An emissions‐based view of climate forcing by methane and tropospheric ozone

Abstract

We simulate atmospheric composition changes in response to increased methane and tropospheric ozone precursor emissions from the preindustrial to present‐day in a coupled chemistry‐aerosol‐climate model. The global annual average composition response to all emission changes is within 10% of the sum of the responses to individual emissions types, a more policy‐relevant quantity. This small non‐linearity between emission types permits attribution of past global mean methane and ozone radiative forcings to specific emissions despite the well‐known non‐linear response to emissions of a single type. The emissions‐based view indicates that methane emissions have contributed a forcing of ∼0.8–0.9 W m−2, nearly double the abundance‐based value, while the forcing from other ozone precursors has been quite small (∼−0.1 for NOx, ∼0.2 for CO + VOCs).