Introduction to Climate Change

4. Long-Range Transboundary Air Pollution

Beginning in the 1970s advances in our understanding of air currents and the regional transport of air pollutants led to increased calls for international control of air pollution, particularly in Europe. One of the first transboundary air pollution problems to gain widespread recognition was acid rain, caused primarily by emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx). SO2 and NOx mix with water and are precipitated out of the atmosphere as acidic compounds. Acid deposition is primarily a regional issue, and given the close link between acid deposition and industrialization, the two regions that have had the longest problems with transboundary acid deposition are Europe and North America (although significant transboundary problems are now regularly occurring in Asia).

High levels of acid deposition can have severe consequences for the environment, especially for aquatic life and trees. Acidification first caused alarm when it was linked to the disappearance of fish in lakes and rivers in which the pH values of the water had dropped substantially.

Acid deposition also has significant impacts on many plant species. In relatively small doses it may have a fertilizing effect, thus stimulating the growth of agricultural crops. However, higher doses can have a harmful if not devastating impact on vegetation, especially forests. In the late 1970s scientists became concerned about unusual damage to Norwegian spruce trees in the forests of southern Germany, a condition that became known by the German term waldsterben (forest death) and later the more neutral term neuartige Waldschäeden (new forest decline).
The processes through which these ecosystems are altered by acid deposition are complex and occur over extended periods. Species in aquatic environments can tolerate different levels of acidity. Numerous smaller species such as algae, zooplankton, phytoplankton, and aquatic insects cannot tolerate pH levels lower than 5.0 or even 5.5, which has serious implications because they are near the bottom of the food chain. As pH levels drop below 5.0, most fish and amphibian species disappear because of reproductive failures caused by the toxic effects of aluminum that acids release into the water. Loss of fish reduces the food supplies of species such as birds that feed upon them.

These same pollutants can also adversely affect human health. Heavy concentrations of air pollutants such as SO2, NOx, particulates, and photochemical oxidants have been found to irritate the respiratory system, cause chronic lung disease, decrease pulmonary function, and increase heart stress, with the impact being the greatest among the young and elderly. Human health may be jeopardized in an indirect way if toxic metals, such as mercury and lead, are released into drinking water.