Climate Forcing Due to Future Ozone Changes: An Intercomparison of Metrics and Methods

This study assesses three different measures of radiative forcing (instantaneous: IRF; stratospheric-temperature adjusted: SARF; effective: ERF) for future changes in ozone. These use a combination of online and offline methods. We separate the effects of changes in ozone precursors and ozone-depleting substances (ODSs) and configure model experiments such that only ozone changes (including consequent changes in humidity, clouds and surface albedo) affect the evolution of the model physics and dynamics.

In the Shared Socioeconomic Pathway 3-7.0 (SSP3-7.0) we find robust increases in ozone due to future increases in ozone precursors and decreases in ODSs, leading to a radiative forcing increase from 2015 to 2050 of 0.268 ± 0.084 W m−2 ERF, 0.244 ± 0.057 W m−2 SARF and 0.288 ± 0.101 W m−2 IRF. This increase makes ozone the second largest contributor to future warming by 2050 in this scenario, approximately half of which is due to stratospheric ozone recovery and half due to tropospheric ozone precursors.

Increases in ozone are found to decrease the cloud fraction, causing an overall negative adjustment to the radiative forcing (positive in the short wave but negative in the long wave). Non-cloud adjustments due to water vapour and albedo changes are positive. ERF is slightly larger than the offline SARF for the total ozone change but approximately double the SARF for the ODS-driven change (0.156 ± 0.071 W m−2 ERF, 0.076 ± 0.025 W m−2 SARF). Hence ERF is a more appropriate metric for diagnosing the climate effects of stratospheric ozone changes.