Defining an Acceptable Level of Environmental Impact
At the heart of EA, and indeed the entire debate over sustainable development, is the question of how much environmental impact should be tolerated. This is a difficult issue even for seasoned planners and environmental specialists. While the overall environmental objective of a project is clearly "sustainability" or "sustainable use," the operational definition of these terms is situational dependant on the type of project being assessed and possibly any number of social, economic and environmental factors.
In an effort to help project managers develop appropriate definitions, the World Bank has issued "Project Level Guides for Environmental Sustainability." Every ecosystem has a threshold for absorbing deterioration and a certain capacity for self-regeneration. These thresholds are defined by the World Bank (1991a: 51) as follows:
- Waste emissions from a project should be within the assimilative capacity of the local environment to absorb without unacceptable degradation of its future waste absorptive capacity or other important services.
- Harvest rates of renewable resource inputs should be within the regenerative capacity of the natural system that generates them; depletion rates of nonrenewable inputs should be equal to the rate at which renewable substitutes are developed by human invention and investment.
The aim of this model is to curb overconsumption and unacceptable environmental degradation. But lacking in a scientific basis, this definition provides only general guidelines for determining the sustainable use of inputs and outputs. The World Bank therefore recommends adopting a "prudent rule of thumb" to serve as a check against overconsumption rather than a "theoretically unique scientifically precise number" (World Bank 1991a: 52).
However, sustainability must also be understood from the perspective of economic and social wellbeing. The price of economic success could entail high levels of environmental depletion. Community members with divergent interests may have conflicting perceptions of sustainability. Changes in patterns of resource consumption affect people in different ways. An impact that is viewed as equitable and sustainable by one group may not be by another. For example, dam and irrigation projects which benefit some by boosting food production may hurt others by flooding valued natural resources. The dissension arising from such differences can threaten a project's success (UNRISD 1992).
As IUCN points out (95:1), assessing sustainability can entail "setting common goals, identifying conflicting interests, devising and applying strategies and ways of measuring. It is a learning process involving reflection, argument, negotiation, strategizing, measurement, action and continuous reassessment."
The Threshold of environmental impact can become more flexible if appropriate mitigation measure can be implemented to eliminate, reduce or control the adverse environmental effects of a project. A tree planting project may compensate for the loss of forest coverage. The air and water pollution caused by a food processing operation may be controlled by using more efficient technologies, and finding a more suitable location. Once the environmental impacts of a project are properly understood, learn about what mitigation measures are appropriate.