Large Regional Shortwave Forcing by Anthropogenic Methane Informed by Jovian Observations

The shortwave radiative forcing of methane is larger than the well-known longwave effects of many less important anthropogenic forcing agents routinely included in radiative forcing assessments. Recently it was recognized that the widely-used formulae to calculate CH₄ radiative forcing systematically underestimated the global forcing by 25% since they did not include these effects. Here we show that shortwave forcing by methane can be accurately calculated despite considerable uncertainty and the existing large gaps in its shortwave spectroscopy. We demonstrate that the forcing is invariant, even when confronted with much more complete empirical methane absorption spectra extending to violet-light wavelengths derived from observations of methane-rich Jovian planetary atmospheres. We undertake the first global, spatially-resolved calculations of this forcing and find that localized annual-mean methane forcing can be as large as +0.25 W/m², ten times the global annualized shortwave forcing and 41% of the total CH₄ forcing. Our observationally-based estimates of shortwave forcing by anthropogenic methane are sufficiently large and accurate to warrant inclusion in analyses of historical climate, projections of future climate, and explorations of climate-change mitigation pathways.