Role of natural halogen chemistry on the evolution of global stratospheric ozone depletion.

The stratospheric ozone layer acts as Earth's primary shield against harmful ultraviolet radiation. Anthropogenic emissions of chlorinated and brominated compounds caused substantial ozone depletion during the 20th century, including the formation of the Antarctic ozone hole. While the role played by these anthropogenic compounds has been well established, the contribution of naturally emitted halogens, particularly bromine and iodine, remains poorly quantified. Here, using chemistry-climate model simulations spanning 1910 to 2100, we show that natural halogen chemistry makes a major contribution to lower-stratospheric ozone loss. Between 1990 and 2020, when anthropogenic halogen loading reached its peak, natural halogens accounted for up to half of the total halogen-induced ozone loss in the extrapolar lower-stratosphere and substantially enhanced ozone destruction along the Antarctic periphery by ~35%. Their relative influence is projected to increase by the end of this century, exceeding 80% of total halogen-induced ozone loss in the extrapolar lower-stratosphere and delaying the recovery of the natural Antarctic seasonal ozone cycle by nearly five decades. These findings reveal that natural halogens play a critical but underrepresented role in shaping the past and future changes in the ozone layer and should be accurately represented in ozone projections.