GWP*is a model, not a metric

Abstract

Abstract To aggregate different greenhouse gases, the UNFCCC common reporting formats, the IPCC inventory guidelines, and the Paris Agreement rulebook use an emission metric that has been around for more than 30 years: the Global Warming Potential (IPCC, 1990). A ‘metric’ establishes an ‘equivalence’ between an amount of CO2 emissions and other greenhouse gases (such as CH4), which can be used to design cost-effective mitigation strategies (Fuglestvedt et al., 2003). As with many established concepts, there has been no shortage of critical views on GWPs (Denison et al., 2019; Plattner et al., 2009;Shine, 2009). More recently, a method that relates emission rate changes of short-lived gases like methane to emissions of CO2 has been suggested, referred to as GWP* (Smith et al., 2012;Allen et al., 2016;Allen et al., 2018;Cain et al., 2019;Smith et al., 2021). This method can usefully approximate the temperature implications of emission time series. Rather mistakenly, though, it has been suggested as an emission metric that can replace the widely used GWPs. The most recent WG1 IPCC report (IPCC, 2021) presents GWP and GWP* both as metrics in the underlying Chapter, although the Summary for Policymakers instead refers to GWP* and related methods as “approaches”. Here, we examine how GWP* falls short on key criteria for a useful emission metric that can usefully be applied in real-world mitigation actions. We show that GWP* can exhibit the wrong sign in terms of the climate effects of a single year of emissions, and that aggregate emissions based on GWP* feature variability which would undermine the stability of any legal framework.