GREENBELT, Md., Oct. 25 (UPI) -- Satellite data show the average area covered by the Antarctic ozone hole this year was the second-smallest in 20 years, NASA reported.
The ozone hole reached its maximum size for the year Sept. 22, covering 8.2 million square miles -- the area of the United States, Canada and Mexico combined.
The ozone layer acts as Earth's natural shield against ultraviolet radiation, which can cause skin cancer, and the ozone hole phenomenon began making a yearly appearance in the early 1980s.
However, the size of the hole has decreased in recent years after an international agreement regulating the production of certain ozone-depleting chemicals.
Warmer temperatures in the Antarctic lower stratosphere helped to keep the hole smaller this year, NASA said.
"The ozone hole mainly is caused by chlorine from human-produced chemicals, and these chlorine levels are still sizable in the Antarctic stratosphere," said Paul Newman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "Natural fluctuations in weather patterns resulted in warmer stratospheric temperatures this year. These temperatures led to a smaller ozone hole."
NASA and the National Oceanographic and Atmospheric Administration have been monitoring the ozone layer from the ground and with a variety of instruments on satellites and balloons since the 1970s.
The Antarctic ozone layer likely will not return to its early pre-1980s state until about 2065, Newman said, because of the long lifetimes of ozone-depleting substances still in the atmosphere.