IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis Testing Models Against Past and Present Climate

Testing models’ ability to simulate ‘present climate’ (including variability and extremes) is an important part of model evaluation (see Sections 8.3 to 8.5, and Chapter 11 for specific regional evaluations). In doing this, certain practical choices are needed, for example, between a long time series or mean from a ‘control’ run with fixed radiative forcing (often pre-industrial rather than present day), or a shorter, transient time series from a ‘20th-century’ simulation including historical variations in forcing. Such decisions are made by individual researchers, dependent on the particular problem being studied. Differences between model and observations should be considered insignificant if they are within:

1. unpredictable internal variability (e.g., the observational period contained an unusual number of El Niño events);

2. expected differences in forcing (e.g., observations for the 1990s compared with a ‘pre-industrial’ model control run); or

3. uncertainties in the observed fields.

While space does not allow a discussion of the above issues in detail for each climate variable, they are taken into account in the overall evaluation. Model simulation of present-day climate at a global to sub-continental scale is discussed in this chapter, while more regional detail can be found in Chapter 11.

Models have been extensively used to simulate observed climate change during the 20th century. Since forcing changes are not perfectly known over that period (see Chapter 2), such tests do not fully constrain future response to forcing changes. Knutti et al. (2002) showed that in a perturbed physics ensemble of Earth System Models of Intermediate Complexity (EMICs), simulations from models with a range of climate sensitivities are consistent with the observed surface air temperature and ocean heat content records, if aerosol forcing is allowed to vary within its range of uncertainty. Despite this fundamental limitation, testing of 20th-century simulations against historical observations does place some constraints on future climate response (e.g., Knutti et al., 2002). These topics are discussed in detail in Chapter 9.