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.
Climate experts have long worried that greenhouse gases are altering the climate on a global scale. But this is the first time they have suggested that industry and power-generation in the north may have disrupted regional weather in poor nations in tropical Africa, affecting a swath of countries from Ethiopia to Senegal.
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鈥檚 surface and leading to dramatic shifts in regional weather patterns.
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In the past thirty to forty years, the Sahel鈥攁 loosely defined band across Africa, just south of the Sahara and including parts of Ethiopia in the east and Guinea in the west鈥攈as 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鈥檚 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.
When the researchers included the huge sulphur emissions from the northern hemisphere during the 1980s in their model, the Earth鈥檚 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.
鈥淚t鈥檚 still speculative, and the model isn鈥檛 very refined, but it鈥檚 very interesting. It鈥檚 the first time we鈥檝e seen a connection between pollution in the mid-latitudes and climate in the tropics,鈥� 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 鈥渃lean air鈥� laws in North America and Europe that reduced sulphur dioxide emissions in response to another environmental crisis, acid rain.
鈥淪ulphur emissions increased rapidly until 1975 due to post-war industrialisation, and then started to decline when emission controls were brought in. Maybe that indirectly helped the people in Africa as well,鈥� he says.
But even if Rotstayn and Lohmann are correct, and cleaner air in Europe and North America will mean fewer droughts in the Sahel, the suffering is likely to continue for many years. During drought, land gets stripped of vegetation and the air fills with dust. Bare earth reflects more solar radiation, and dust aerosols trap heat while reflecting some back. So some climate experts suspect those changes may also have helped to prolong the droughts. What鈥檚 more, when the rains return, the bare soil gets washed away as there is no vegetation to hold it in place.
鈥淚t鈥檚 a vicious cycle. A land degradation issue,鈥� says Habiba Gitay, an ecologist at the Australian National University in Canberra who worked on the latest report from the Intergovernmental Panel on Climate Change. Even in a wet year following the droughts, almost 4 million people in the Sahel are likely to go hungry, she says.
And even if sulphate emissions over the northern hemisphere have diminished, 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.
That may wake the world up to the harm that aerosols can do. In the push to convince people of the reality of global warming, climate experts may neglect the impact of aerosols, partly because they can also have a cooling effect.
