Paul Eccleston, The Telegraph 4 Nov 08;
The ozone hole over Antarctica covered an area as big as North America at its peak in September, according to the latest figures.
The hole was the fifth biggest since satellite monitoring began in 1979.
Nasa satellites measured the maximum area of this year's ozone hole at 10.5m square miles and four miles deep on 12th September.
Ozone loss was at its worst in 2006 when the hole covered more than 11.4m square miles at its peak but by last year the ozone hole had returned to average size and depth and was 30 per cent smaller than the record size.
The figures were released by the National Oceanic and Atmospheric Administration (NOAA) which has been monitoring the ozone layer since 1962.
Ozone, a type of molecular oxygen, forms a protective layer measuring between six and 30 miles in the stratosphere that absorbs harmful ultra-violet (UV) radiation from the Sun.
Scientists from the British Antarctic Survey were the first to discover in 1985 that ozone levels were declining and that a hole had appeared above the Antarctic.
The depletion at certain times of the year was blamed on the release of man-made chemicals containing chlorine such as ChloroFluoroCarbons (CFC's) used in refrigeration and air conditioning systems, aerosols and solvents.
Under the 1987 Montreal Protocol production was controlled and countries who signed up agreed that no more would be produced after 1995 except for essential uses such as medical sprays.
CFCs in the stratosphere peaked in about 2000 and are declining at an estimated one per cent per year But the World Meteorological Organisation calculates that it will be 2050 before the ozone layer recovers to its former levels.
According to NOAA scientists, the ozone hole was bigger this year because of colder than average temperatures.
"Weather is the most important factor in the fluctuation of the size of the ozone hole from year-to-year," said 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.
"How cold the stratosphere is and what the winds do determine how powerfully the chemicals can perform their dirty work."
The cycle starts in May, according to the NOAA, as Antarctica moves into a period of 24-hour-a-day darkness. Rotating winds the size of the continent create a vortex of cold, stable air centred near the South Pole that isolates CFCs over the continent.
When spring sunshine returns in August, the sun's ultraviolet light sets off a series of chemical reactions inside the vortex that consume the ozone. The colder and more isolated the air inside the vortex, the more destructive the chemistry.
By late December when the southern hemisphere summer is at its height, the vortex has crumbled and the ozone has returned until the process begins again the following winter.
Craig Long, a research meteorologist at NOAA's National Centres for Environmental Prediction, said: "These chemicals - and signs of their reduction - take several years to rise from the lower atmosphere into the stratosphere and then migrate to the poles.
"The chemicals also typically last 40 to 100 years in the atmosphere. For these reasons, stratospheric CFC levels have dropped only a few per cent below their peak in the early 2000s."
And NOAA atmospheric chemist Stephen Montzka said: "The decline of these harmful substances to their pre-ozone hole levels in the Antarctic stratosphere will take decades. We don't expect a full recovery of Antarctic ozone until the second half of the century."
Ozone hole over Antarctica grows again
Stratospheric levels of harmful CFCs will take between 40 and 100 years to dissipate and have only dropped a few per cent since reaching a peak in 2000, scientists warn
John Vidal, guardian.co.uk 4 Nov 08;
The ozone hole over Antarctica grew to the size of North America this year – the fifth largest on record – according to the latest satellite observations.
US government scientists from the National Oceanic and Atmospheric Administration (NOAA) say this year's ozone hole reached its maximum level on September 12, extending to 10.5m sq miles and four miles deep. That is bigger than 2007 but smaller than 2006, when the hole covered over 11.4m sq miles.
Scientists blamed colder-than-average temperatures in the stratosphere for the ozone hole's unusually large size this year. "Weather is the most important factor in the fluctuation of the size of the ozone hole from year to year," said Bryan Johnson, a scientist at NOAA's Earth System Research Laboratory in Boulder, which monitors ozone, ozone-depleting chemicals, and greenhouse gases around the globe. "How cold the stratosphere is and what the winds do determine how powerfully the chemicals can perform their dirty work."
The main cause of the ozone hole is human-produced compounds called chlorofluorocarbons, or CFCs, which release ozone-destroying chlorine and bromine into the atmosphere. The Earth's protective ozone layer acts like a giant parasol, blocking the sun's ultraviolet-B rays. Though banned for the past 21 years to reduce their harmful build up, CFCs still take many decades to dissipate from the atmosphere
The 1987 Montreal Protocol and other regulations banning CFCs reversed the build-up of chlorine and bromine, first noticed in the 1980s.
"These chemicals – and signs of their reduction – take several years to rise from the lower atmosphere into the stratosphere and then migrate to the poles," said NOAA's Craig Long, a research meteorologist at NOAA's National Centers for Environmental Prediction. "The chemicals also typically last 40 to 100 years in the atmosphere. For these reasons, stratospheric CFC levels have dropped only a few per cent below their peak in the early 2000s."
"The decline of these harmful substances to their pre–ozone hole levels in the Antarctic stratosphere will take decades," said NOAA atmospheric chemist Stephen Montzka of the Earth System Research Laboratory. "We don't expect a full recovery of Antarctic ozone until the second half of the century."
Starting in May, as Antarctica moves into a period of 24-hour-a-day darkness, winds create a vortex of cold, stable air centred near the South Pole that isolates CFCs over the continent. When spring sunshine returns in August, the sun's ultraviolet light sets off a series of chemical reactions inside the vortex that consume the ozone. The colder and more isolated the air inside the vortex, the more destructive the chemistry. By late December the southern summer is in full swing, the vortex has crumbled, and the ozone has returned – until the process begins anew the following winter.
