TOO MUCH WATER IN DRYLANDS

Sahel will see floods in the future, followed by droughts

IN THE past few decades, the semi-arid tropical Savanna region Sahel, stretching from Mauritania in the west to Eriteria in the east, has seen several devastating floods. The Niger floods of 2010 and 2012 are two such deluge witnessed at Niamey weather station since record keeping began in 1929. In 1995, 1998 and 1999, five, eight and 11 countries in the region were hit by heavy rainfall respectively. A 2008 study by the University of South Wales and University of Ghana suggests that Sahelian countries "lay to rest the desertification narrative" and "consider ther possibility of both floods and droughts, and mobilise local memory for anticipatory learning and practical adaptation". That suggestion has gained much relevance over the years, with the IPCC report predicting that high-intensity rainfall events could increase by 20 per cent over the next decades.

Scientists attribute this weather anomaly to global warming. As the surface temperature of the Atlantic Ocean in the west and the Mediterranean Sea in the north increases, more water evaporates. The moist air drifts onto land, where vapour is released as rainfall. A study by climate scientists at Potsdam University, Germany, and Columbia University, US, shows moisture flux from the Atlantic into Sahel will increase more strongly than from the Mediterranean by the end of the 21st century. "We looked at 30 climate models to understand projections of summer rainfall in Sahel. Out of those, seven models showed a doubling of average summer rainfall by 2100, including three models that project an increase of over 100 per cent in average summer (July-September) rainfall across the central and eastern Sahel," says Jacob Schewe, co-author of the study. The increase in rainfall is also attributed to a northward shift in West African monsoon circulation dynamics. "West African monsoon, which generally covers the region between latitudes 9o N and 20oN, tapers off as it moves further north. But in future, it can make inroads into new territories," says Schewe. What explains this shift is the fact that the northern hemisphere has been heating up faster than southern hemisphere since 1980, largely because the former has more land and less ocean, and greenhouse trapping is larger over land than ocean at the same temperature.