From your link
But beginning in the late 1960s, the region experienced a series of devastating droughts. As the rains increasingly failed to come, the region began undergoing desertification. At the same time, local people became more and more dependent on the lake as a source of water to replace the water they had previously obtained from the monsoons.
While obviously human water use is a major factor, I don't quite see why 40 years of drought might not have something to do with global warming, and the drought certainly relates to the increased dependence of locals on the remaining lake water.