"These glaciers supply between 15 and 30 percent of the water to major cities like La Paz and its neighboring sister city of El Alto," Cook said. "Combined, they have a population of something like 2.3 million inhabitants and growing, because you have a lot of urban migration going on in Bolivia. These glaciers have retreated quite rapidly, and the very rough, almost back-of-the-envelope calculation is that these glaciers will be more or less gone by the end of the century. So the problem there is that in 50, 60 years' time, during the dry season, where is the water coming from? There will still be water coming from groundwater, precipitation and runoff and so on, but the glaciers are really a buffer, or a store. If you take that away, you become immediately more vulnerable to changes in precipitation, or drought."
There is another, potentially catastrophic, scenario. As glaciers form, Cook observed, "they take big bite marks out of the landscape." As they melt and recede, the "bite marks" fill with meltwater and become lakes. That wouldn't necessarily be a problem, Cook said, except that "these lakes can burst, and when they do they can wash away villages or roads."