The main loss of hydrogen (H2) is believed to be through surface deposition, but about 25% is lost through oxidation by OH. In the stratosphere, this enhances the water vapour concentrations and thus also affects the ozone concentrations. In the troposphere, the chemical effects are similar to those of CO, leading to ozone production and CH4 enhancements (Prather, 2003). Derwent et al. (2001) calculated an indirect 100-year GWP for the tropospheric effects of H2 of 5.8, which includes the effects of CH4 lifetime and tropospheric ozone.
2.10.4 New Alternative Metrics for Assessing Emissions
While the GWP is a simple and straightforward index to apply for policy makers to rank emissions of different greenhouse gases, it is not obvious on what basis ‘equivalence’ between emissions of different species is obtained (Smith and Wigley, 2000; Fuglestvedt et al., 2003). The GWP metric is also problematic for short-lived gases or aerosols (e.g., NOx or BC aerosols), as discussed above. One alternative, the RF index (RFI) introduced by IPCC (1999), should not be used as an emission metric since it does not account for the different residence times of different forcing agents.