Contribution of CH4 to Multi-Gas Emission Reduction Targets

Abstract

Weighting indices can be used to express emissions of different greenhouse gases (GHGs) in CO2-equivalent ternis. The choice of a specific weighting index is often thought to determine the emphasis on non-CO2 GHGs in a multi-gas target. Although the choice of a 100-year Global Warming Potential (GWP) for the Kyoto Protocol narrows the range of possible index values, we use the example of CH4 to show that this choice does not constrain the GWP to a single number. Projected changes in atmospheric CH4 concentrations due to interactions with CO, NOx, and OH produce direct GWPs ranging from 12.5–15.5. This increases to 14–24 when the range of uncertainty in indirect effects due to tropospheric ozone (O3) and stratospheric water vapour is included. We examine the impact of the calculated range in GWPs on the optimal contribution of CH4 to U.S. emission reductions up to 1.0 GtCeq. Our results show that for a 100-year GWP there is still significant variation in effective CH4 reductions for a multi-gas target. This has implications for the overall contribution of CH4 and other non-CO2 GHGs to both short-term and long-term strategies focussed on the mitigation of climate change impacts.