Scientists have detected a dramatic thinning of the ozone layer in the northern hemisphere in the first three months of the year.
Over latitudes that include London and San Francisco they found that ozone, which filters harmful ultraviolet radiation from the sun, was up to 14 per cent lower than the average for previous years.
Globally Nasa scientists found ozone levels in 1992 were the lowest on record. Satellite data show that ozone around the world had thinned by 2- 3 per cent on the lowest levels previously recorded. For the first time, the scientists detected the simultaneous depletion of ozone in the northern and southern hemispheres.
The Nasa scientists said that they can offer no explanation for the dramatic fall in ozone, except that the gradual loss caused by ozone-destroying pollutants may have been accelerated by unusual atmospheric effects that have lingered on from the 1991 Mount Pinatubo volcanic explosion in the Philippines.
Jim Gleason, a member of the Nasa reseach team, said the effect was unexpected and extremely large. 'We are seeing lower global ozone values than we've ever seen before. We predicted lower ozone in 1992, but nothing like the values we actually observed.'
He said that ozone was the major filter of UV-B light, which can cause skin cancer and is believed to be capable of damaging crops and the delicate marine plankton on which all ocean life depends. The Total Ozone Mapping Spectrometer, an instrument on the Nimbus-7 satellite, has been recording daily ozone levels since 1979 and has charted a steady decline.
The Nasa researchers say in today's issue of the journal Science that the daily global average for ozone last year 'is significantly lower than in any of the earlier 13 years'. The ozone depletion was 'much larger and of considerably longer duration' than previous years when levels were low.
Rich McPeters, another member of the Nasa team, said more recent data, yet to be published, showed that ozone continues to be even lower in the first three months of this year. 'We are seeing very low ozone over all of Europe and especially the western US.'
He said ozone was 12 to 14 per cent below normal levels last month. 'We don't see any signs of recovery. The measurements show that ozone has been going down for some time. It's not just a random fluctuation.
'It takes a while to work out the cause. Pinatubo is just a guess. There may be two or three things going on at the same time.'
A different set of measurements, taken from a network of ground stations, support the Nasa findings.
They have detected a 20 to 25 per cent drop in ozone below long-term averages in the first half of last month over much of the northern
Rumen Bojkov, special adviser on ozone to the UN's World Meteorological Organisation, said that Nasa's satellite measurements were independent confirmation that the ozone depletion is continuing to increase and that these were the lowest ozone levels in 37 years of ground-based
He added that because man-made pollutants, such as CFCs, that destroy the ozone layer are active for so long in the atmosphere, 'we haven't seen the worst'.
Ozone will continue to decline before it begins to level out, he said.
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