The hole in the ozone layer, the earth's protective shield against ultraviolet rays, is expected to be smaller this year over the Antarctic than last, showing how a ban on harmful substances has stopped its depletion, the United Nations said.
But the hole is probably larger than in 2010 and a complete recovery is still a long way off.
The signing of the Montreal Protocol 25 years ago to phase out chemicals that deplete the ozone layer has helped prevent millions of cases of skin cancer and eye cataracts as well as harmful effects on the environment, the UN weather agency said.
"The temperature conditions and the extent of polar stratospheric clouds so far this year indicate that the degree of ozone loss will be smaller than in 2011 but probably somewhat larger than in 2010," the World Meteorological Organization (WMO) said in a statement.
The Antarctic ozone hole, which currently measures 19 million square kilometers, most likely would be smaller this year than in the record year of 2006, it said. The annual occurrence typically reaches its maximum surface area during late September and maximum depth in early October.
But the banned chlorofluorocarbons (CFCs), once used in refrigerators and spray cans, have a long lifetime in the atmosphere and it will take several decades before their concentrations are back to pre-1980 levels, the WMO said.
The Montreal protocol has been a "great success", UN weather agency expert Geir Braathen said.
"This has prevented a major environmental disaster and globally ozone depletion has leveled off. We haven't really seen any kind of unequivocal ozone recovery yet," he said.
"Ozone depletion has stopped, leveled off," Braathen said.
In the Arctic stratosphere, there was record ozone loss in spring of 2011, but it has returned to more normal conditions this year, he said.