2.10.2 Direct Global Warming Potentials

All GWPs depend on the AGWP for CO₂ (the denominator in the definition of the GWP). The AGWP of CO₂ again depends on the radiative efficiency for a small perturbation of CO₂ from the current level of about 380 ppm. The radiative efficiency per kilogram of CO₂ has been calculated using the same expression as for the CO₂ RF in Section 2.3.1, with an updated background CO₂ mixing ratio of 378 ppm. For a small perturbation from 378 ppm, the RF is 0.01413 W m^–2 ppm^–1 (8.7% lower than the TAR value). The CO₂ response function (see Table 2.14) is based on an updated version of the Bern carbon cycle model (Bern2.5CC; Joos et al. 2001), using a background CO₂ concentration of 378 ppm. The increased background concentration of CO₂ means that the airborne fraction of emitted CO₂ (Section 7.3) is enhanced, contributing to an increase in the AGWP for CO₂. The AGWP values for CO₂ for 20, 100, and 500 year time horizons are 2.47 × 10^–14, 8.69 × 10^–14, and 28.6 × 10^–14 W m^–2 yr (kg CO₂)^–1, respectively. The uncertainty in the AGWP for CO₂ is estimated to be ±15%, with equal contributions from the CO₂ response function and the RF calculation.

Updated radiative efficiencies for well-mixed greenhouse gases are given in Table 2.14. Since the TAR, radiative efficiencies have been reviewed by Montzka et al. (2003) and Velders et al. (2005). Gohar et al. (2004) and Forster et al. (2005) investigated HFC compounds, with up to 40% differences from earlier published results. Based on a variety of radiative transfer codes, they found that uncertainties could be reduced to around 12% with well-constrained experiments. The HFCs studied were HFC-23, HFC-32, HFC-134a and HFC-227ea. Hurley et al. (2005) studied the infrared spectrum and RF of perfluoromethane (CΩF₄) and derived a 30% higher GWP value than given in the TAR. The RF calculations for the GWPs for CH₄, N₂O and halogen-containing well-mixed greenhouse gases employ the simplified formulas given in Ramaswamy et al. (2001; see Table 6.2 of the TAR). Table 2.14 gives GWP values for time horizons of 20, 100 and 500 years. The species in Table 2.14 are those for which either significant concentrations or large trends in concentrations have been observed or a clear potential for future emissions has been identified. The uncertainties of these direct GWPs are taken to be ±35% for the 5 to 95% (90%) confidence range.