Investigating the relationship between stratospheric temperature and intermodel CO2 radiative forcing spread

Addressing the cause of intermodel spread in carbon dioxide (CO₂) radiative forcing is essential for reducing uncertainty in estimates of climate sensitivity. Recent studies demonstrate 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 the significance of intermodel differences in stratospheric ozone (O₃) as a cause of intermodel differences in stratospheric temperature, and hence its role as a contributing factor to intermodel spread in CO₂ radiative forcing. We use the Community Earth System Model 2 and the Norwegian Earth System Model 2 to analyse the impact of systematic increases/decreases in stratospheric O₃ on the magnitude of 2xCO₂ and 4xCO₂ effective radiative forcing (ERF). Corresponding rapid adjustments and instantaneous radiative forcing (IRF) are diagnosed using radiative kernels and the Parallel Offline Radiative Transfer code, respectively.

We demonstrate that differences in base state stratospheric O₃ lead to significant differences in base state stratospheric temperature, ranging from +6 K to -8 K given a 50% increase and decrease in stratospheric O₃ concentration. However, this does not result in a correspondingly large spread in CO₂ IRF and ERF due to the compensating greenhouse effect of CO₂ and O₃. Intermodel differences in stratospheric O₃ concentration are therefore not predominantly responsible for intermodel spread in CO₂ IRF and ERF.