Melting glaciers pose threat beyond water scarcity: floods

The tropical glaciers of South America are dying from soot and rising temperatures, threatening water supplies to communities that have depended on them for centuries.
But experts say that the slow process measured in inches of glacial retreat per year also can lead to a sudden, dramatic tragedy.
The melting of glaciers like Peru's Pastoruri has put cities like Huaraz, located downslope from the glacier about 55 kilometers away, at risk from what scientists call a "glof" glacial lake outburst flood.
A glof occurs when the weak walls of a mountain valley collapse under the weight of meltwater from a glacier. Recent examples include the rapid draining in 2013 of a lake at Chile's Ventisquero glacier in the Bernardo O'Higgins National Park, six years after another, nearby lake essentially disappeared there.

Those sites are in remote, sparsely populated Patagonia. But if the glacial Palcacocha lake collapsed, it could cause a damaging flood, say experts in Peru, sort of like a smaller, modern cousin of the ancient glof that is thought to have carved the English Channel.
"As glaciers disappear around the world, there is less water available for use for hydroelectric power, as a renewable resource for agriculture, for human consumption," said Benjamin Orlove, a professor of international and public affairs at Colombia University in New York.

"The glacier retreat also brings many disasters. Entire slopes are destabilised, creating landslides that travel many miles and have destroyed entire towns."

Benjamin Morales Arnao, the head of Peru's National Institute for Glacier Research, said that while the country's glaciers "are a source of life, due to their water resources and biodiversity these glaciers are also a source of glacial catastrophes."
The problem is that glacial lakes are often fragile structures, created when rocks and rubble carried by a glacier form a moraine that dams up its water outflow. The dam can also be created by chunks of a glacier's own ice. These inherently unstable structures can collapse quickly, especially in a place like Peru that is prone to frequent, violent earthquakes.

At a conference last week on the glacier retreat in Peru, Morales Arnao said that Huaraz, a city of about 100,000 people, is particularly at risk from the glacial Palcacocha lake, just 20 kilometers up the mountain above the city, and called for resources to mitigate the risk. In the past, dams, spillways and other waterworks have helped in other places.