THE seasonal hole in the ozone layer above the Antarctic this year was the second smallest in two decades, but still covers an area roughly the size of North America, US experts say.
The average size of the Earth's protective shield was 18 million square kilometres, according to satellite measurements by the National Oceanic and Atmospheric Administration (NOAA) and US space agency NASA.
"It happened to be a bit warmer this year high in the atmosphere above Antarctica, and that meant we didn't see quite as much ozone depletion as we saw last year," said Jim Butler of the NOAA.
The Antarctic ozone hole, which forms in September and October, reached its largest size for the season at 13.2 million sq km, roughly the combined area of the United States, Mexico and Canada - on September 22, the NOAA said.
In comparison, the largest ozone hole recorded to date was one of 18.5 million sq km in 2000.
The ozone layer - which helps protect the Earth from potentially dangerous ultraviolet rays that can cause skin cancer and cataracts - began developing holes on an annual basis starting in the 1980s due to chlorofluorocarbons, or CFCs.
CFCs, once commonly used in refrigerators and aerosol cans, are almost non-existent thanks to an international treaty signed in 1987.
Still, it could take another decade before scientists detect early signs that the ozone over the Antarctic is returning, the NOAA said.
The ozone layer above Antarctica will not return to its early 1980s state until about 2060, according to NASA scientist Paul Newman.
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