Climate Forcing due to Future Ozone Changes: An intercomparison of metrics and methods

Abstract. We use Earth system models and a chemistry transport model to determine the radiative forcing due to changes in ozone Three different measures of radiative forcing (instantaneous: IRF, stratospheric-temperature adjusted: SARF, effective: ERF) are compared using both online and offline calculations for the IRF and SARF, and online calculations for the ERF. To isolate the ozone radiative forcing, we configure the model experiments such that only the ozone changes (including respective changes in water vapour, clouds etc.) affect the evolution of the model physics and dynamics. We find robust changes in ozone due to future changes in ozone precursors and ODSs. These lead to a positive radiative forcing of 0.27±0.09 Wm^-2 ERF, 0.24 ± 0.021 W m^-2 offline SARF, 0.29 ± 0.10 Wm^-2 online IRF. Increases in ozone lead to an overall decrease in cloud fraction (although there are increases at some levels). This decrease causes an overall negative adjustment to the radiative forcing (positive in the short-wave (SW), but negative in the long-wave (LW)). Non-cloud adjustments (excluding stratospheric temperature) are positive (both LW and SW). The opposing signs of the cloud and non-cloud adjustments mean the overall adjustment to the SARF is slightly positive.

We find general agreement between models in the impact of the ozone changes on temperature and cloud fractions and agreement in the signs of the individual adjustment terms when split into SW and LW. However, the overall difference between the ERF and SARF is smaller than the inter-model variability.