Nov. 2 (UPI) -- The hole in Earth's ozone layer shrunk to its smallest size since 1988, scientists with NASA and NOAA announced on Thursday. Researchers believe warm air worked to limit the rate of ozone depletion.
Every year, the hole in the ozone layer above Antarctica gets larger during the Southern Hemisphere's winter and shrinks, or repairs itself, during the summer.
Since 1991, the average Antarctic ozone hole maximum measured 10 million square miles. In 2017, the maximum -- reached in early September -- measured 7.6 million square miles.
"The Antarctic ozone hole was exceptionally weak this year," Paul A. Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland, reported in a news release. "This is what we would expect to see given the weather conditions in the Antarctic stratosphere."
The ozone hole is created when solar rays catalyze manmade chemicals like chlorine and bromine. These chemical reactions destroy ozone molecules. The chemical reactions are encouraged by polar stratospheric cloud formation, but warmer air in the stratosphere in 2016 and 2017 helped limit these reactions and curtailed the growth of the ozone hole.
The lack of stratospheric cloud patterns above the Arctic is one reason why ozone depletion above the Arctic is much less severe.
NASA and NOAA researchers use weather balloons outfitted with ozone sensors to regularly measure and map the size and shape of the ozone layer above the poles. The balloons can track the concentration of ozone molecules at different altitudes. Their measurements showed the ozone layer above Antarctica wasn't as thin as usual this year.
Researchers suggest the recent lows in Antarctic ozone hole extent and the improved thickness are evidence of natural variability, not permanent repair.
The depletion of the ozone in the 1970s and 80s was caused by the proliferation of chlorofluorocarbons, chlorine derivatives used in aerosol cans, refrigerators and many other products and appliances. After the 1987 passage of the Montreal Protocol, the first treaty unanimously ratified by all members of the United Nations, CFCs were almost entirely phased out.
But chlorofluorocarbons have a long half-life, and as such can continue to damage the ozone for decades after their release into the atmosphere. There is also some concern that other unregulated CFC-like molecules are accumulating in the atmosphere and delaying the repair of the ozone hole.