Comment on egusphere-2022-1260

Abstract

This study evaluates the future evolution of atmospheric ozone between 2015 and 2099 simulated with the Earth System Model (ESM) SOCOLv4. Simulations have been performed based on two potential Shared Socioeconomic Pathways (SSP): the “middle-of-the-road” (SSP2-4.5) and “fossil-fueled” (SSP5-8.5) scenarios. In both scenarios, the model projects a decline in tropospheric ozone in the future that starts in the 2030s in SSP2-4.5 and after the 2060s in SSP5-8.5 due to a decrease in concentrations of ozone precursors like NOx and CO. The results also suggest a very likely ozone increase in the mesosphere, upper and middle stratosphere, as well as at high latitudes of the lower stratosphere. Under SSP5-8.5, the ozone increase in the stratosphere is higher because of stronger cooling (> 1 °K/decade) induced by the greenhouse gases (GHG), which slows the catalytic ozone destruction cycles. In contrast, in the tropical lower stratosphere ozone concentrations decrease in both experiments and increase over the middle and high latitudes of both hemispheres due to the intensification of meridional transport, which is stronger in SSP5-8.5. No evidence was found of a decline in ozone levels in the lower stratosphere at mid-latitudes. In both future scenarios, the total column ozone is expected to be distinctly higher than present in mid-to-high latitudes and might be lower in the tropics, which causes a decrease/increase in the surface level of UV radiation. The results of SOCOLv4 suggest that the stratospheric ozone evolution throughout the 21st century is strongly governed not only by a decline in halogen concentration, but also by future GHGs forcing. In addition, the tropospheric ozone column changes, mainly due to the changes in anthropogenic emissions of ozone precursors, also have a strong impact on the total column. Therefore, even though the anthropogenic halogen loading problem has been brought under control to date, the sign of future ozone column changes, globally and regionally, is still unclear and largely depends on diverse future human activities. The results of this work are, thus, relevant for developing future strategies for socioeconomic pathways.