By Steve Painter
The US space agency, NASA, claims the danger of ozone depletion over the northern hemisphere has eased due to a short Arctic winter, but a seven-month study by the agency confirms the ozone shield is "increasingly vulnerable to depletion by man-made chemicals". The degree of ozone loss over the Arctic depends on the severity and length of the polar winter, says Harvard professor James Anderson, who headed the NASA study.
Cold temperatures in December and early January primed the Arctic region chemically for serious ozone loss, but early warming in late January prevented major depletion. NASA also found evidence that ozone thinning at mid-latitudes of the northern hemisphere is related to increased levels of chlorine and bromine in the stratosphere. NASA says chemicals released by the Mt Pinatubo volcanic eruption probably made a minor contribution to ozone depletion, and there have been suggestions that oil fires in Kuwait also contributed.
A NASA survey of the stratosphere over the Arctic in February found extremely high levels of the most potent ozone-destroying chemicals, particularly chlorine monoxide, leading to predictions that an ozone hole would open up over vast areas of Canada, Russia, northern Europe, and parts of the US, causing higher rates of skin cancer and cataracts in humans and stunting the growth of many plants.
This prompted George Bush to announce a four-year advance in the timetable for phase-out of ozone destroying chemicals, particularly chlorofluorocarbons (CFCs). The date is now December 1995, four years earlier than required by the 1987 Montreal Protocol. US military forces and military-related industries use massive amounts of the solvent CFC-113, one of the leading ozone destroyers.
Winter is the time of greatest ozone depletion over the poles because clouds forming at that time can hold microscopic ice particles with high levels of harmful chemicals. Ozone depletion will remain a serious threat in coming winters because chlorine sources break down slowly, says Waters. "Conditions in the upper atmosphere are in a very delicate balance. With so much chlorine in the stratosphere, a slight temperature difference can make an enormous difference in the potential for ozone depletion."
While present levels of ozone depleting chemicals are lower, they are still too high. Serious ozone depletion over the Antarctic is more frequent because prevailing winds make it more difficult for warmer air to enter the region and disperse ice clouds.