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The main device used by researchers to measure ozone levels in the upper
atmosphere confuses the gas with another pollutant, sulphur dioxide, according
to a leading American sceptic about thinning of the ozone layer. Fred Singer,
a geophysicist from the University of Virginia, told a meeting in London
last week: ‘What we may really be seeing is a fall in the amount of sulphur
dioxide in the air, not ozone.’

Sulphur dioxide and ozone absorb light at similar frequencies. And
the Dobson meter, which measures concentrations of gases from their absorption
spectra, cannot easily distinguish between the two, says Singer. Over Europe
and North America this confusion over gases could obscure what is really
going on. Singer’s claim follows a Belgian study two years ago, which
found that an apparent loss of ozone above Brussels was largely due to
improved controls on sulphur dioxide emissions from power stations.

But environmentalists attacked Singer’s conclusions. ‘Researchers have
known about this potential confusion since at least the early 1960s,’ said
Douglas Parr of Greenpeace. ‘But you cannot infer global trends from results
at a single station. Brussels is likely to be very untypical.’

Joe Farman of the British Antarctic Survey, the man who discovered the
ozone hole over Antarctica in the early 1980s, said: ‘What Singer says is
quite possible, but it’s not a big deal. There is uncertainty about how
much the general ozone layer has thinned. But it is the episodes of very
low ozone over Antarctica and the Arctic that really matter. And you can’t
explain them with sulphur dioxide.’

Singer, whose scepticism on environmental issues extends to global warming
and theories of nuclear winter, is also director of the Science and Environmental
Policy Project, a think-tank in Washington DC. He and Farman debated the
ozone layer at the Society of Chemical Industry.

Singer agreed that man-made chemicals such as CFCs destroy ozone in
the stratosphere. But he claimed that natural and man-made sources of chlorine
are equally to blame.

He suggested that a doubling in the number of major volcanic eruptions
in the past 20 years could have added to chlorine in the stratosphere, partly
explaining the formation of ozone holes. But ozone concentrations follow
a natural variation linked to the 11-year sunspot cycle, he said, and it
is still too early to be able to separate out this effect from other factors
affecting ozone.

Farman denied this. ‘We actually have a very clear idea of the chlorine
budget now,’ he said. ‘And there is no direct evidence that any volcano
has ever thrown chlorine into the stratosphere in a place where it matters
to ozone.’

Farman accused Singer of being ‘extremely selective’ with his data.
‘The facts of the ozone holes are unavoidable,’ he said. ‘And we know that
CFCs are doing most of the damage.’