Investigating the role of stratospheric ozone as a driver of inter-model spread in CO2 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 (CO3) 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 CO3 on the magnitude of 4×CO2 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 CO3 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 CO3 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 CO3 concentration are, therefore, not predominantly responsible for inter-model spread in CO2 ERF.