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Working Group I: The Scientific Basis |
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Working Group I: The Scientific Basis |
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6.12.2 Direct GWPs
The CO2 response function used in this report is the same as that in WMO (l999) and the SAR and is based on the “Bern” carbon cycle model (see Siegenthaler and Joos, 1992; Joos et al., l996) run for a constant future mixing ratio of CO2 over a period of 500 years. The Bern carbon cycle model was compared to others in IPCC (l994), where it was shown that different models gave a range of as much as 20% in the CO2 response, with the greatest differences occurring over time-scales greater than 20 years. The radiative efficiency per kilogram of CO2 has been updated compared to previous IPCC assessments (IPCC, 1994; SAR). Here we employ the approach discussed in WMO (l999) using the simplified formula presented in Table 6.2. We assume a background CO2 mixing ratio of 364 ppmv, close to the present day value (WMO (1995) used 354 ppmv). For this assumption, this expression agrees well with the adjusted total-sky radiative forcing calculations of Myhre and Stordal (1997); see also Myhre et al. (1998b). The revised forcing is about 12% lower than that in the SAR. For a small perturbation in CO2 from 364 ppmv, the radiative efficiency is 0.01548 Wm-2/ppmv. This value is used in the GWP calculations presented here. We emphasise that it applies only to GWP calculations and cannot be used to obtain the total radiative forcing for this key gas since pre-industrial times, due to time-dependent changes in mixing ratio as noted above. Because of this change in the CO2 forcing per mass, the CO2 AGWPs are 0.207, 0.696, and 2.241 Wm-2/yr /ppmv for 20, 100, and 500 year time horizons, respectively. These are smaller than the values used in the SAR by 13%. AGWPs for any gas can be obtained from the GWP values given in the tables presented here by multiplying by these numbers. The decreases in the CO2 AGWPs will lead to proportionately larger GWPs for other gases compared to previous IPCC assessments, in the absence of other changes. In Tables 6.7 and 6.8, GWPs (on a mass basis) for 93 gases are tabulated for time horizons of 20, 100, and 500 years. The list includes CH4, N2O, CFCs, HCFCs, HFCs, hydrochlorocarbons, bromocarbons, iodocarbons, fully fluorinated species, fluoroalcohols, and fluoroethers. The radiative efficiencies per kilogram were derived from the values given per ppbv in Section 6.3. Several of these have been updated since the SAR, most notably that of CFC-11. Since the radiative forcings of the halocarbon replacement gases are scaled relative to CFC-11 in GWP calculations, the GWPs of those gases are also affected by this change. As discussed further in Section 6.3, the change in radiative forcing for CFC-11 reflects new studies (Pinnock et al., 1995; Christidis et al., l997; Hansen et al., l997a; Myhre and Stordal, 1997; Good et al., 1998) suggesting that the radiative forcing of this gas is about 0.25 Wm-2 ppbv-1, an increase of about 14% compared to the value adopted in earlier IPCC reports, which was based on the study of Hansen et al. (l988).
The lifetimes and adjustment times used in Tables 6.7 and 6.8 come from Chapter 4 except where noted. For some gases (including several of the fluoroethers), lifetimes have not been derived from laboratory measurements, but have been estimated by various other means. For this reason, the lifetimes for these gases, and hence the GWPs, are considered to be much less reliable, and so these gases are listed separately in Table 6.8. NF3 is listed in Table 6.8 because, although its photolytic destruction has been characterised, other loss processes may be significant but have not yet been characterised (Molina et al., 1995). Note also that some gases, for example, trifluoromethyl iodide (CF3I) and dimethyl ether (CH3OCH3) have very short lifetimes (less than a few months); GWPs for such very short-lived gases may need to be treated with caution, because the gases are unlikely to be evenly distributed globally, and hence estimates of, for example, their radiative forcing using global mean conditions may be subject to error. Uncertainties in the lifetimes of CFC-11 and CH3CCl3 are thought to be about 10%, while uncertainties in the lifetimes of gases obtained relative to CFC-11 or CH3CCl3 are somewhat larger (20 to 30%) (SAR; WMO, l999). Uncertainties in the radiative forcing per unit mass of the majority of the gases considered in Table 6.7 are approximately ± 10%. The SAR suggested typical uncertainties of ± 35% (relative to the reference gas) for the GWPs, and we retain this uncertainty estimate for gases listed in Table 6.7. In addition to uncertainties in the CO2 radiative efficiency per kilogram and in the response function, AGWPs of CO2 are affected by assumptions concerning future CO2 abundances as noted above. Furthermore, as the CO2 mixing ratios and climate change, the pulse response function changes as well. In spite of these dependencies on the choice of future emission scenarios, it remains likely that the error introduced by these assumptions is smaller than the uncertainties introduced by our imperfect understanding of the carbon cycle (see Chapter 3). Finally, although any induced error in the CO2 AGWPs will certainly affect the non-CO2 GWPs, it will not affect intercomparisons among non-CO2 GWPs. |
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