region includes countries in Asia between 18°N and the Arctic Circle, including the Japanese islands, the Korean peninsula, Mongolia, most parts of China, and Siberia in Russia. The east-west distance of the area is about 8,000 km, and its north-south extent is about 5,000 km. The largest plateau in the world-the Qing-Zang plateau (Tibetan plateau), with an average elevation of more than 4,000 m-is located in southWest China. Inner Siberia, with a mean monthly temperature in January of about -50°C, is the coldest part of the northern hemisphere in winter; this area is called the "cold pole." On the other hand, extremely dry, hot climate prevails in the Taklamakan Desert in China.

Human activities through the ages have brought profound changes to the landscape of this area. Except for boreal forests in Siberia, other natural forests in the region have long been destroyed. Broad plains have been cultivated and irrigated for thousands of years, and natural grasslands have been used for animal husbandry. In recent years, many countries in this region have shown marked economic development. Their gross national products (GNPs) and populations are increasing at an extremely rapid rate. The region's population is expected to grow from 1.42 billion in 1995 to 1.72 billion by 2025; the environment in this region already is under great stress. The impacts of expected climate change may exacerbate existing environmental problems.

Climate Characteristics

Climate differs widely within Temperate Asia. It has a tropical monsoon climate in the far south; a humid, cool, temperate climate in the north; and a desert climate or steppe climate in the west and northWest. In the rest of the area-where most of the population of the region is concentrated-a humid, temperate climate prevails.

The east Asian monsoon has great influence on temporal and spatial variations in the hydrological cycle over parts of the region. For example, the summer monsoon accounts for 70% of the total annual runoff in China; for northern China, this precipitation often concentrates in a few storms during the flood season. At time scales longer than 100 years, the 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.

Tropical cyclones (typhoons) frequent the coastal regions. They are important not only because they cause disasters along the coasts 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 El Niño-Southern Oscillation (ENSO) and non-ENSO years.

Over the past century, the average annual temperature in Temperate Asia has increased by more than 1°C. This increase has been most evident since the 1970s; seasonally, the warming is evident mainly in winter. 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 some parts of southeasten China, except for the coastal area. These trends also are reflected in corresponding seasonal temperature distributions, except that summer temperatures in central Siberia actually are decreasing. In terms of precipitation, large decadal variability seems to have masked a smaller positive trend in annual precipitation. Over the past 100 years, there has been a 20-50% increase 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 in annual precipitation in northeast China is evident mainly in spring and summer rainfall; in south China, annual winter precipitation shows a slight positive trend, in contrast to a negative trend in total annual precipitation.

Future Climate Projection

Numerical experiments with coupled atmosphere-ocean general circulation models (AOGCMs) that include transient increases in greenhouse gases [i.e., increases in equivalent carbon dioxide (CO₂) at a rate of 1% per year] project a warming of between 2°C and 3°C over the annual mean of the region at the time of CO₂ doubling. Warming was more pronounced in the arid/semi-arid and Siberian regions than in the coastal monsoon region. Recent simulations, wherein GCMs also include the offsetting effects of sulfate aerosols, project a temperature rise of about 0.8°C over the eastern part of the region, about 1°C over most parts of eastern China, and close to 2°C in the Siberian region on an annual mean basis by the middle of the next century. It should be noted that the projections that account for sulfate aerosols also are highly uncertain.

In equilibrium and transient-response numerical experiments with GCMs, precipitation is projected to marginally increase (<0.5 mm/day) at the time of CO₂ doubling during the winter (DJF) throughout the region. In the summer (JJA), the spatial pattern of projected changes in precipitation is not uniform over the region. Model projections suggest that precipitation will increase slightly (0.5-1.0 mm/day) in the northern part of the region (Siberia), and by more than 1 mm/day over the Korean peninsula, the Japanese islands, and the southWestern part of China. In contrast, precipitation may decline in the northern, western, and southern parts of China. The projected decline in rainfall over most of China is substantial in numerical experiments that include the effects of sulfate aerosols.

Summary Points

• For boreal forests, which are concentrated mainly in the Russian Federation, models suggest large shifts in distribution (e.g., area reductions of up to 50%) and productivity. In the boreal region, grasslands and shrublands may expand significantly, whereas the tundra zone may decrease by up to 50%, according to model predictions. Climatic warming would increase the release of methane from deep peat deposits-particularly from tundra soils, since they would become wetter. The release of CO₂ is expected to increase, though not by more than 25% of its present level.
• It is likely that by 2050, up to a quarter of the existing mountain glacier mass would disappear. For areas with very large glaciers, the extra runoff may persist for a century or more. By 2050, the volume of runoff from glaciers in central Asia is projected to increase threefold. Eventually, however, glacial runoff will taper off or even cease. Projected future glacier runoff is about 68 km³/yr in 2100, compared with the present value of 98 km³/yr. Permafrost in northeast China is expected to disappear if the temperature were to increase by 2°C or more. Over the Qing-Zang plateau, estimates of the impact of climate change on permafrost range from its complete disappearance as a result of a temperature increase of 2°C to a raising of its elevation limit to 4,600 m as a result of a warming of 3°C.
• Hydrologically, model results suggest that the areas most vulnerable to climate change would be in the northern part of China. Projected changes in runoff are due mainly to changes in precipitation in spring, summer, and autumn because of the strong influence of the monsoon climate. The most critical uncertainty is the lack of credible projections of the effect of climate change on either the Asian monsoon or the ENSO phenomenon, both of which strongly influence river runoff. In moderately and extremely dry years, the projected potential water deficiency caused by climate change-although less than that caused by population growth and economic development-may exacerbate seriously the existing water shortage.
• Different GCMs' estimated impacts on agricultural yields vary widely in range. Possible large negative impacts on rice production as a consequence of climate change would be of concern in the face of expected population increases. In China, across different scenarios and different sites, yield changes for several crops by 2050 are projected to be -78% to +15% for rice, -21% to +55% for wheat, and -19% to +5% for maize. An increase in productivity may occur if the positive effects of CO₂ on crop growth are considered, but the magnitude of the fertilization effect remains uncertain.

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