© NZ Herald

The Antarctic ozone hole grew to the size of North America in September, the fifth largest recorded in nearly 30 years.

The information released yesterday by the United States National Oceanic and Atmospheric Administration (NOAA) said the ozone hole fluctuated in response to temperature and sunlight.

It had grown to the size of North America in a one-day maximum in September, making it the fifth largest since NOAA satellite records began in 1979.

The primary cause of the ozone hole was human-produced compounds called chlorofluorocarbons, or CFCs, which released ozone-destroying chlorine and bromine into the atmosphere.

Earth's protective ozone layer acted like a giant umbrella, blocking the sun's ultraviolet-B rays. Though banned for the past 21 years to reduce their harmful build-up, CFCs still took many decades to dissipate.

The NOAA scientists found that colder than average temperatures in the stratosphere played a part in allowing the ozone hole to develop more fully this year.

Nasa satellites measured the maximum area of this year's ozone hole at 27.2 million sq km and 6.4 km deep on September 12.

Bryan Johnson, a scientist at NOAA's Earth System Research Laboratory in Boulder, Colorado, which monitors ozone, ozone-depleting chemicals and greenhouse gases around the globe, said weather was the most important factor in the fluctuation of the size of the hole from year to year.

"How cold the stratosphere is and what the winds do determine how powerfully the chemicals can perform their dirty work."

Greg Bodeker, Niwa principal scientist, told the Herald the ozone hole was shrinking, but very slowly. He said the chemicals that mostly depleted ozone, chlorine and bromine, were decreasing at about 1 per cent a year but there were annual variations.

Dr Bodeker said it was estimated that ozone would return to 1980 levels in the next 60 to 70 years. Increases in greenhouse gases had contributed to the problem as they trapped infra-red radiation which warmed up the lower atmosphere, or troposphere, up to 15km altitude.

The upper atmosphere, or stratosphere, became cooler as a result, which promoted Antarctic ozone depletion and delayed the recovery of the ozone hole.