The destruction of ozone over Antarctica this autumn was much less than the last few years, but experts are not predicting an early slowdown in annual ozone loss, writes Dick Ahlstrom
A much smaller than expected ozone hole has opened up over Antarctica, the smallest since 1988. The apparent improvement relates to unusual weather conditions over the South Pole rather than reduced output of ozone-destroying chemicals however, experts indicate.
Watching the ozone hole open up over Antarctica has become an annual activity for atmospheric scientists, who measure ozone loss caused by chemical pollutants in the upper atmosphere.
This loss, mainly in layers between 10 and 20 kilometres above the Earth's surface, peaks between August and October when stratospheric temperatures over the pole are at their lowest. This is when the polar vortex forms, a ring-shaped swirl of high altitude air inside of which temperatures plummet.
Unusually warm conditions have disturbed this pattern, however, and slowed ozone destruction this year, according to Dr Craig Long, a meteorologist at the US National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Centre. "The southern hemisphere's stratosphere was unusually disturbed this year," he said.
The warmer air broke down the edges of the vortex and was responsible for the reduced ozone loss, he said. The effect was so strong that it split the ozone hole into two smaller pieces. The hole is usually seen as a single large gap in our ozone cover. "This is the first time we've seen the polar vortex split in September," says Long.
Ozone is a pollutant at the Earth's surface, but plays an essential role in the stratosphere. There it reduces the penetration of the sun's ultraviolet radiation, which causes sunburn. Too much can cause skin cancer and cataracts in the eyes. It could also impact on plant growth, disturbing the complex web of species which support life on this planet.
In 2001 the Antarctic ozone hole was larger than the combined area of North America. Over the last six years it has averaged about 24 million square kilometres, but this year measures about 15 million square kilometres.
The destruction varies depending on altitude and temperature conditions in the vortex. While some ozone layers will experience a near complete ozone loss during the vortex, the warmer conditions have slowed ozone loss in others. More ozone than usual has remained in place above 25 kilometres, according to NOAA data.
The root cause of the loss is chlorine and bromine pollution caused by gases containing these chemicals that have been carried into the stratosphere. Key groups include the chlorofluorocarbons (CFCs) and bromine-containing halons. These were banned under the 1995 Montreal Protocol once their destructive power became clear, but it is far too soon for this ban to have an impact on reducing ozone loss, according to Dr Paul Newman, an ozone researcher at NASA's Goddard Space Flight Centre in Maryland.
"While chlorine and bromine chemicals cause the ozone hole, temperature is also a key factor in ozone loss," he says. The polar vortex allows thin clouds to form in the cold conditions and these provide a substrate for the ozone destroying chemical reactions to occur. If the vortex is disturbed there is less cloud and so less ozone loss.
Newman put the lower loss down to weather conditions and not to any effect attributed to the Montreal Protocol. CFCs and halons are extremely long-lived, stable chemicals and it will take some decades for them to disappear.
Once the vortex has passed, the atmospheric chemistry that encourages the formation of ozone in the upper atmosphere takes hold once again and the hole gradually refills with ozone to await the next vortex.