The chief source of new problems is solutions to old problems. The ammonia that we used in domestic fridges as a coolant in the early 20th century was poisonous if it leaked, so in the 1930s we replaced it with chlorofluorocarbons (CFCs), which you can breathe all day without harm. Problem solved.
Unfortunately, it turned out that CFCs, when they leaked, eventually rose into the stratosphere where they began destroying ozone. The ozone layer is the only thing protecting us from the sun's harmful ultraviolet radiation, so countries responded quickly in the 1980s when scientists discovered a spreading "ozone hole" over the Antarctic.
In only a few years the world's nations negotiated the Montreal Protocol of 1987, which mandated the elimination of CFCs from all industrial processes by 1996. The deadline was met, and the latest projection is that the ozone layer will recover to 1980 levels between 2050 and 2070. Problem solved.
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