Direct and indirect global warming potentials of source gases
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- CICERO Reports 
Global Warming Potentials for emissions of CH4, CO and NOx that take into account indirect chemical effects have been estimated for five time horizons. The indirect effects of methane emissions that have been considered are the ability of methane to increase its own lifetime (through the CH4/OH-feedback) and the effects on the levels of tropospheric ozone and stratospheric water vapour. All these effects have Positive signs, and our estimate shows that the indirect effects together are of the same magnitude as the direct radiative effect of methane. For emissions of NOx, the picture is somewhat more complicated. Emissions of NOx generally increase the levels of tropospheric ozone. Ozone is an important GHG and therefore this mechanism constitutes a positive indirect effect. On the other hand, NOx emissions increase the levels of OH, thereby reducing the lifetime of gases that are removed by OH. Of these, methane is the most important in this context. The effect on CH4, through changes in OH, constitutes a negative indirect effect on climate. Difficulties may therefore be connected to estimating the net effect of these opposing effects. However, for NOx emitted from airplanes the radiative forcing from changes in ozone is significantly larger than the radiative forcing from the changes in methane. We may therefore conclude that the net effect of NOx emitted from aeroplanes is positive. Our estimates also indicate that the climate impact of NOx emissions from aeroplanes, expressed as GWPS, is relatively large. One should, however, bear in mind that while the GWPs for NOx from aeroplanes are relatively large, the emissions are small compared to the surface emissions of this component. For NOx emitted from surface sources the picture is not so clear. Contrary to the emissions from aeroplanes, surface emissions of NOx affect OH in a region of the troposphere that is very important for the oxidation of CH4. The negative effect through reduced CH4 levels are therefore more important than in the case of NOx emitted from aeroplanes. The estimated net effect is slightly negative since the positive effect through increased O3 levels is somewhat smaller than the negative effect. This estimate may be model dependent and should only be considered a preliminary finding. Emissions of CO increase the tropospheric levels of ozone, and, through lowered levels of OH, CO increases the levels of methane. Thus, the considered indirect effects of CO are positive. The effects of NMHC emissions on the radiative balance are assessed by comparison with the effects of CO. This assessment is dependent on the chemical composition of NHMC. In the IPCC assessment from 1992, only the direct GWPs for methane were given and the estimates of the indirect GWP from the 1990 assessment were abandoned. In IPCC 1990, GWPs for NOx from surface sources were also given, but only the effect through increased levels of tropospheric ozone were considered. This estimate was also left out in the 1992 report. There are large uncertainties connected to the estimates of the indirect GWPS. The estimated GWPs represent what we are able to infer with the present model tools, and they should be considered as preliminary results which will change. Efforts should be made to develop these model further and to refine the methods for estimating the GWPS. The GWPs for CF4 and C2F6 given in this current report are also different from earlier estimates (WMO, 1992 and EPCC, 1992). The estimates are taken from Isaksen et al. (1992b), and a summary of the present state of knowledge in this area is also given. In addition, a short review of the knowledge of SF6 is also given. CF4 , C2 F6 and SF6 , all have very long atmospheric lifetimes and strong absorbtion of IR, giving very high GWP values.