Earth's upper atmosphere is still so saturated with ozone-eating chlorine that it will take about another decade for evidence that a nearly 25-year-old ban on such destructive chemicals is working, scientists say.
Full recovery of the ozone layer, which shields Earth from the sun's harmful ultraviolet radiation, should occur about 2070, atmospheric scientist Natalya Kramarova, with Nasa's Goddard Space Flight Center in Greenbelt, Maryland, told reporters at the American Geophysical Union conference in San Francisco last week.
"Currently, we do not see that the ozone hole is recovering," she said. "It should become apparent in 2025."
Researchers report puzzlingly large variations in the size of the annual ozone hole over Antarctica.
In 2012 for example, the ozone hole was the second smallest on record, an apparently positive sign that the 1989 Montreal Protocol agreement - which called for the phasing out of Freon and other damaging chlorofluorocarbons, or CFCs - was working.
But scientists say that meteorological effects masked the hole's true size. The year before, they point out, the ozone hole was nearly as big as it was in 2006, the largest on record.
"Currently, small declines in levels of ozone-depleting substances are far too small to show ozone recovery, in comparison with year to year variability," Kramarova said.
With the stratosphere still flush with ozone-destroying chlorine, the size of the annual hole over Antarctica is more dependent on temperature and upper atmospheric winds, scientists said.
As chlorine levels drop, however, the annual ozone holes over Antarctica will consistently decrease in size, they said.
"We've still got so much chlorine up there that the ozone hole area just doesn't depend on chlorine," said atmospheric scientist Susan Strahan, also with Nasa's Goddard Space Flight Center.
As a result of the Montreal Protocol, scientists expected chlorine levels to decrease by about 5 per cent this decade.
Instead, measurements from instruments aboard satellites show chlorine levels increase or decrease by 5 per cent every year, Strahan said.
Chlorine is gradually declining, she said, "but it's bumpy road down - some years it's higher, some years it's lower".
By the mid-2030s, chlorine levels are forecast to be 20 per cent lower than current levels, leading to consistently smaller ozone holes. A full recovery is expected between 2058 and 2090 and most likely about 2070, scientists said.
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