Ozone Depletion

The thinning of the stratospheric ozone layer that shields the Earth's surface from harmful ultraviolet radiation, caused chiefly by chlorine and bromine released from long-lived industrial chemicals such as chlorofluorocarbons breaking down ozone molecules in a catalytic chain reaction. Depletion is most severe over Antarctica each spring, where extreme cold enables reactions that produce the seasonal ozone hole, and the 1987 Montreal Protocol's phase-out of the responsible chemicals has since allowed the layer to begin a slow, still-incomplete recovery.