African droughts triggered by Western pollution

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African droughts " triggered by Western pollution "

19:00 12 June 02

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.

Nowak, Melbourne