Drivers of Tropical Upper Tropospheric Ozone Increases in 1995-2014 and Radiative Forcing Implications

Changes in tropospheric O₃ can alter Earth’s radiative energy budget, particularly in the tropical upper troposphere (TUT), where observations suggest concentrations have increased during recent decades. To understand which precursor emissions to target to reduce O₃ radiative forcing, we conduct a set of ‘‘all-but-one-forcing’’ initial-condition ensembles over 1995-2014 in the WACCM6 chemistry-climate model, with one forcing agent held constant at 1995 levels. For each ensemble, three members are generated using sea surface temperatures and sea ice prescribed from fully-coupled CESM2-WACCM6 historical simulations. Here, we quantify the time-evolving impacts of all global surface anthropogenic emissions (SfcA), global SfcA of NO_x alone, global SfcA of CO+NMVOCs, Asian SfcA of NO_x+CO+NMVOCs, and Asian SfcA of NO_x alone, as well as global CH₄ prescribed as a latitudinal varying lower boundary condition. The ensemble-mean net stratospheric-adjusted radiative forcing (SARF) from tropospheric O₃ and CH₄, quantified with an O₃ radiative kernel (developed by a radiative transfer model with idealized O₃ perturbations) and the most recent IPCC expression for CH₄, increases by 0.14 W m^-2 over 1995-2014. After accounting for internal climate variability, we find a dominant role for global SfcA, primarily NO_x emissions, in driving TUT O₃ increases, with Asian SfcA contributing nearly half of the rise and Asian NO_x emissions alone accounting for two thirds of this contribution. The increase in tropospheric O₃ SARF in response to global SfcA of NO_x is 0.078 W m^-2, five times that from CO+NMVOCs emissions, and three times that from global CH₄ rise. However, when accounting for the OH feedback on CH₄, the net SARF response is minimal (0.011 W m^-2), and smaller than that from CO+NMVOCs emissions (0.016 W m^-2). Asian SfcA of O₃ precursors contribute to a much greater net SARF over the two decades than global SfcA (0.042 vs 0.0082 W m^-2), due to their smaller impact on CH₄, and NO_x emissions alone accounts for a 0.029 W m^-2 increase in net SARF over this period. Our study highlights the significant influence of SfcA of NO_x—particularly from Asia—on TUT O₃ over 1995-2014, while from the perspective of radiative forcing, mitigation efforts targeting CO and VOCs may yield greater benefits because both O₃ and CH₄ SARF decrease.