Environmental foresight and structural change

Policy-makers and the public, it has famously been said [Brooks, H. (1986) Sustainable Development of the Biosphere, Cambridge University Press, p. 325], are more interested in the possibility of non-linear dislocations and surprises in the behavior of the environment than in smooth extrapolations of current trends. How indeed should we design our models to generate environmental foresight, to detect, in particular, threats to our environment lying 'just beyond the horizon'? In facing this prospect of potentially profound dislocations in behavior, the problem is that the number of state variables in the model, whether they interact, how they interact, and the form of their interactions, may be evolving over time. What may have appeared to have been an insignificant mode of behavior in the past-buried within the uncertainty of the model and the historical data-may come to dominate behavior in the future. Technically, we may call this a change of structure. The concern of the paper is to address the challenge of constructing and employing models to generate environmental foresight in the presence of structural change. A number of case histories, ranging across lake eutrophication, urban ozone levels, the restoration of ecosystems, the circulation of waters in the North Atlantic, and the invasion of exotic species, are used to construct a much more immediate sense of the nature of structural change and, therefore, the character of the challenge of generating environmental foresight. Some mathematical and logical formalities are then introduced, both to define the issues more sharply and to open up the means with which to address them. This provides an opportunity to take stock of three rather different programs of model-building used, over the decades, to generate environmental foresight. We close by illustrating a set of possible responses to the essential challenge through a number of contemporary case studies: in assessing, inter alia, the reachability of the lay community's hopes and fears for the future of their cherished piece of the environment; in apprehending and diagnosing the possibility of imminent structural change; and in examining the record of the past for emergence of the seeds of any such structural change.