One of the nice things the atmosphere does is provide a layer of ozone -- molecules of three oxygen atoms -- that shield living things from ultraviolet solar radiation. Basically, it's gaseous sunblock.
In the 1970s, scientists began to suspect that manmade chemicals called chlorofluorocarbons, which were used in refrigeration, aerosol sprays and elsewhere, were breaking down the ozone layer. Not until 1985 could researchers document an expanding "hole" in the ozone layer. The public snapped to attention.
The resulting Montreal Protocol on Substances that Deplete the Ozone Layer is "perhaps the single most successful international agreement to date," according to Kofi Annan, former secretary general of the United Nations. All 197 UN member nations agreed to legally binding standards to cut ozone-eating pollution.
The hole grows and shrinks seasonally and stands now near its record of almost 30 million square kilometers. But the problematic trend has reversed, and scientists expect ozone levels to return to pre-1980s levels by 2070.
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