Introduction to climate dynamics and climate modelling

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Technical Note

6.2.1 Changes in global mean surface temperature

Nearly all the simulations covering the 21^st century available up to now have been obtained using the SRES scenarios (see Section 6.1.2). The average of the results of the General Circulation Models (GCMs) is a warming of nearly 2°C by 2100 for scenario B1, a bit less than 3°C for scenario A1B and about 3.5°C for scenario A2 (Fig. 6.7). Fewer models have been driven by the other scenarios, but A1T and B2 generally lead to a forecast warming intermediate between those obtained with scenarios B1 and A2, while scenario A1F1 predicts more warming than A2. An additional scenario that has been widely tested is the so-called constant commitment scenario in which the concentration of greenhouse gases is held constant at year 2000 values for the whole of the 21^st century. Even in this extreme case, the GCM simulations predict an average warming of more than 0.5°C by 2100 on average (Fig. 6.7). This is because the climate was far from equilibrium with the forcing in 2000.

**Figure 6.7:** Multi-model means of surface warming (relative to 1980-1999) for the SRES scenarios A2, A1B and B1, shown as continuations of the 20^th-century simulation. Values beyond 2100 are for the stabilisation scenarios in which the forcing in 2100 is kept constant for the 22^nd and 23^rd centuries. For the constant composition commitment, the composition in 2000 in maintained during the whole 21^st century. Linear trends from the corresponding control runs have been removed from these time series. Lines show the multi-model means, shading denotes the 1 standard deviation range of individual-model annual means. Discontinuities between different periods have no physical meaning and are caused by the fact that the number of models that have run a given scenario is different for each period and scenario, as indicated by the coloured numbers given for each period and scenario at the bottom of the panel. Figure 10.4 of Meehl et al. (2007) with a modified legend, reproduced with permission from IPCC.

Figure 6.7 illustrates two sources of uncertainty in climate projections. The first is related to the scenario, as discussed above. A second is due to model uncertainty, different models displaying a different response to the same forcing. This is indicated on Figure 6.7 by the range of the results of all the models. Additional uncertainty is related to the internal variability of the system, i.e. the natural fluctuations that would occur even in the absence of any change in radiative forcing.

**Figure 6.8:** The fraction of total variance in decadal mean surface air temperature predictions explained by the three components of total uncertainty is shown for (a) a global mean and (b) a British Isles mean. Green regions represent scenario uncertainty, blue regions represent model uncertainty, and orange regions represent the internal variability component. As the scope of the model is reduced (e.g. from the world to the British Isles), the relative importance of internal variability increases. Figure form Hawkins and Sutton (2009), copyright American Meteorological Society 2009. .

The relative importance of the three sources of uncertainty can be estimated for projections over different time periods (also known as lead times). For estimates of the global mean temperature over the next decade, the influence of the uncertainty about future emissions of greenhouse gases is small. This is consistent with Figure 6.7, where the curves for all the SRES scenarios lie close to each other until 2030–2040. On a global scale (Figure 6.8a), the relative importance of the scenario uncertainty increases with time, and is dominant in projections for the end of the 21^st century. The internal variability only plays a role for a few decades, the natural fluctuations in global mean temperatures over decades and centuries being much smaller than the changes expected by 2100. The model uncertainty is dominant for projections up to 40 years ahead, but its relative contribution then decreases, although it is still significant in 2100.

When analysing temperature changes over a smaller region such as the British Isles (Figure 6.8b), each source of uncertainty has more or less the same behaviour as discussed for the Earth as a whole. The only clear change is that internal variability makes a larger contribution to the total uncertainty. Natural fluctuations also have a much larger amplitude on a regional scale than on the global one (see Section 5.5.2.2).