Comment on ‘Unintentional unfairness when applying new greenhouse gas emissions metrics at country level’

Rogelj and Schleussner (2019) (RS19) raise a number of important issues in their critique of the use of warming-equivalent emissions to relate very long-lived (and hence cumulative) climate pollutants (LLCPs) and short-lived climate pollutants (SLCPs), but in the process introduce some errors and misrepresentations that merit a response. Although RS19 focuses on GWP* (Allen et al2016, 2018, Cain et al 2019), their arguments would apply to any metric that accounts for the key difference between SLCPs and LLCPs. Increasing emissions of an SLCP have a much greater impact on global mean surface temperature (GMST) per tonne of SLCP emitted than constant emissions of that SLCP while LLCPs have approximately the same impact on GMST per tonne of LLCP emitted, regardless of emission trajectory. Metrics that capture this difference include forcing-equivalent emissions (Wigley 1998, Jenkins et al 2018), global temperature-change potential for a sustained emission (GTP_S) (Shine et al 2005), mixed metrics (Lauder et al 2013), or combined global temperature-change potential (CGTP) (Collins et al 2020). The point of GWP* was not to capture this behaviour per se, which was already well known, but to demonstrate that it could be captured relatively simply, and retain the use of the familiar global warming potential (GWP) values.