The atmosphere’s protective ozone layer plays a vital role in filtering out dangerous ultraviolet rays from the Sun that can cause skin cancers.
Each year over the southern summer, depletion of the ozone layer over the Antarctic speeds up.
During the cold and dark winter months, polar stratospheric clouds form, which contain ice crystals.
Trapped on the surface of the crystals are compounds derived from chlorofluorocarbons (CFCs).
Once sunlight returns to the South Pole, those compounds are released and transformed into substances that destroy ozone molecules.
A single molecule of chlorine can break down thousands of molecules of ozone.
Since the advent of industrialisation, the amount of CFCs released into the atmosphere has risen sharply – mostly used in refrigeration and aerosol sprays.
But with the adoption of the Montreal Protocol in 1987, steps have been introduced to stem the use of CFCs.
Already, there have been signs over the past two years that damage to the ozone layer has slowed down, although a return to pre-industrial ozone levels is not expected to be seen for another 50 years.
In a warning that recovery is some way off, this year’s ozone hole – effectively a thinning of the layer – has grown to an area of 10 million square kilometres, roughly the size of Europe.
Envisat’s data has shown that the Antarctic hole was bigger in mid-August this year than at the same time in any year since 2000.
“We expect that the depletion will continue for another month or so,” Geir Braathen, senior scientific officer with the World Meteorological Organisation (WMO) told the BBC’s online news service.
“Typically, it reaches a maximum around mid-September, though the exact date varies from year to year.”
“The biggest holes on record came in 2000 and 2003. This year’s may be as big, but we will have to wait another two or three weeks to be sure,” Mr Braathen said.