Stanford, California
Global change can be defined as change to the Earth system that either occurs globally or occurs regionally and sufficiently often to be of global significance. Schneider focused his comments on the component of global change attributable to human activity.
Probability estimates of particular events' occurring do not come from objective methods but rather depend on assumptions that are themselves subjective (i.e., physical, biological or social assumptions underlying "objective" methods used to calculate probabilities are usually intuitive). In the final analysis, probability estimates are the subjective judgments of experts. From medical care to insurance, nearly every aspect of human life depends on speculation by experts; why do we demand from global change science a far higher level of objectivity and certainty than we demand in other areas of our lives? Once we accept that probability estimates are subjective judgments and intuitions of experts, key issues then become: 1) what is the credibility of the sources for scenarios and their probabilities and 2) how do we establish methods to sample reliable intuitions?
The definition of what constitutes a high or low probability is also important. Clear delineation of the numbers that go with terms such as "very probable," "not enough evidence," etc., is critical. The recent attachment of ranges of numbers to such terms by the Intergovernmental Panel on Climate Change (IPCC) is a welcome improvement.
Referring, by way of example, to a model of predicted changes in precipitation patterns that might result from global warming, Schneider stressed the importance of taking model results "seriously, but not literally."
The process of multidisciplinary exchange will lead us to the discovery of surprises at the interfaces of subjects - to ideas unlikely to be uncovered within a single discipline. This discovery marks the transition from a multi- to an interdisciplinary perspective.
Schneider used various examples to demonstrate a large degree of consensus among the accepted experts - indeed the big global change questions spark far less controversy than is portrayed in the media. Although everyone admits a large degree of uncertainty, several surveys of experts judgments reveal fairly widespread agreement on major global change issues, except in the case of budget allocation for the US Global Change Research Program.
The synergism of fragmentation of habitat and climate change will cause already stressed creatures and systems to be further stressed and this is one of the greatest environmental threats we face. Other points Schneider discussed include the importance of scale, the need to look at things at the "tails" of distributions, and how to identify potential issues that aren't even on our "radar screen" yet.