LONDON — The ozone layer is showing signs of recovering, thanks to a drop in ozone-depleting chemicals, but it is unlikely to stabilise at pre-1980 levels, researchers said on Wednesday.

Depletion of the earth's protective ozone layer is caused by the chemical action of chlorine and bromine released by man-made chlorofluorocarbons (CFCs), which are used in aerosol sprays and cooling equipment.

Ozone-depleting chemicals were banned by the 1987 Montreal Protocol which has now been ratified by 180 nations.

"We now have some confidence that the ozone layer is responding to the decreases in chlorine levels in the atmosphere due to the levelling off and decrease of CFCs," said Dr Betsy Weatherhead, of the University of Colorado in Boulder.

"Not only is the ozone layer getting better, we feel it is due to the Montreal Protocol," she added in an interview.

The depletion of the ozone layer, which absorbs most of the harmful effects of the sun's ultraviolet radiation, increases the risk of skin cancer and cataracts in humans and may harm crop yields and sea life.

Despite the signs of recovery, Weatherhead, who reported the findings in the journal Nature, said people should still protect themselves from harmful ultraviolet rays.

Weatherhead and Signe Bech Anderson of the Danish Meteorological Institute in Copenhagen analysed data from satellites and ground stations and information from 14 modelling studies.

They found that ozone levels have stabilised or increased slightly in the past 10 years. But full recovery is still decades away.

The researchers said depletion has been most severe at the poles and to a lesser extent at mid-latitudes covering bands of North America, South America and Europe.

Shifting temperatures, greenhouse gases, nitrous oxide (N20) and atmospheric dynamics, which can influence ozone levels, are going to change in the future, they added.

"Therefore we really don't think ozone is going to stabilise back to its pre-ozone-depleting-substance levels," Weatherhead said.

Volcanic activity on Earth also has an impact. The 1991 Mount Pinatubo eruption in the Philippines caused ozone levels to backslide for several years, according to the researchers.

Source: Reuters