Present global warming: a justifiable and stable metric for evaluating short-lived climate pollutants

Abstract

The impacts of short-lived climate pollutants (SLCPs) such as methane are typically described using metrics that compare their climate impacts to carbon dioxide’s climate impact. The metrics consider a climate pollutant’s atmospheric heat-trapping effectiveness and atmospheric lifetime. Here we introduce an alternative metric called the Present Global Warming (PGW), which uses economic exponential discount modelling to characterize short-term and long-term effects simultaneously, resulting in a justifiable, familiar, and stable metric for evaluating SLCPs. We recommend quantifying the climate impacts of methane emissions using 2.5% annual discounting—consistent with the discount rates recently proposed in academic work and traditionally used in climate policy—corresponding to methane PGW2.5% = 50. In this context, one ton of emitted methane has the same climate impact as 50 tons of emitted carbon dioxide.