declined by 75% between 1978 and 1995 due to bans on the use of leaded gasoline and lead-based paint; fish have returned to rivers and lakes once considered "dead"; the air in many cities is clearer than it was 30 years ago; and the production of ozone-destroying chemicals has been reduced drastically (EPA, 1995). Environmental research played an important part in these successes, as illustrated by the example of the restoration of the Kesterson National Wildlife Refuge in Box 1-1. However, a problem-oriented approach also has limitations that are increasingly apparent.
It has become increasingly evident that environmental systems are complex and interconnected. In looking back at early environmental successes it appears that, in many cases, attempts to correct one environmental problem unwittingly created or exacerbated others. The interplay of atmospheric particulates and sulfur oxides provides one example.
Acid deposition in the northeastern United States increased substantially over the same time period in which EPA regulations reduced powerplant emissions of particles larger than 2mm by 78% (EPA, 1996). Although the removal of larger particulates from atmospheric emissions resulted in substantially clearer urban air, the bulk of those particulates were alkaline and tended to neutralize the acidic sulfur oxides. To remedy the ecosystem and health effects associated with acidic aerosols, the sulfur in atmospheric emissions needs to be removed. But the removal of sulfur seems to lead to another unanticipated effect. Very small sulfuric acid aerosols also serve as nuclei for the formation of clouds—the more nuclei, the more small cloud droplets will be present. These droplets scatter incoming solar radiation before it reaches the earth's surface, resulting in a global cooling effect that partially offsets the warming effect of greenhouse gas buildup. A decrease in sulfuric acid in the atmosphere, aimed at reducing acid deposition and its harmful effects, may affect this global heat balance with unknown consequences (NRC, 1996b).
Another example of the complexity of environmental interactions is shown in Figure 1-1. The widespread use of chlorofluorocarbons (CFCs) as apparently safe, inert refrigerants and propellants led to depletion of the earth's protective ozone layer. CFC use provided a safer home environment but led to a serious global environmental problem.
Still another example of the interconnectedness of environmental problems is the rapid invasion of the non-native zebra mussel into Lake Erie, which has caused fouling of structures such as water intakes, outfall pipes, and piers. Engineers have experienced some success in designing materials that resist mussel colonization. However, this solution will not help remedy another, more surprising problem: it appears that the mussels are increasing the concentration of chemical contaminants throughout the aquatic food web in the lake and adjacent rivers.