Separating the influence of halogen and climate changes on ozone recovery in the upper stratosphere

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Abstract
While halogens from chlorofluorocarbons (CFCs) dominated past ozone changes, our simulations show climate change playing an increasingly important role over coming decades. Including potential climate‐induced stratospheric water vapor increases, the ozone change relative to 1980 attributable to climate change surpasses that due to halogens in the 2030s for both the upper stratosphere and the total column. Its overall impact may delay recovery of total ozone to 1980 values. These results emphasize that ozone recovery is not only a detection issue but also a detection and attribution issue, as trends must be ascribed to climate and halogens. Exploiting their different spatial patterns, we demonstrate a technique to isolate the halogen signature in the upper stratosphere by contrasting equatorial and midlatitude ozone trends at 40‐km altitude. This may allow early detection of the effectiveness of halogen emission reductions, which would show that a delayed recovery does not represent a CFC treaty failure but rather highlights the growing influence of greenhouse gases on the Earth's atmosphere.