Stratospheric dynamics modulates ozone layer response to molecular oxygen variations

Abstract

Photolysis of molecular oxygen (O₂) sustains the stratospheric ozone layer and is thereby protecting living organisms on Earth by absorbing harmful ultraviolet radiation. In the past, atmospheric O₂ levels were not constant, and their variations are thought to be responsible for the extinction of some species due to the thinning of the ozone layer. Over the Phanerozoic Eon (last similar to 500 Mio years), the O₂ volume mixing ratio ranged between 10% and 35% depending on the level of photosynthetic activity of plants and oceans. Previous estimates, mostly performed by simplified 1-D models, showed different ozone (O₃) responses to atmospheric O₂ changes within this range, such as monotonically positive or negative correlations, or displaying a maximum in the O₃ column around a certain O₂ level. Here, we assess the ozone layer sensitivity to atmospheric O₂ varying between 5% and 40% with a state-of-the-art 3-D chemistry-climate model (CCM). Our findings show that the O₃ layer thickness maximizes around the current mixing ratio of O₂, 21% +/- 5%, while lower or higher levels of O₂ result globally in a reduction of total column O₃. At low latitudes, the total column O₃ is less sensitive to O₂ variations, because of the "self-healing" effect, namely, a vertical dipole in the tropical ozone response. Mid- and high-latitude O₃ columns that are largely affected by transport of O₃ from the tropics, however, are much more sensitive to O₂ with changes up to 20 DU even for small (+/- 5%) O₂ perturbations. We show that these variations are largely driven by the radiative impact of O₃ on stratospheric temperatures and on the strength of the Brewer-Dobson circulation (BDC), indicating chemistry-radiation-transport feedback. High O₂ cases result in an acceleration of the BDC and vice versa, which always works in favor of the negative part of the O₃ anomaly dipole in the tropics being more effectively transported to the mid- and high-latitudes than the positive one. Although there are other factors strongly influencing O₃/O₂ relationship on the Phanerozoic Eon timescales that have not been considered here, our results and the presented mechanism bring useful insights for other studies focusing on the long-term O₃/O₂ relationship.