Managing Uncertainty in Climate Change Projections – Issues for Impact Assessment

Climate change projection is the term the IPCC Second Assessment Report (SAR) uses for model estimates of future climate. In that report, projections are presented in two forms: as single model scenarios and as projected ranges of uncertainty. In climate studies, scenarios are commonly regarded as being plausible, but have no further probability attached. Projected ranges of uncertainty can have probabilities attached to the range and within the range, so are more likely to occur than individual scenarios. However, as there is significant remaining uncertainty beyond the projected range, such projections cannot be regarded as forecasts. An appropriate terminology is required to communicate this distinction. The sources of uncertainty in projected ranges of global temperature to 2100 are analysed by Visser et al. (2000), who recommend that all major sources of uncertainty be incorporated into global warming projections. This will expand its projected range beyond that of the IPCC SAR. Further sources of uncertainties are contained within projections of regional climate. Several strategies that aim to manage that uncertainty are described. Uncertainty can also be managed where it is unquantifiable. An example is rapid climate change, where discarding the term climate ‘surprises’ in favour of more precise terminology to aid in identifying possible adaptation strategies, is recommended.