Introduction

Global mean temperatures, whether for times in the past or the future, have become a touchstone for ‘global change’. As the previous chapter has shown, the fact that they have been reconstructed for the past and modelled in a reasonably well-constrained, probabilistic way for the future gives them special value. As with temperature, so with sea-level; the global picture makes a realistic point of departure for further analyses. Once we move beyond global temperature and sea-level and seek either spatially to disaggregate global values or consider aspects of climate other than temperature, everything becomes much more complicated and even more uncertain.

This chapter looks at the ways in which future projections of climate change may be resolved zonally and regionally, paying particular attention to the likely implications of future climate change for the hydrological cycle. Ideally, downscaling future climate scenarios to the regional level should involve a logical sequence of inferences that include the testing and calibration of regional models against the instrumental record from the recent past at the very least. Model projections might then be developed for any chosen emission scenario, with attached probabilities, for target times and places. These steps would pave the way for a closer look at the future interactions between climate and the full range of human activities. They are, however, far from simple and they continue to provide formidable challenges in an area where, despite impressive progress, there is still a very long way to go.