Global temperature goals should determine the time horizons for greenhouse gas emission metrics

Authors: Abernethy S, RB Jackson

Emission metrics, a crucial tool in setting effective exchange rates between greenhouse gases, currently require an arbitrary choice of time horizon. Here, we propose a novel framework to calculate the time horizon that aligns with scenarios achieving a specific temperature goal. We analyze the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 ¡C Scenario Database to find that time horizons aligning with the 1.5 ¡C and 2 ¡C global warming goals of the Paris Agreement are 24 [90% prediction interval: 7, 41] and 58 [90% PI: 41, 74] years, respectively. We then use these time horizons to quantify time-dependent emission metrics for methane. We find that the Global Warming Potential (GWP) values that align with the 1.5 ¡C and 2 ¡C goals are GWP1.5 ¡C = 75 [90% PI: 54, 107] and GWP2 ¡C = 42 [90% PI: 35, 54]. For the Global Temperature change Potential (GTP) they are GTP1.5 ¡C = 41 [90% PI: 16, 102] and GTP2 ¡C = 9 [90% PI: 7, 16]. The most commonly used time horizon, 100 years, underestimates methane's GWP and GTP by 34% and 38%, respectively, relative to the values we calculate that align with the 2 ¡C goal and by 63% and 87%, respectively, relative to the 1.5 ¡C goal. To best align emission metrics with the Paris Agreement 1.5 ¡C goal, we recommend a 24 year time horizon, using 2045 as the endpoint time, with its associated GWP1.5 ¡C = 75 and GTP1.5 ¡C = 41.