Surface anthropogenic NOx 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.

Plain Language Summary

Tropospheric ozone affects Earth's energy balance and has continued to increase in recent decades. We use a chemistry-climate model to determine how near-surface emissions of different ozone precursors from human activities affect tropospheric ozone from 1995 to 2014. After accounting for natural climate fluctuations like El Niño, we find that nitrogen oxide emissions, especially from South and East Asia, are the main driver of the simulated ozone increases. These Asian emissions explain about 40% of the global nitrogen-oxide-driven tropospheric ozone increase, but a much larger share of the perturbation to Earth's energy balance due to combined ozone and methane changes. This occurs because these emissions increase ozone widely across the Northern Hemisphere but only affect methane lifetime over a narrow latitude band. Our ensemble approach simulates a range of responses that might have been observed; observed trends contain an unknown contribution from climate variability on top of the “forced” response from ozone precursor emissions. We find that natural climate variability can reverse the apparent ozone response to past carbon monoxide, non-methane volatile organic compounds emissions, and rising global methane, whereas the nitrogen-oxide-driven ozone increase remains positive and detectable across all realizations, suggesting its signal should be identifiable in the observational record.