NEW YORK • Nearly three decades after the world banned chemicals that were destroying the atmosphere's protective ozone layer, scientists say there are signs the atmosphere is on the mend.
The researchers said they had found "fingerprints" indicating that the seasonal ozone hole over Antarctica, a cause of concern since it was discovered in 1984, was getting smaller. Although the improvement has been slight so far, it is an indication that the Montreal Protocol - the 1987 treaty signed by almost every nation that phased out the use of chemicals known as chlorofluorocarbons, or CFCs - is having its intended effect.
Full recovery of the ozone hole is not expected until the middle of the century. "This is just the beginning of what is a long process," said Dr Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology and lead author of the study published in the journal Science on Thursday.
Ozone high in the stratosphere protects life on Earth by absorbing damaging ultraviolet rays from the Sun. But ozone is destroyed by reactions with chlorine and other atoms that are released by CFCs and similar chemicals, which had been used for decades as refrigerants and propellants. More ultraviolet radiation leads to increased incidence of skin cancers, cataracts and other health problems.
Scientists who pushed for the treaty always acknowledged that recovery of the ozone layer would be very slow, because CFCs linger in the stratosphere for a long time.
Think of it like a patient with a disease. First, it was getting worse. Then it stopped - it was stable but still in bad shape. Now as molecules slowly decay away from the atmosphere, it's getting just a little bit better.
DR SUSAN SOLOMON, an atmospheric chemist at the Massachusetts Institute of Technology and lead author of the study published in the journal Science.
"Think of it like a patient with a disease," Dr Solomon said. "First, it was getting worse. Then it stopped - it was stable but still in bad shape." Now, she said, "as molecules slowly decay away from the atmosphere, it's getting just a little bit better".
Dr David Fahey, a research physicist at the National Oceanic and Atmospheric Administration who was not involved in the study, said Dr Solomon's work "gives us a critical level of confidence that we are moving in the direction we want to see".
It also reinforces that the Montreal Protocol has been a "resounding success", Dr Fahey said. "It stands head and shoulders above any other environmental treaty."
While ozone has been depleted in the Arctic and mid-latitude regions as well, the destruction over Antarctica is greater, in part because temperatures there are so cold. Technically, the depleted area is not a hole, but rather a large region of the stratosphere where the concentration of ozone is below a certain thres- hold. The depleted area can be vast in the Southern Hemisphere, covering all of Antarctica and extending into the Southern Ocean.
Because the reactions that cause ozone to be destroyed require sunlight, this thinning begins each year in late August, when winter in the Southern Hemisphere is ending, and reaches its maximum by September and October. The ozone layer recovers later in the year, and then the cycle repeats itself.
The study found that the ozone hole had shrunk by about 1.5 million square miles (3.9 million sq km), or about one-third the area of the United States, from 2000 to 2015. This reduction occurred despite the effects of some volcanic eruptions, including one last year in the Chilean Andes that led to one of the largest holes ever measured.
NEW YORK TIMES