With the declining levels of ozone-destroying chlorine, scientists have for the first time observed less ozone depletion.

The findings showed that an international ban on chlorine-containing manmade chemicals called chlorofluorocarbons(CFCs) has resulted in about 20 per cent less ozone depletion during the Antarctic winter from 2005 to 2016.

Specifically, chlorine levels declined by an average about 0.8 per cent annually.

“We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it,” said lead author Susan Strahan, an atmospheric scientist from NASA’s Goddard Space Flight Centre in Maryland, US.

CFCs are long-lived chemical compounds that eventually rise into the stratosphere, where they are broken apart by the Sun’s ultraviolet radiation, releasing chlorine atoms that go on to destroy ozone molecules.

Stratospheric ozone protects life on the planet by absorbing potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and damage plant life.

The Antarctic ozone hole forms during September in the Southern Hemisphere’s winter as the returning sun’s rays catalyse ozone destruction cycles involving chlorine and bromine that come primarily from CFCs.

The change in ozone levels above Antarctica from the beginning to the end of southern winter — early July to mid-September — was computed daily from the Microwave Limb Sounder (MLS) aboard the Aura satellite every year from 2005 to 2016.

When ozone destruction is ongoing, chlorine is found in many molecular forms, most of which are not measured. But after chlorine has destroyed nearly all the available ozone, it reacts instead with methane to form hydrochloric acid — a gas measured by MLS.

The Antarctic ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades, the researchers said, in the paper published in the journal Geophysical Research Letters.

“CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time,” said Anne Douglass, a fellow atmospheric scientist at the Goddard Centre.

“As far as the ozone hole being gone, we’re looking at 2060 or 2080. And even then there might still be a small hole.”