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African droughts "triggered by Western pollution"
19:00 12 June 02
Exclusive from New Scientist Print Edition
Emissions spewed out by power stations and factories in North America
and Europe may have sparked the severe droughts that have afflicted the
Sahel region of Africa. The droughts have been among the worst the world has
ever seen, and led to the infamous famines that crippled countries such as
Ethiopia in the 1980s.
Sahel dries out
The cause appears to be the clouds of sulphur belched out alongside
the soot, organic carbon, ammonium and nitrate produced when fossil fuels
are burnt, according to researchers in Australia and Canada. As these
compounds move through the atmosphere, they create aerosols that affect
cloud formation, altering the temperature of the Earth's surface and leading
to dramatic shifts in regional weather patterns.
In the past thirty to forty years, the Sahel--a loosely defined band
across Africa, just south of the Sahara and including parts of Ethiopia in
the east and Guinea in the west--has suffered the most sustained drought
seen in any part of the world since records began, with precipitation
falling by between 20 and 50 per cent.
Although the droughts have had climate experts scratching their heads,
the impacts have been obvious. During the worst years, between 1972 and
1975, and 1984 and 1985, up to a million people starved to death.
Now Leon Rotstayn of the CSIRO, Australia's national research agency,
thinks he knows what caused them. Rotstayn and his colleague Ulrike Lohmann
of Dalhousie University in Halifax, Nova Scotia, ran a simulation of global
climate that included interactions between sulphur dioxide emissions and
cloud formation. Sulphur dioxide creates sulphate aerosols that provide
condensation nuclei for clouds. With more nuclei, clouds form from smaller
droplets than usual, and are more efficient at reflecting solar radiation,
cooling the Earth below.
Acid rain
When the researchers included the huge sulphur emissions from the
northern hemisphere during the 1980s in their model, the Earth's surface in
the north cooled relative to the south, driving the tropical rain belt south
and causing droughts in the Sahel. Their results will be reported soon in
the Journal of Climate.
"It's still speculative, and the model isn't very refined, but it's
very interesting. It's the first time we've seen a connection between
pollution in the mid-latitudes and climate in the tropics," says Johann
Feichter of the Max Planck Institute for Meteorology in Hamburg. Feichter,
who has run similar simulations but cannot talk about the results because
the research is being peer-reviewed for a major journal, says the sulphur
emissions probably worsen the natural cycle of droughts that would have
happened anyway.
During the past few years, the droughts have become less severe, a
change that Rotstayn puts down to the "clean air" laws in North America and
Europe that reduced sulphur dioxide emissions in response to another
environmental crisis, acid rain.
But the problems in Asia may be just beginning. Climate researchers
around the world are beginning to study other types of aerosols, such as the
clouds of black soot and sulphate being churned out by rapidly
industrialising India and China, in the hope that they may shed light on
other regional weather anomalies. For instance, northern China has had
unusually dry summers in the past few years, while it has been particularly
wet in the south.
Rachel Nowak, Melbourne
