Scientists in Australia and Canada say that pollution from western countries may have caused the droughts which ravaged Africa's Sahel region in the 1970s and 1980s.

Millions died in the droughts, which hit Ethiopia hardest in 1984.

It's the first time we've seen a connection between pollution in the mid-latitudes and climate in the tropics

Johann Feichter Climate expert

Other Sahelian countries, from Senegal in the west stretching east to the Red Sea, were also devastated by the lack of rain and the southwards spread of the Sahara desert.

The research says that sulphur dioxide from factories in Europe and the United States has cooled the Northern Hemisphere, driving the tropical rain belt south - away from the Sahel.

Rainfall in the region has declined by between 20% and 50%, leading to severe droughts in 1972, 1975, 1984 and 1985.

This was the most sustained drought in any part of the world since records began, according to the research which is reported in the New Scientist magazine.

But climate experts had been unable to explain the drastic change.

Cold north

The research was carried out by Leon Rotstayn from Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) and his colleague Ulrike Lohmann from Canada's Dalhousie University.

They ran a simulation of global weather including the interaction between sulphur dioxide emissions and cloud formation.

Sulphur emissions from power stations and factories prevent cloud formation, cooling the Earth below.

As this pollution mainly happened in the industrialised north, the Northern Hemisphere became relatively cooler than the south.

This caused the rain belt to move south - away from the Sahel.

Johann Feichter of the Max Planck Institute for Meteorology in Hamburg, Germany, says the sulphur emissions probably worsened the natural cycle of droughts that would have happened anyway.

"It's still speculative, and the model isn't very refined, but it's very interesting.

Have Western factories changed Africa's climate?

"It's the first time we've seen a connection between pollution in the mid-latitudes and climate in the tropics," he said.

During the past few years, rainfall has increased in the Sahel.

Mr Rotstayn explains this improvement by the "clean air" laws introduced in North America and Europe.

This legislation reduced sulphur dioxide emissions in response to another environmental crisis - acid rain.