Revealed: how the smoke stacks of Europe and America have brought the world's worst drought to Africa

To those who live there, it is as if the rich have stolen the rain. For more than 30 years, the Sahel region of Africa has suffered the longest sustained droughts in the world. In some places, rainfall has fallen by between 20 and 50 per cent.

To those who live there, it is as if the rich have stolen the rain. For more than 30 years, the Sahel region of Africa has suffered the longest sustained droughts in the world. In some places, rainfall has fallen by between 20 and 50 per cent.

As a consequence, crops have failed on a huge scale; in the worst years, between 1972 and 1975 and between 1984 and 1985, up to a million people starved to death.

However, for President George Bush, who has only recently accepted that global warming and climate change are the result of human influences – such as the burning of fossil fuels – the idea of a cause involving the developed world is unwelcome.

But new research indicates that pollution from factories and power stations, especially in North America and Europe, has exacerbated drought in countries south of the Sahara.

Researchers have little doubt that the two are connected and also that the effect of the drought will last long after any clean-up.

There are also warnings that growing industrialisation in India and China is likely to create the same problems on the Indian subcontinent – with potentially disastrous effects for millions more people.

According to a report in New Scientist magazine today, climate modelling studies by scientists in Australia and Canada have fingered the clouds of sulphur poured out by vehicles and power stations when they burn fossil fuels for pushing the Saharan rain-belt south.

The effect is complex, which is why it has only just emerged from the analyses. New Scientist explains that Leon Rotstayn of CSIRO, the national research agency in Australia, and Ulrike Lohmann, of Dalhousie University, in Halifax, Nova Scotia, created a computer model that simulated the interactions between sulphur dioxide emissions from power plants and other sources and cloud.

A key element here is that those emissions create huge volumes of "aerosols" – tiny particles about one micrometre across that can remain floating in the atmosphere for days. They are very efficient at scattering light and forming clouds, which reflect sunlight; both effects tend to cool the atmosphere and the Earth below.

The vast amount of aerosols produced especially in the 1980s lingered over the northern hemisphere and tended to cool it down, say the researchers. But it is the final step – to the shifting fortunes of the rain clouds that should linger over the Sahel – which is the subtle one.

David Roberts, the head of the aerosol modelling group at the Meteorological Office's Hadley Centre for Climate Prediction and Research, said: "It's an effect of the thermal balance between the two hemispheres. There has to be a rough balance between the north and south hemispheres – you can't have spare energy in one place or the other. If the Earth was completely symmetrical, then the point of thermal equilibrium, where the total energy on either side of a line was equal, would be the Equator. But because the Northern hemisphere isn't the same as the south [because of the vast energy reservoir of the Pacific, which retains energy more efficiently than land] we find that the Northern hemisphere is warmer than the South."

However, aerosol-driven cooling of the Northern hemisphere pushes that point of thermal equilibrium south – and with it go the rainclouds that people depend on for their crops in the Sahel.

The historical evidence tends to back up the findings.

One key change that the researchers point to is that in the 1980s, improvements in emission laws meant that sulphur emissions in particular dropped – because they were blamed for acid rain, which was noticed far more keenly in the industrialised countries than droughts in sub-Saharan Africa.

Dr Rotstayn and Professor Lohmann said that droughts have become less severe during the past few years. But that does not mean that they have disappeared. Far from it; the whole of southern Africa is facing a "regional food crisis", according to a recent report that notes that a total of six countries in southern Africa have roughly 11 million people who need emergency food assistance. Ironically, the note came from the United States Agency for International Development.

But the cleaning of the air in the US and Europe (and the closure through economic failure of many of the worst polluters in eastern Europe) does not mean that the threat is over. If anything, it could get worse.

Although Dr Roberts says he is "cautious" about taking the interpretation of the link between aerosols in the northern hemisphere and the weather in the Sahel as gospel, he says that if it is correct, then there are other areas around the globe that could be threatened by a new wave of fossil-fuel burning.

"The emissions from China and India are rising," he said yesterday. "And that could affect, for example, the monsoon season in India, which would have an important regional impact. If it weakens the monsoon, then that would be a concern. The subcontinent is very dependent on it; it would be of serious concern to them if that moved. But the aerosols would alter the balance between north and south in the same way."

What is unclear is how the world would cope with change like that. In the Sahel, drought is becoming ingrained: without water, vegetation dies, and the the dry land and the dust thrown up from it into the atmosphere both reflect heat, keeping the thermal equilibrium point in its southerly place. And when it does rain, the lack of plant life means that water just runs away.

"It's a vicious cycle, a land degradation issue," Habiba Gitay, an ecologist who worked on the Intergovernmental Panel on Climate Change told New Scientist. And the blame for starting the cycle turning appears to lie with the developed world. The problem, though, is that nobody knows quite how to stop it and bring back the rain to the places that need it.

Some people think that the first signs of the thermal changes caused by the increasing industrialisation of the developing countries are already being seen. In a worrying echo of the situation in the Sahel, northern China has had unusually dry summers in the past few years, note s New Scientist – while the south has had a notably wet time.

The new theory about the connection between aerosols and rainfall also means that the world will have to expand the sophistication of its understanding of climate change. Because aerosols cool the Earth, it had been thought that they might in some way be beneficial in easing the warming caused by carbon dioxide. But now it can be seen that they also contribute to changing weather patterns in ways that are potentially disastrous for millions of people, as surely as rising sea levels caused by simple global warming.