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Working Group I: The Scientific Basis |
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9.3.2 Patterns of Future Climate Change
For the change in annual mean surface air temperature in the various cases, the model experiments show the familiar pattern documented in the SAR with a maximum warming in the high latitudes of the Northern Hemisphere and a minimum in the Southern Ocean (due to ocean heat uptake) evident in the zonal mean for the CMIP2 models (Figure 9.8) and the geographical patterns for all categories of models (Figure 9.10). For the zonal means in Figure 9.8 there is consistent mid-tropospheric tropical warming and stratospheric cooling. The range tends to increase with height (Figure 9.8, middle) partly due to the variation in the level of the tropopause among the models. Ocean heat uptake also contributes to a minimum of warming in the North Atlantic, while land warms more rapidly than ocean almost everywhere (Figure 9.10). The large war ming in high latitudes of the Northern Hemisphere is connected with a reduction in the snow (not shown) and sea-ice cover (Figure 9.9). The ensemble mean temperature divided by its standard deviation { There is relatively good agreement between the models for the lower latitude response, with larger range and less certain response at higher latitudes (Figure 9.10). For example, most models show a minimum of warming somewhere in the North Atlantic but the location is quite variable. There is a tendency for more warming (roughly a degree) in the tropical central and east Pacific than in the west, though this east-west difference in warming is generally less than a degree in the multi-model ensemble and is not evident with the contour interval in Figure 9.10 except in the B2 experiment in Figure 9.10e. This El Nino-like response is discussed further in Section 9.3.5.2. The biggest difference between the CMIP2 G (Figure 9.10a,b) and GS experiments (Figure 9.10c) is the regional moderating of the warming mainly over industrialised areas in GS where the negative forcing from sulphate aerosols is greatest at mid-21st century (note the regional changes discussed in Chapter 10). This regional effect was noted in the SAR for only two models, but Figure 9.10c shows this is a consistent response across the greater number of more recent models. The GS experiments only include the direct effect of sulphate aerosols, but two model studies have included the direct and indirect effect of sulphate aerosols and show roughly the same pattern (Meehl et al., 1996; Roeckner et al., 1999). The simulations performed with and without the direct sulphate effect (GS and G, respectively) with the same model are more similar to each other than to the other models, indicating that the individual response characteristics of the various models are dominating the response pattern rather than differences in the forcing. With greater CO2 forcing, the simulated patterns are more highly correlated in the G simulations than in the GS simulations (Table 9.2, 26 of 36 possible model combinations for temperature, 22 of 36 for precipitation).
The SRES A2 and B2 integrations (Figure 9.10d,e) show a similar pattern of temperature change as the CMIP2 and G experiments. Since the positive radiative forcing from greenhouse gases overwhelms the sulphate aerosol forcing at the end of the 21st century in A2 and B2 compared to the GS experiments at mid-21st century, the patterns resemble more closely the G simulations in Figure 9.10a,b. The amplitude of the climate change patterns is weaker for the B2 than for the A2 simulations at the end of the 21st century (Figure 9.10d,e).
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