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Although there has been major progress in controlling acid-forming emissions in some countries, the global threat from acid rain is far from over yet. In fact, the dimensions of the acid rain problem are growing rapidly in Asia, with sulfur dioxide (SO2) emissions expected to as much as triple from 1990 levels by 2010 if current trends continue. Curtailing the already substantial acid rain damage in Asia and avoiding much heavier damages in the future will require investments in pollution control on the order of those made in Europe and North America over the past 20 years [1].
Even in developed countries where there have been serious efforts to control acid rain, the story is more complicated than it once appeared. Questions remain in regards to how much damage has been done to forests, lakes, and streams over the years; whether current progress is sufficient to protect the most vulnerable ecosystems; and how soon acid-damaged areas will recover.
Acid rain emerged as a concern in the 1960s with observations of dying lakes and forest damage in northern Europe, the United States, and Canada. It was one of the first environmental issues to demonstrate a large-scale regional scope, with the chief pollutants – oxides of sulfur (SOx) and nitrogen (NOx) from combustion of fossil fuels – able to be carried hundreds of miles by winds before being washed out of the atmosphere in rain, fog, and snow.
As evidence grew of the links between air pollution and environmental damage, legislation to curb emissions was put in place. The 1979 Geneva Convention on Long-Range Transboundary Air Pollution and its subsequent amendments set targets for reductions of sulfur and nitrogen emissions in Europe that have largely been achieved. The 1970 and 1990 Clean Air Acts have led to similar improvements in the United States.
Scientific uncertainties about acid rain persist, however. In the case of forest damage, the contribution of acid rain is hard to isolate from other stresses such as drought, fire, and pests that figure heavily in forest health. In Canada, for example, losses to fires and insects exceed the volume of timber harvested for industrial use [2]. For this reason, the contribution of air pollution to forest damage is a controversial subject, particularly in North America. The most recent and authoritative assessment of forest conditions in Europe reports that 25 percent of trees sampled in more than 30 countries were rated as damaged (having lost more than 25 percent of their leaves). Damage has been increasing over the past 20 years and, while the report notes the difficulty of identifying definitive causes, nearly one half of the countries participating in the survey mentioned air pollution as a cause [3].
Acid rain is now emerging as a major problem in the developing world, especially in parts of Asia and the Pacific region where energy use has surged and the use of sulfur-containing coal and oil – the primary sources of acid emissions – is very high. An estimated 34 million metric tons of SO2 were emitted in the Asia region in 1990, over 40 percent more than in North America [4] [5]. Acid deposition levels were particularly high in areas such as southeast China, northeast India, Thailand, and the Republic of Korea, which are near or downwind from major urban and industrial centers. The effects are already being felt in the agriculture sector. Researchers in India found that wheat growing near a power plant where SO2 deposition was almost five times greater than the critical load (the amount the soil can safely absorb without harm) suffered a 49-percent reduction in yield compared with wheat growing 22 kilometers away [6]. In southwestern China, astudy in Guizhou and Sichuan provinces revealed that acid rain fell on some two thirds of the agricultural lands, with 16 percent of the crop area sustaining some level of damage. Other ecosystems are also beginning to suffer. A study of pines and oaks in acid rain-affected areas of the Republic of Korea, both rural and urban, showed significant declines in growth rates since 1970 [7].
| SO2 Emissions in Asia Could Triple | |
| Past and Projected Sulfur Dioxide Emissions for Asia, Europe, and the United States and Canada | |
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| Source: R. Downing, R. Ramankutty, and J. Shah, RINS-ASIA: An Assessment Model for Acid Deposition in Asia (The World Bank, Washington, D.C., 1997), p. 11. | |
1. R. Downing, R. Ramankutty, and J. Shah, RAINS-ASIA: An Assessment Model for Acid Deposition in Asia (The World Bank, Washington, D.C., 1997), pp. 11, 48, 54; Table 3, p. 27.
2. Food and Agriculture Organization of the United Nations (FAO), State of the World’s Forests 1997 (FAO, Rome, 1997), p. 157.
3. European Commission and United Nations Economic Commission for Europe (EC-UN/ECE), Forest Conditions in Europe: Results of the 1995 Survey (EC-UN/ECE, Brussels and Geneva, 1996), pp.23, 42-43.
4. Op. cit. 1, p. 38.
5. World Resources Institute in collaboration with the United Nations Environment Programme, the United Nations Development Programme, and the World Bank, World Resources 1996-97 (Oxford University Press, New York, 1996), p. 331.
6. Tara Pattel, “Rampant Urban Pollution Blights Asia’s Crops,” New Scientist (June 14, 1997), p. 11.
7 . Op. cit. 1, p. 6.