Bonn - Rising greenhouse gases have boosted rainfall in the Sahel region of Africa, easing droughts that killed 100 000 people in the 1970s and 1980s, in a rare positive effect of climate change, a study said on Monday.The report adds to debate about the causes of a greening of the Sahel region, south of the Sahara Desert from Senegal to Sudan. It said a continued rise in greenhouse gas emissions was likely to help more rainfall in the region in future."Amounts of rainfall have recovered substantially," said Rowan Sutton, a professor at the National Centre for Atmospheric Science at Britain's Reading University and co-author of the study in the journal Nature Climate Change."And it was a surprise that the increase in greenhouse gases appears to have been the dominant factor," he told Reuters. Sahel summer rainfall was0.3 mm a day higher from 1996-2011 than the drought period of 1964-93.The scientists, using a supercomputer climate simulator, said heat-trapping emissions accounted for three-quarters of the recovery in rainfall, rather than other suggested factors such as changes in sea temperature or acid rain air pollution.Warming by greenhouse gases means that air can hold more moisture, bringing more rains, and can shift winds, influencing the pattern of the monsoon. Acute droughts have caused famine in the region, mainly in the 1970s and 1980s.Sutton cautioned that the change in rainfall was only local and that many parts of Africa face mounting problems from warming, ranging from desertification and floods to rising sea levels."It would be naïve to conclude that this is a good thing for Africa," he said. "And in future, there are other effects - the rise in temperatures can be detrimental to crops."Still, the scientists wrote that models indicated that a projected continued rise in greenhouse gas emissions "is favourable for sustaining, and potentially amplifying, the recovery of Sahel rainfall."Separately, the Stockholm Resilience Centre says a greening of the Sahel is also caused by changes in farming. "Farmers have actively managed their land in ways that have enhanced its productivity," it says.Almost 200 nations are meeting in Bonn this week to work on a deal to limit climate change that is due to be agreed in Paris in December.