On the potential for alternative greenhouse gas equivalence metrics to influence sectoral mitigation patterns
Equivalence metrics used to quantify the relative climate impacts of different atmospheric forcers serve an essential function in policy and economic discussions about global climate change. The 100-year global warming potential (GWP-100), the most established greenhouse gas (GHG) equivalence metric, is used within the Kyoto Protocol, and in most emissions inventory, trading and offset mechanisms, to assign the mitigation value of non-carbon dioxide greenhouse gases relative to carbon dioxide. In recent literature the GWP-100 and alternative metrics have been used to compare various anthropogenic climate forcers with respect to a wide range of environmental and economic goals. Building on this work, we examine how 16 different static and time-varying CO2-equivalence schemes might influence GHG mitigation across sectors and gases in a perfect and fluid global mitigation regime. This mitigation regime is guided by achieving a global mean radiative forcing (RF) of 5.7 Wm−2 in 2100 from 1765 levels through a mitigation policy of prescribed emissions reductions in each decade. It was found that static metrics defined on 20- instead of 100-year time horizons favor mitigation strategies that maximize the abatement of short-lived gases (e.g. methane), on average resulting in an RF from methane in 2100 of 0.5 Wm−2 instead of 1.1 Wm−2 from 100-year metrics. Similarly, metrics that consider integrated rather than end-point climate impacts imply mitigation strategies that maximize mitigation of shorter-lived GHGs, resulting in higher abatement of agriculture and waste emissions. Comparing extreme scenarios, these mitigation shifts across gases and sectors result in a nearly 30% difference in the representation of methane in global cumulative emissions reductions. This shift across gases and sectors to mitigate shorter-lived GHGs, in lieu of longer-lived GHGs like carbon dioxide, has implications for the long-term warming commitment due to 21st century emissions.