Predicting regional climate change: living with uncertainty

Regional climate prediction is not an insoluble problem, but it is a proble m characterized by inherent uncertainty. There are two sources of this unce rtainty: the unpredictability of the climatic and global systems. The clima te system is rendered unpredictable by deterministic chaos; the global syst em renders climate prediction uncertain through the unpredictability of the external forcings imposed on the climate system. It is commonly inferred f rom the differences between climate models on regional scales that the mode ls are deficient, but climate system unpredictability is such that this inf erence is premature; the differences are due to an unresolved combination o f climate system unpredictability and model deficiencies. Since model defic iencies are discussed frequently and the two sources of inherent uncertaint y are discussed only rarely, this review considers the implications of clim atic and global system unpredictability for regional climate prediction. Co nsequently we regard regional climate prediction as a cascade of uncertaint y, rather than as a single result process sullied by model deficiencies. We suggest three complementary methodological approaches: (1) the use of mult iple forcing scenarios to cope with global system unpredictability; (2) the use of ensembles to cope with climate system unpredictability; and (3) the consideration of the entire response of the climate system to cope with th e nature of climate change. We understand regional climate change in terms of changes in the general circulations of the atmosphere and oceans; so we illustrate the role of uncertainty in the task of regional climate predicti on with the behaviour of the North Atlantic thermohaline circulation. In co nclusion we discuss the implications of the uncertainties in regional clima te prediction for research into the impacts of climate change, and we recog nize the role of feedbacks in complicating the relatively simple cascade of uncertainties presented here.