Biggest hole in ozone forecast
Wednesday 30 September 1998
Jonathan Shanklin, a meteorologist at the British Antarctic Survey, which first detected the development of the hole in the 1980s, also said that the northern hemisphere will suffer equally bad ozone depletion in the next 20 years, raising the incidence of skin cancer in northern Europe.
Preliminary satellite data collated by New Zealand teams in the Antarctic suggest that the hole in the layer of the upper atmosphere, which protects the Earth from the Sun's ultra violet rays, will be up to 25 per cent greater than the previous record in 1996. Then it stretched over 27m sq km - an area twice as large as Europe. It may take another 75 years for the ozone layer to recover.
The layer usually begins about 16km up, where oxygen molecules absorb high-energy ultraviolet light from the Sun. The ozone loss is caused by the presence of chlorofluorocarbons. These CFCs are molecules used as a coolant in fridges and aerosol can, which break down the oxygen. Each chlorine atom in a CFC molecule destroys an estimated 100,000 atmospheric ozone molecules, and lingers for up to 20 years.
Mr Shanklin added: "This is happening as a consequence of El Nino [a warm Pacific current that alters global weather]. The stratosphere has been much colder than usual, which has led to more stratospheric clouds, and those in turn are a big factor in ozone depletion." The reaction that destroys ozone occurs on cloud surfaces.Global warming could also contribute to ozone depletion because it causes the upper atmosphere to cool. The survey team reported earlier this month that the upper atmosphere has dropped by 8km because it has cooled as more heat is kept close to the ground by greenhouse gases.
"That causes a vicious feedback cycle which increases depletion," Mr Shanklin said.
The depth of the hole is measured in "Dobson Units". The maximum reading, which occurs in the spring, is about 300 to 350 units. Each winter it falls, as the air cools and the CFCs catalyse the breakdown. The previous confirmed record low is 105 Dobson Units. Mr Shanklin said it could now fall into double digits - "which is a psychologically important figures, even though it doesn't have a physical significance".
He added: "Each DU lost means about a 2 per cent rise in the ultraviolet levels on the ground. We are talking here about 70 per cent depletion, which means more than 100 per cent increase in radiation levels." Normal ozone levels would mean being able to ski for an hour without sunscreen with only a mild risk of sunburn. The record low Dobson Unit levels would mean that, even wearing factor 30 sunscreen, a skier would burn in about 20 minutes.
Although developed nations have almost phased out their use of CFCs, developing nations, such as China, still use them, although they are committed to stopping by 2000. Yet CFC smuggling is reckoned to be one of the biggest undercover businesses.
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