 The climate has altered drastically since the onset of the prolonged drought in the 1960s, part of a recurrent pattern of wet and dry cycles common to Sahelian Africa.Experts agree, however, that overgrazing, deforestation, denuding of ground cover around wells, poor farming methods, and overpopulation have aggravated the drought.In Mauritania the isohyet indicating annual rainfall of 150 millimetres—considered the minimum for pastoralism—has shifted southward about 100 kilometres to a point well south of Nouakchott.