Surface Anthropogenic NO _x Emissions Drive 1995–2014 Increase in Tropospheric Ozone: Implications for Ozone and Methane Radiative Forcing

Abstract Tropospheric ozone influences Earth's radiative energy budget and has increased in recent decades. With initial‐condition ensembles from a single chemistry‐climate model, we show that global surface anthropogenic NO x emissions explain about 90% of the simulated 1995–2014 tropospheric ozone increase (1.8 DU) and that this increase exceeds those arising internally from natural climate variability. South and East Asian NO x emissions account for about 40% of the global NO x ‐driven ozone increase but contribute about 80% of the associated net positive stratospheric‐adjusted radiative forcing (SARF; tropospheric ozone plus methane), reflecting a weaker methane response to Asian NO x than to tropical surface NO x emissions. Considering both CO + NMVOC and NO x emissions from Asia produces a larger net SARF than from global emissions (+0.032 vs. +0.023 W m −2 ). Tropospheric ozone changes over 1995–2014 from global CO + NMVOC emissions or methane concentrations alone are too small to be detected relative to internal climate variability.