Global Warming Potentials (GWPs) serve as an “exchange rate” among greenhouse gases, enabling emitters to compare the climate benefits of reducing emissions of different gases, such as carbon dioxide (CO2) or methane (CH4). The methane GWP is defined as the total radiative forcing (i.e. heating) over 100 years (or another time horizon) caused by emitting methane, compared to emitting an equal amount of CO2. Since methane is a precursor of ozone and stratospheric water vapor, both of which are greenhouse gases, the methane GWP customarily includes the radiative forcing caused by these decay products. Our analysis suggests that methane breakdown in the atmosphere produces more ozone and more radiative forcing than previously recognized, with most of the additional ozone residing in the stratosphere. New data also revise the methane lifetime upwards (9.1 yr). Taken together, we calculate the methane GWP to be 32, which is 25% larger than past assessments. This high GWP implies that methane emissions are more harmful to the climate than previously thought, but also that reducing those emissions will have greater benefits. Our full analysis of atmospheric methane lifetime, future abundances, and GWP is published in Atmospheric Chemistry and Physics.
GWP provides a basis for comparing climate heating caused by different greenhouse gases that have different absorbances (heat-trapping ability) and atmospheric lifetimes. Our work suggests that the methane GWP (100 yr) is 32, significantly larger than the value of 25 that that was recommended by the Intergovernmental Panel on Climate Change (2007).