IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis Other Methods of Evaluation

Simulations of climate states from the more distant past allow models to be evaluated in regimes that are significantly different from the present. Such tests complement the ‘present climate’ and ‘instrumental period climate’ evaluations, since 20th-century climate variations have been small compared with the anticipated future changes under forcing scenarios derived from the IPCC Special Report on Emission Scenarios (SRES). The limitations of palaeoclimate tests are that uncertainties in both forcing and actual climate variables (usually derived from proxies) tend to be greater than in the instrumental period, and that the number of climate variables for which there are good palaeo-proxies is limited. Further, climate states may have been so different (e.g., ice sheets at last glacial maximum) that processes determining quantities such as climate sensitivity were different from those likely to operate in the 21st century. Finally, the time scales of change were so long that there are difficulties in experimental design, at least for General Circulation Models (GCMs). These issues are discussed in depth in Chapter 6.

Climate models can be tested through forecasts based on initial conditions. Climate models are closely related to the models that are used routinely for numerical weather prediction, and increasingly for extended range forecasting on seasonal to interannual time scales. Typically, however, models used for numerical weather prediction are run at higher resolution than is possible for climate simulations. Evaluation of such forecasts tests the models’ representation of some key processes in the atmosphere and ocean, although the links between these processes and long-term climate response have not always been established. It must be remembered that the quality of an initial value prediction is dependent on several factors beyond the numerical model itself (e.g., data assimilation techniques, ensemble generation method), and these factors may be less relevant to projecting the long-term, forced response of the climate system to changes in radiative forcing. There is a large body of literature on this topic, but to maintain focus on the goal of this chapter, discussions here are confined to the relatively few studies that have been conducted using models that are very closely related to the climate models used for projections (see Section 8.4.11).