Natural Halogens Modulate the Evolution of Global Stratospheric Ozone Depletion

The ozone layer acts as Earth’s primary shield against harmful ultraviolet radiation. Since the mid-20th century, anthropogenic emissions of long-lived chlorinated compounds have disrupted the photochemical balance controlling stratospheric ozone abundance, leading to severe depletion most evident with the formation of the Antarctic ozone hole. While the dominant role of anthropogenic chlorine in global stratospheric ozone depletion is well established, the influence of naturally emitted short-lived halogens, particularly bromine and iodine, on the timing, magnitude and regional variability of ozone loss has been so far overlooked. Using comparative chemistry-climate modeling experiments that include and exclude natural halogen sources for the 1910-2100 period, we show that natural halogen chemistry accounts for up to half of the total halogen-induced ozone loss in the lower extrapolar (60º S – 60º N) stratosphere during recent decades (1990-2020). This contribution is projected to rise sharply, exceeding 80% by the end of the century (2080-2099). Natural halogens also amplify lower-stratospheric ozone loss along the Antarctic periphery by about 35% in recent decades, most notably delaying the projected recovery of its natural seasonal ozone cycle by nearly five decades. Our results demonstrate the disproportionate per-unit-of-mass efficiency of natural bromine and iodine relative to anthropogenic chlorine in controlling lower stratospheric ozone loss. These findings underscore the key but previously overlooked role of natural halogens in the past-to-future spatiotemporal evolution of global stratospheric ozone balance and highlight the need to accurately represent natural halogen chemistry in past and future projections of the stratospheric ozone layer.