Stratospheric ozone chemistry feedbacks are not critical for the determination of climate sensitivity in CESM1(WACCM)

The Community Earth System Model-Whole Atmosphere Community Climate Model (CESM1-WACCM) is used to assess the importance of including chemistry feedbacks in determining the equilibrium climate sensitivity (ECS). Two 4×CO₂ model experiments were conducted: one with interactive chemistry and one with chemical constituents other than CO₂ held fixed at their preindustrial values. The ECS determined from these two experiments agrees to within 0.01 K. Similarly, the net feedback parameter agrees to within 0.01 W m⁻² K⁻¹. This agreement occurs in spite of large changes in stratospheric ozone found in the simulation with interactive chemistry: a 30% decrease in the tropical lower stratosphere and a 40% increase in the upper stratosphere, broadly consistent with other published estimates. Off-line radiative transfer calculations show that ozone changes alone account for the difference in radiative forcing. We conclude that at least for determining global climate sensitivity metrics, the exclusion of chemistry feedbacks is not a critical source of error in CESM.