9.2.1 Radiative Forcing Estimates Used to Simulate Climate Change

9.2.1.1 Summary of ‘Forward’ Estimates of Forcing for the Instrumental Period

Estimates of the radiative forcing (see Section 2.2 for a definition) since 1750 from forward model calculations and observations are reviewed in detail in Chapter 2 and provided in Table 2.12. Chapter 2 describes estimated forcing resulting from increases in long-lived greenhouse gases (carbon dioxide (CO₂), methane, nitrous oxide, halocarbons), decreases in stratospheric ozone, increases in tropospheric ozone, sulphate aerosols, nitrate aerosols, black carbon and organic matter from fossil fuel burning, biomass burning aerosols, mineral dust aerosols, land use change, indirect aerosol effects on clouds, aircraft cloud effects, solar variability, and stratospheric and tropospheric water vapour increases from methane and irrigation. An example of one model’s implemented set of forcings is given in Figure 2.23. While some members of the MMD at PCMDI have included a nearly complete list of these forcings for the purpose of simulating the 20th-century climate (see Supplementary Material, Table S9.1), most detection studies to date have used model runs with a more limited set of forcings. The combined anthropogenic forcing from the estimates in Section 2.9.2 since 1750 is 1.6 W m^–2, with a 90% range of 0.6 to 2.4 W m^–2, indicating that it is extremely likely that humans have exerted a substantial warming influence on climate over that time period. The combined forcing by greenhouse gases plus ozone is 2.9 ± 0.3 W m^–2 and the total aerosol forcing (combined direct and indirect ‘cloud albedo’ effect) is virtually certain to be negative and estimated to be –1.3 (90% uncertainty range of –2.2 to –0.5 W m^–2; see Section 2.9). In contrast, the direct radiative forcing due to increases in solar irradiance is estimated to be +0.12 (90% range from 0.06 to 0.3) W m^–2. In addition, Chapter 2 concludes that it is exceptionally unlikely that the combined natural (solar and volcanic) radiative forcing has had a warming influence comparable to that of the combined anthropogenic forcing over the period 1950 to 2005. As noted in Chapter 2, the estimated global average surface temperature response from these forcings may differ for a particular magnitude of forcing since all forcings do not have the same ‘efficacy’ (i.e., effectiveness at changing the surface temperature compared to CO₂; see Section 2.8). Thus, summing these forcings does not necessarily give an adequate estimate of the response in global average surface temperature.