Greenhouse gas emissions – timing matters!

Cooper, S.; Green, R.; Hattam, L.; Röder, M.; Welfle, A.; McManus, M.

Greenhouse gases (GHGs) cause climate change. However, the actual climatechange effects occur after the emissions that cause them and the variation with time of these effects is important in understanding their potential impact. The principal GHG is carbon dioxide (CO2) but there are many others such as methane (CH4) and nitrous oxide (N2O). For simplicity, the effects of these GHGs are often combined in terms of CO2- equivalent mass (CO2e). However, what does “equivalent” really mean? And, given that the various GHGs have different potencies and lifetimes, does this “equivalence” mask anything? To complicate matters further, while some systems cause emissions at a single point in time, others (such as bioenergy) cause various emissions and absorptions of GHGs over a period of many years. Their GHG emissions might balance out overall, but what effect does their timing have on their impact? In this note, we will clarify these issues and look at a couple of case studies involving bioenergy, based on recently published work. While CO2 equivalence is a sensible metric for lots of applications, in some cases it doesn’t tell the full story. The timing of GHG emissions, and what those emissions are, matters!