Investigating the role of stratospheric ozone as a driver of inter-model spread in CO ₂  effective radiative forcing

Abstract. Addressing the cause of inter-model spread in carbon dioxide (CO2) radiative forcing is essential for reducing uncertainty in estimates of climate sensitivity. Recent studies have demonstrated that a large proportion of this spread arises from variance in model base-state climatology, particularly the specification of stratospheric temperature, which itself plays a dominant role in determining the magnitude of CO2 forcing. Here, we investigate stratospheric ozone (O3) as a cause of inter-model differences in stratospheric temperature, and hence its role as a contributing factor to spread in CO2 radiative forcing. We use the Norwegian Earth System Model 2 (NorESM2) to analyse the impact of systematic increases and decreases in stratospheric O3 on the magnitude of 4xCO2 effective radiative forcing (ERF) and its components. Firstly, we demonstrate that the accurate estimation of instantaneous radiative forcing (IRF) requires the use of host-model radiative transfer calculations. Secondly, we show that a 50 % increase or decrease in the stratospheric O3 concentration leads to significant differences in the base-state stratospheric temperature, ranging from +6 to −9 K, respectively. These experiments impact IRF primarily due to the influence of the base-state stratospheric temperature on the emission of outgoing longwave radiation, with the spectral overlap of CO2 and O3 playing a subsidiary role. However, the impact on IRF does not result in a correspondingly large spread in CO2 ERF. We conclude that inter-model differences in stratospheric O3 concentration are, therefore, not predominantly responsible for inter-model spread in CO2 ERF.