Mechanisms and feedback causing changes in upper stratospheric ozone in the 21st century

Stratospheric ozone is expected to increase during the 21st century as the abundance of halogenated ozone-depleting substances decrease to 1960 values. However, climate change will likely alter this recovery of stratospheric ozone by changing stratospheric temperatures, circulation, and abundance of reactive chemical species. Here we quantify the contribution of different mechanisms to changes in upper stratospheric ozone from 1960 to 2100 in the Goddard Earth Observing System chemistry-climate model, using multiple linear regression analysis applied to simulations using either A1b or A2 greenhouse gas (GHG) scenarios. In both scenarios, upper stratospheric ozone has a secular increase over the 21st century. For the simulation using the A1b GHG scenario, this increase is determined by the decrease in halogen amounts and the GHG-induced cooling, with roughly equal contributions from each mechanism. There is a larger cooling in the simulation using the A2 GHG scenario, but also enhanced loss from higher NOy and HOx concentrations, which nearly offsets the increase because of cooler temperatures. The resulting ozone evolutions are similar in the A2 and A1b simulations. The response of ozone caused by feedback from temperature and HOx changes, related to changing halogen concentrations, is also quantified using simulations with fixed-halogen concentrations.