Metrics and methods for aggregating greenhouse gas emissions: GWP* is a useful tool, but is not a metric

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 (GWP). 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*.  The most recent WG1 IPCC report (IPCC, 2021) presents GWP and GWP* both as metrics or approaches. This paper evaluates the idea of using GWP* as a metric for measuring changes in aggregate emissions. It concludes that, although GWP* can be a useful method for approximating the temperature implications of emission time series, it risks undermining legal and policy stability if adopted as a metric.