It was a rare piece of good news about the health of the planet. In September, a United Nations scientific panel declared that earth's protective ozone layer was beginning to recover, largely because man-made chlorine emissions were outlawed 27 years ago.
But it is stubbornly remaining the size of North America, data from Nasa suggests.
The hole in the thin layer of gas, which helps shield life on earth from harmful ultraviolet solar radiation that can cause skin cancers, grows and contracts throughout the year but reached its maximum extent on September 9 when monitors at the south pole showed it to cover 24.1 million sq km. This is about 9 per cent below the record maximum in 2000 but almost the same as in 2010, 2012 and 2013.
But scientists are unsure why the hole has not reduced more since countries signed the Montreal Protocol agreement in 1987.
The global treaty banned the use of ozone-depleting chlorofluorocarbons (CFCs), substances that were widely-used in household and industrial products such as refrigerators, spray cans and insulation foam.
In September, the UN Environment Programme and the World Meteorological Organisation said there were "positive indications" that the ozone layer was on track to recovery, but warned it might take a further 35 years or more to recover to 1980 levels.
They said that without the Montreal Protocol atmospheric levels of ozone depleting substances could have increased tenfold by 2050.
"It's broadly on track [to reduce]," said Dr Jonathan Shanklin, emeritus professor at the British Antarctic Survey in Cambridge, one of three scientists who discovered the hole.