Chapter 9
Stratospheric Ozone in the 21^st Century

This chapter discusses the future evolution of stratospheric ozone and its return to historical values from state-of-the-art three-dimensional coupled chemistry-climate models. These models have been developed by international research groups and evaluated by the atmospheric science community over the last two decades and include a detailed representation of the physical, dynamical, and chemical processes of the atmosphere, with a focus on the stratosphere. This chapter describes the models and shows examples on how these models are evaluated with observations. Factors that affect stratospheric ozone abundance, specifically ones that relate to uncertainties in the return of ozone to historical values (e.g., stratospheric halogen loading, and climate-induced temperature and transport trends) are discussed. The projected evolution of ozone is shown for different latitude regions (e.g., tropics, mid-latitudes, and polar), for different altitudes and for the total column. Hemispheric differences are also highlighted. In general models that have a better representation of transport processes measured by the models ability to represent the mean age of air and inorganic chlorine abundances are better suited to project the evolution of ozone in the 21^st century. Uncertainties in current modelling approaches and recommendations for future projection simulations are also discussed.