26 Jan 2024
 | 26 Jan 2024

Investigating the role of stratospheric ozone as a driver of inter-model spread in CO2 effective radiative forcing

Rachael Byrom, Gunnar Myhre, Dirk Olivié, and Michael Schulz

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 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 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/decreases in stratospheric O3 on the magnitude of 4xCO2 effective radiative forcing (ERF) and its components.

Firstly, we demonstrate that accurate estimation of instantaneous radiative forcing requires the use of host-model radiative transfer calculations. Secondly, we show that a 50 % increase and decrease in stratospheric O3 concentration leads to significant differences in base state stratospheric temperature, ranging from +6 K to -9 K, respectively. However, this does not result in a correspondingly large spread in CO2 ERF due to the impact of base-state stratospheric temperature on the emission of outgoing longwave radiation and the spectral overlap of CO2 and O3. We conclude that inter-model differences in stratospheric O3 concentration are therefore not predominantly responsible for inter-model spread in CO2 ERF.

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Rachael Byrom, Gunnar Myhre, Dirk Olivié, and Michael Schulz

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-111', Anonymous Referee #1, 08 Feb 2024
  • RC2: 'Comment on egusphere-2024-111', Anonymous Referee #2, 20 Feb 2024
  • RC3: 'Comment on egusphere-2024-111', Anonymous Referee #3, 22 Feb 2024
Rachael Byrom, Gunnar Myhre, Dirk Olivié, and Michael Schulz
Rachael Byrom, Gunnar Myhre, Dirk Olivié, and Michael Schulz


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Short summary
Addressing the cause of model spread in CO2 effective radiative forcing is important for reducing uncertainty in climate change. We investigate stratospheric ozone as a driver of this spread by changing its concentration by 50 % and analysing the impact on CO2 forcing. Our idealised experiments show a significant impact on stratospheric temperature but a minimal impact on CO2 forcing due to the combined effect on longwave emission and gaseous spectral overlap.