9.3.1.2 Projections of future climate from forcing scenario experiments (IS92a)

Figure 9.5: (a) The time evolution of the globally averaged temperature change relative to the years (1961 to 1990) of the DDC simulations (IS92a). G: greenhouse gas only (top), GS: greenhouse gas and sulphate aerosols (bottom). The observed temperature change (Jones, 1994) is indicated by the black line. (Unit: °C). See Table 9.1 for more information on the individual models used here. (b) The time evolution of the globally averaged precipitation change relative to the years (1961 to 1990) of the DDC simulations. GHG: greenhouse gas only (top), GS: greenhouse gas and sulphate aerosols (bottom). (Unit: %). See Table 9.1 for more information on the individual models used here.

Please note that the use of projections for forming climate scenarios to study the impacts of climate change is discussed in Chapter 13.

These experiments include changes in greenhouse gases plus the direct effect of sulphate aerosol using IS92a type forcing (see Chapter 6 for a complete discussion of direct and indirect effect forcing from sulphate aerosols). The temperature change (Figures 9.5a and 9.7a, top) for the 30-year average 2021 to 2050 compared with 1961 to 1990 is +1.3°C with a range of +0.8 to +1.7°C as opposed to +1.6°C with a range of +1.0 to +2.1°C for greenhouse gases only (Cubasch and Fischer-Bruns, 2000). The experiments including sulphate aerosols show a smaller temp-erature rise compared to experiments without sulphate aerosols due to the negative radiative forcing of these aerosols. Additionally, in these simulations CO₂ would double around year 2060. Thus for the averaging period being considered, years 2021 to 2050, the models are still short of the CO₂ doubling point seen in the idealised 1%/yr CO₂ increase simulations. These sensitivity ranges could be somewhat higher (about 30%) if the positive feedback effects from the carbon cycle are included interactively but the magnitude of these feedbacks is uncertain (Cox et al., 2000; Friedlingstein, 2001). The globally averaged precipitation response for 2021 to 2050 for greenhouse gases plus sulphates is +1.5% with a range of +0.5 to +3.3% as opposed to +2.3% with a range of +0.9 to +4.4% for greenhouse gases only (Figures 9.5b and 9.7a, bottom).

9.3.1.3 Marker scenario experiments (SRES)

As discussed in Section 9.1.2, only the draft marker SRES scenarios A2 and B2 have been integrated with more than one AOGCM, because the scenarios were defined too late to have experiments ready from all the modelling groups in time for this report. Additionally, some new versions of models have been used to run the A2 and B2 scenarios that have not had time to be evaluated by Chapter 8. Therefore, we present results from all the model simulations and consider them all as possible realisations of future climate change, but their ranges are not directly comparable to the simple model results in Section 9.3.3 (range: 1.4 to 5.8°C), because in the simple model analysis seven somewhat different versions of the nine models have been considered. Additionally, for the AOGCMs the temperature changes are evaluated for an average of years 2071 to 2100 compared with 1961 to 1990, while the simple model results are differences of the year 2100 minus 1990.

The average temperature response from nine AOGCMs using the SRES A2 forcing (Figures 9.6a and 9.7b, top) for the 30-year average 2071 to 2100 relative to 1961 to 1990 is +3.0°C with a range of +1.3 to +4.5°C, while using the SRES B2 scenarios it amounts to +2.2°C with a range of +0.9 to +3.4°C. The B2 scenario produces a smaller warming which is consistent with its lower positive radiative forcing at the end of the 21st century. For the 30-year average 2021 to 2050 using the A2 scenario, the globally averaged surface air temperature increase compared with 1961 to 1990 is +1.1°C with a range of +0.5 to +1.4°C, while using the SRES B2 scenarios it amounts to +1.2°C with a range of +0.5 to +1.7°C. The values for the SRES scenarios for the mid-21st century are lower than for the IS92a scenarios for the corresponding period due to differences in the forcing.

The average precipitation response using the SRES A2 forcing (Figures 9.6b and 9.7b, bottom) for the 30-year average 2071 to 2100 compared with 1961 to 1990 is an increase of 3.9% with a range of 1.3 to 6.8% , while using the SRES B2 scenarios it amounts to an increase of 3.3% with a range of 1.2 to 6.1%. The lower precipitation increase values for the B2 scenario are consistent with less globally averaged warming for that scenario at the end of the 21st century compared with A2. For the 30-year average 2021 to 2050 the globally averaged precipitation increases 1.2% for the A2 scenario, and 1.6% for B2 which is again consistent with the slightly greater global warming in B2 for mid-21st century compared with A2. Globally averaged changes of temperature and precipitation are summarised in Figure 9.7b. A more extensive analysis of globally averaged temperature changes for a wider range of SRES forcing scenarios using a simple climate model is given in Section 9.3.3.

Figure 9.6: (a) The time evolution of the globally averaged temperature change relative to the years (1961 to 1990) of the SRES simulations A2 (top) and B2 (bottom) (Unit: °C). See Table 9.1 for more information on the individual models used here. (b) The time evolution of the globally averaged precipitation change relative to the years (1961 to 1990) of the SRES simulations A2 (top) and B2 (bottom) (Unit: %). See Table 9.1 for more information on the individual models used here.

Figure 9.7: (a) The global mean, the maximum and minimum simulated by the respective models and the standard deviation for the CMIP2 experiments at the time of CO2-doubling and for the DDC experiments during the years 2021 to 2050 relative to the years 1961 to 1990 for temperature (top) (Unit: °C) and precipitation (bottom) (Unit: %). G: greenhouse gases only, GS: greenhouse gases and sulphate aerosols. See Table 9.1 for more information on the individual models used here. (b) The global mean, the maximum and minimum simulated by the respective models and the standard deviation for the SRES scenario experiments A2 and B2 performed by the AOGCMs, for the years 2021 to 2050 and 2071 to 2100 relative to the years 1961 to 1990 for temperature (top) (Unit: °C) and precipitation (bottom) (Unit: %). See Table 9.1 for more information on the individual models used here.

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