We now apply the results of a number of climate models to reflect uncertainty in climate modeling. In order to do so, we have diagnosed a range of different response time scale parameters and climate sensitivities from sophisticated atmosphere-ocean climate models in den Elzen and Schaeffer (2000). Using this range results in the spread in projected tem-
perature response shown in Figure 45.3 (gray area). Here the solid line represents the temperature response of our meta-IMAGE reference case, which is not necessarily the most likely. The global surface temperature increase for the reference projection is about 3.1°C for the period 1751 (pre-industrial) to 2100 for the reference case. The innermost, dark gray area depicts the range of results if all the different IRF time scale parameters are applied using the same IPCC 'best guess' climate sensitivity of 2.5°C. The uncertainty range broadens significantly if the IRF time scale parameters are combined with their respective climate sensitivities, ranging from 1.58 to 3.7°C. Finally, the range broadens even further if the IRF time scale parameters are arbitrarily combined with climate sensitivities from the full IPCC range of 1.5-4.5°C (light gray). Figure 45.3 clearly shows that the climate sensitivity plays a dominant role in determining the range of absolute temperature increase.
Figure 45.3(a) illustrates the uncertainty of carbon cycle modeling as shown by terrestrial and oceanic carbon sink fluxes (dark gray), and in the sink and land-use sources (light gray). Figure 45.3(b) illustrates uncertainty in the climate system response as estimated using different IRFs of AOGCMs. Dark gray: range of outcomes for IPCC's 'best guess' climate sensitivity of 2.5 °C combined with the IRF time scale parameters of the AOGCMs. Grey: IRF time scale parameters combined with their respective climate sensitivities. Light gray: IRF time scale parameters arbitrarily combined with climate sensitivities in the full IPCC range of 1.5-4.5°C.
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