Climate Change 2001: Working Group I: The Scientific Basis

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6.3.1 Carbon Dioxide

IPCC (1990) and the SAR used a radiative forcing of 4.37 Wm^-2 for a doubling of CO₂ calculated with a simplified expression. Since then several studies, including some using GCMs (Mitchell and Johns, 1997; Ramaswamy and Chen, 1997b; Hansen et al., 1998), have calculated a lower radiative forcing due to CO₂ (Pinnock et al., 1995; Roehl et al., 1995; Myhre and Stordal, 1997; Myhre et al., 1998b; Jain et al., 2000). The newer estimates of radiative forcing due to a doubling of CO₂ are between 3.5 and 4.1 Wm^-2 with the relevant species and various overlaps between greenhouse gases included. The lower forcing in the cited newer studies is due to an accounting of the stratospheric temperature adjustment which was not properly taken into account in the simplified expression used in IPCC (1990) and the SAR (Myhre et al., 1998b). In Myhre et al. (1998b) and Jain et al. (2000), the short-wave forcing due to CO₂ is also included, an effect not taken into account in the SAR. The short-wave effect results in a negative forcing contribution for the surface-troposphere system owing to the extra absorption due to CO₂ in the stratosphere; however, this effect is relatively small compared to the total radiative forcing (< 5%).

The new best estimate based on the published results for the radiative forcing due to a doubling of CO₂ is 3.7 Wm^-2, which is a reduction of 15% compared to the SAR. The forcing since pre-industrial times in the SAR was estimated to be 1.56 Wm^-2; this is now altered to 1.46 Wm^-2 in accordance with the discussion above. The overall decrease of about 6% (from 1.56 to 1.46) accounts for the above effect and also accounts for the increase in CO₂ concentration since the time period considered in the SAR (the latter effect, by itself, yields an increase in the forcing of about 10%).

While an updating of the simplified expressions to account for the stratospheric adjustment becomes necessary for radiative forcing estimates, it is noted that GCM simulations of CO₂-induced climate effects already account for this physical effect implicitly (see also _{Chapter 9}). In some climate studies, the sum of the non-CO₂ well-mixed greenhouse gases forcing is represented by that due to an equivalent amount of CO₂. Because the CO₂ forcing in the SAR was higher than the new estimate, the use of the equivalent CO₂ concept would underestimate the impact of the non-CO₂ well-mixed gases, if the IPCC values of radiative forcing were used in the scaling operation.