The threat posed by climate change must be considered in the context of efforts by countries around the world to achieve sustainable development (see Section 1.1). Improved analysis of impacts of and adaptation to climate change is important for the development of appropriate policy measures. However, the chain of events from human behaviors that give rise to disturbances to the climatic system; to atmospheric changes; to impacts on humans, societies, other species, ecosystems, and their adaptive responses is very complex (as noted in Chapters 2 and 19). Uncertainty is a common feature in the discussion of complexity, and it is compounded by the complex interactions of many subsystems that constitute the socionatural system, each of which has its own inherent uncertainties (see Box 1-2). This section summarizes some of the complexities that make it difficult to provide very many highly confident projections about climatic impacts—assessments that are directly relevant to the oft-asked policy question: “What should we do about climate change?” (see Chapter 2 and references therein for more complete treatment).

At the regional level, there is a wide range of projected changes in temperature and precipitation simulated from a doubling of CO₂ concentrations because of large model-to-model differences. Annex B of the Special Report on Regional Impacts of Climate Change (IPCC, 1998) provides the following conclusion regarding the confidence that can be placed in regional climate projections:

“Analysis of surface air temperature and precipitation results from regional climate change experiments carried out with AOGCMs indicates that the biases in present-day simulations of regional climate change and the inter-model variability in the simulated regional changes are still too large to yield a high level of confidence in simulated change scenarios. The limited number of experiments available with statistical downscaling techniques and nested regional models has shown that complex topographical features, large lake systems, and narrow land masses not resolved at the resolution of current GCMs significantly affect the simulated regional and local change scenarios, both for precipitation and (to a lesser extent) temperature (IPCC, 1996a). This adds a further degree of uncertainty in the use of GCM-produced scenarios for impact assessments. In addition, most climate change experiments have not accounted for human-induced landscape changes and only recently has the effect of aerosols been vigorously investigated. Both these factors can further affect projections of regional climate change.”

The wide range of projected changes in temperature and precipitation would affect the degree of exposure of systems and populations to climatic stimuli and hence their vulnerability to climate change. This range suggests that high confidence will not often be assigned to any regional impact assessments that are based on GCM results. Difficulty in obtaining many highly confident outcomes is why the term “climate scenarios” has been adopted in most impact assessments. Such scenarios should be regarded as internally consistent patterns of plausible future climates, not predictions carrying assessed probabilities (see Section 2.6 and Chapter 3). Decisionmakers need to be aware of the large range of plausible climate projections when they formulate strategies to cope with the risks of climate change. However, in the absence of some explicit estimation of the likelihood of various scenarios by those who produce them, users of the many decision frameworks in the literature (see Box 1-2 and Section 2.7) often have to impute likelihood to various scenarios to apply many of these methods.

The review chapters in this report summarize impact studies that are based on a range of climate scenarios, when available. As noted earlier, transient scenarios are particularly valuable because the Earth currently is undergoing a transient response to global change disturbances. Great care is required in interpreting and comparing results from research or assessments that use different climate scenarios, particularly when some conclusions follow from static scenarios and others from transient scenarios. Unfortunately, such mixed use of scenarios is still a problem in the literature and in assessments of it.

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