WASHINGTON • The giant hole in the Earth's protective ozone layer is shrinking and has shrivelled to its smallest peak since 1988, Nasa scientists said.

The largest the hole became this year was about 19.7 million sq km wide, about 21/2 times the size of the United States, in September. But it was still 3.4 million sq km smaller than last year, scientists said, and has shrunk more since September.

Warmer-than-usual weather conditions in the stratosphere are to be thanked for the shrinkage since last year, as the warmer air helped fend off chemicals such as chlorine and bromine that eat away at the ozone layer, scientists said. But the hole's overall reduction can be traced to global efforts since the mid-1980s to ban the emission of ozone-depleting chemicals.

"Weather conditions over Antarctica were a bit weaker and led to warmer temperatures, which slowed down ozone loss," said chief Earth scientist Paul Newman at Nasa's Goddard Space Flight Centre in Maryland.

The news comes just after the 30th anniversary of the hole's discovery, which led to the 1987 Montreal Protocol - a landmark international agreement that led to major global efforts to phase out the use of ozone-depleting chemicals.

In 2014, scientists at the United Nations credited the recovery of the ozone layer to the phasing out of chemicals used in refrigerators, air-conditioners and aerosol cans in the 1980s. But chlorofluorocarbons have long lifetimes, and could still float around in the atmosphere 100 years from now, Dr Newman said. Scientists predict the ozone layer will not return to its 1980s form until about 2070.

In June, scientists identified a possible threat to the recovery, believing dichloromethane - an industrial chemical with the power to destroy ozone - doubled in the atmosphere over the past 10 years. If its concentrations keep growing, it could delay the Antarctic ozone layer's return to normal by up to 30 years, according to the study published in the journal Nature Communications.

The ozone hole was largest in 2000, when it was 29.8 million sq km wide, according to the National Aeronautics and Space Administration (Nasa).

WASHINGTON POST