One of the prominent environmental storylines of the 1980s and 1990s was the hole in the ozone layer. After a sweeping ban on ozone-depleting chemicals in 1987, the degradation of the ozone layer appears to have slowed, and is now poised for recovery.
Ozone is a molecule composed of three oxygen atoms. A layer of ozone sits about 6 miles above the earth, in the stratosphere, and protects earth from harmful wavelengths of ultraviolet radiation. Ozone reacts very easily with other chemicals, however. Chlorine and bromine can knock an oxygen atom off of an ozone molecule, leaving an oxygen atom that cannot perform the same protective function as ozone.
Chloroflurocarbons, chemicals used in refrigeration, are particularly effective in destroying ozone molecules. After a seasonal hole in the ozone layer was first discovered in 1985, however, international cooperation yielded an agreement in 1987 to phase out ozone-depleting chemicals. Since then, the amount of chlorine in the atmosphere that poses a danger to ozone has peaked and is beginning to decline. The size of the ozone hole reached its largest recorded extent in 2006, at 30 million square kilometers, but hasn't surpassed that number since. A running five-year average of ozone hole size from NASA data shows values ranging from 20.7-24 million square kilometers post-2000. The size of the hole appears to have plateaued. Forecasts point to a slow, steady recovery by mid-century, but there is not yet convincing evidence that the ozone hole has actually decreased in size.
Will the annual maximum Antarctic ozone hole size begin its recovery by 2020?
For this question to resolve as positive, an atmospheric science report, NASA, (or data from another credible group that reproduces the same existing NASA data) must report results showing that the running five-year average of the mean September-October ozone hole size drops below 20.7 million square kilometers on or before December 31, 2020.