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are often either obscure or complex. Human impacts on stratospheric ozone and greenhouse gases provide examples. Unlike a belching smokestack, the effect of chlorofluorocarbons on stratospheric ozone is apparent only to those with the expertise to understand the critical atmospheric chemistry. Meanwhile, a plethora of human activities contribute greenhouse gases to the atmosphere and thereby have the potential to alter climates. As a result of such obscure and complex relationships, it is difficult to predict the future environmental consequences of human activities.
As more obscure and complex relationships between humans and the environment are recognized, those charged with reducing and monitoring human impacts face greater challenges. They must shift their attention from obvious and relatively simple impacts (e.g., lead in gasoline or oil floating on a river) to more complicated processes (e.g., the effects of manufactured chemical compounds on animal development and human health [Sharpe, 1995]). Thus, they need to improve their ability to predict the ecosystem consequences of changes in human activities. As usual, better predictive abilities will require better measurement abilities.
One can envision a continuum of progress, from a past when there was little concern for the obvious environmental impacts of human activities, to the present when some obvious effects have been reduced but new understanding has led to new concerns, and, finally, to a future when comprehensive understanding of the environmental implications of human activities makes it possible to eliminate any activities that have unacceptable environmental consequences. The latter is probably too much to expect, given the diversity of ecological impacts, the potential for interaction between these impacts, and the complexity of the ecosystems involved, but it is a worthy target.
Over the past few decades, two new groups of professionals have emerged. One is responsible for managing and reducing the environmental impacts of human activities. The other is responsible for assessing and monitoring the conditions of ecosystems that are affected by human activities.
Those charged with managing and reducing environmental impacts have developed a suite of metrics for gauging environmental performance. Those who assess the status of affected environments have developed metrics for determining ecosystem conditions. However, surprisingly little interaction appears to take place between those working to improve environmental performance and those trying to monitor the condition of ecosystems. With so little interaction, those who work to improve environmental performance are rarely able to assess the marginal environmental effects of their improvements. Meanwhile, those who measure the condition of impacted ecosystems often lack information on the particular human activities that are responsible for changes in ecosystem conditions. As attention turns to more obscure and complex environmental impacts, the lack of communication between these two groups could substantially impede progress. How can a plant manager know which of two alternatives in a production process