In this Aug. 12, 2016 photo, a group of tourists walk in front of the Pastoruri glacier in a tour called “The Route of Climate Change” in Huaraz, Peru. The melting of glaciers like the Pastoruri has put cities like Huaraz, located downslope from the glacier about 35 miles (55 kilometers) away, at risk from what scientists call a “glof,” or glacial lake outburst flood.
A glof occurs when the weak walls of a mountain valley collapse under the weight of meltwater from a glacier. (AP Photo/Martin Mejia)

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