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Increasing Asian smog blocks out the Sun

11:32 12 July 01
Fred Pearce, Amsterdam

A year-round smog over the Indian Ocean and Asia is
blotting out 10 to 15 per cent of the Sun's rays, with
potentially "very major consequences" for the atmosphere.

The warning comes from the 1995 Nobel prize-winner for
chemistry, Paul Crutzen, in a devastating analysis of the scale
of air pollution in a once-pristine region of the tropics. He
was speaking at a conference on global climate change in
Amsterdam, a week before political talks resume on tackling
the problem.

The smog comes mostly from farmers using fire to clear fields
across Asia and Africa. In field experiments last year, says
Crutzen, "we found thick brown smog 4000 metres up in the
Himalayas, over the Maldives in the Indian Ocean, and
widely across south and east Asia. We were shocked."

Crutzen, from the Max Planck Institute for Chemistry in
Mainz, Germany, won his Nobel prize for his pioneering
investigations of the thinning ozone layer. He warned that
biomass burning in the tropics had the potential to cause an
unpleasant environmental "surprise" on a par with the
discovery of the ozone hole 15 years ago.

Crutzen says the theoretical cooling effect of the smoke
blocking the Sun's warmth is largely ignored in current climate
models. He calculates it to be almost ten times greater than
the warming from greenhouse gases.

But there is no evidence that the region is in fact cooling. This
could be because much of the pollution is black soot, which
absorbs the sunlight and then itself radiates heat towards the
ground.


Cleansing agent

Even so, Crutzen warned that the tropical smogs could have
"very major consequences" for the atmosphere. They could
upset the hydrological cycle that maintains the Asian
monsoon, for instance. They might also use up large amounts
of the atmosphere's main cleansing agent, the hydroxyl
radical, so damaging the ability of the atmosphere to cleanse
itself.

The pollution is set to grow in coming decades. "In southern
Africa we are very worried that the predicted decease in
rainfall could cause an extreme increase in fires," said Mary
Scholes of Witwatersrand University, South Africa.

Meinrat Andreae, also of the Max Planck Institute for
Chemistry, said recent studies had found that more than
seven billion tonnes of biomass was burned in the tropics
each year, for farming our fuel. A typical rural inhabitant of
the developing world consumes a tonne of biomass fuel a
year. "Poverty pollutes, too," said Crutzen.

But the conference also heard that some rich nations are also
burning. Mike Flannigan of the Canadian Forest Service said
fires in its northern forests, mostly triggered by lightning, were
responsible for some 20 per cent of his country's carbon
dioxide emissions.