Leslie Reid and Tom Lisle, RWU-4351, Pacific Southwest Research Station
The Council on Environmental Quality defined cumulative impact as "...the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions...." The "cumulative" impact is simply the overall impact a resource experiences from the combination of impact mechanisms to which it is subjected. The issues of global climate change and cumulative impacts are closely related:
1. Human-induced climate change is itself a cumulative impact of multiple human activities. Prediction of the local magnitude, style, and timing of climate changes will require an understanding of how the many influences on climate interact.
2. Outcomes from this episode of climate change will differ from those of previous episodes in part because of interactions with environmental changes that humans have already caused-outcomes will be a cumulative effect. For example, Pleistocene climate changes resulted in elevational and latitudinal shifts of ecosystem boundaries. However, ecosystems now are highly fragmented by land-use activities, so climate change is more likely to result in extirpations than in the past because incremental shifts along a gradient may no longer be possible. In addition, geomorphic and ecosystem processes have been extensively modified by land-use activities, impairing some systems' mechanisms for resilience and thereby increasing their sensitivity to change.
3. Potential effects of climate change must be considered as a component of cumulative impacts during environmental impact evaluations for particular land-use plans. Changes in climate cause changes in vegetation, water availability, water demand, and frequencies and modes of environmental disturbance. Until these changes are better understood, it will be difficult to reliably predict the environmental outcomes of particular land-use activities.
Climate changes influence vegetation, water, and disturbance frequencies, and these changes, in turn, influence one another. A change in one aspect causes a cascade of responses that in some cases counteract and in others magnify the initial change. Such interactions make prediction of the likely effects of climate change difficult at particular locations even if the nature of the climate change is known. Ordinarily we rely on the past for evidence needed to predict the future, but when the context for a change is unprecedented, we must instead use our basic understanding of physical, ecological, and sociological processes and their interactions. At this point we can be certain that changes will occur, but we do not yet know the mode, timing, or magnitude of changes or environmental responses even at a regional scale.
We can, however, begin to identify the kinds of changes that different styles and magnitudes of climatic change are likely to cause. For example, we know that an activity's impacts on watershed processes are strongly influenced by events following the activity. Erosion rates are particularly high if a major storm or wildfire occurs soon after logging. Climate change will affect the frequency of storms and wildfires, so the "usual" patterns of response-those we have based land-use plans and practices on-are likely to change in the future. If we can identify the changing conditions early enough, we can adjust land-use plans and practices to take the new conditions into account. In any case, however, we face increased uncertainty concerning the environmental outcomes of our activities.
Options for Management
Several approaches to land management have been developed in the face of high uncertainty concerning the outcomes of management actions. A widely cited example is the "precautionary principle": if the outcome of an activity is uncertain and harmful effects are possible, be conservative until outcomes are better understood. Development of such understanding in this case would require detection and documentation of the climate-mediated changes that are occurring and recognition of potential "threshold" behaviors, in which a system begins to respond in a new way once conditions change beyond a particular point.
A second approach is "adaptive management," which is most broadly interpreted to mean that management actions are administered as experiments, with appropriate monitoring to evaluate the efficacy of the action and to allow redesign to improve future applications. Such an approach will be particularly useful as the climatic context for management activities changes.
Under changing conditions, the most useful kind of knowledge is that which furthers our understanding of basic processes. If we know the likely response of a system as a function of controlling conditions, we can predict outcomes for a variety of conditions, even if they are unprecedented. In contrast, simple empirical correlations between treatment and effect implicitly assume that driving variables will not change, so this kind of information may be misleading. It will be important to reevaluate management tools that are based on empirical correlations to ensure that the underlying assumptions remain valid under changing climatic conditions.