Climate change has caused mountain glaciers in the Bolivian Andes to retreat by 43 percent between 1986 and 2014, threatening rural and urban populations alike — in South America's poorest country — with long-term water shortages and possibly even catastrophic flooding.

According to Simon Cook of Manchester Metropolitan University in the U.K., Bolivia holds 20 percent of the world's tropical glaciers — which, despite being at the latitude of the tropics, are able to exist because of their high altitudes.

"Bolivia has a distinct wet and dry season, rather than a hot and cold season," Cook explained to Seeker. "During the dry season, which is during the Southern Hemisphere winter, they tend to lose more mass. So the weird thing is they tend to lose more mass in winter because you've got clearer skies and less precipitation falling as snow; and counter-intuitively they tend to gain mass in summer, which is the wet season."

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However, in a paper published in the journal The Cryosphere, Cook and four co-authors describe how, using satellite images from Landsat, the U.S. Geological Survey's and NASA's Earth Observation Program, they determined that the area of the Bolivian Andes covered by glaciers decreased from a little over 200 square miles in 1986 to only around 115 square miles in 2014. One important consequence is that fewer and smaller glaciers means less meltwater in rivers and thus a diminished water supply for some of the country's biggest towns and cities.

"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."

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If that sounds overly apocalyptic — well, it's happened. On Dec. 13, 1941, a large piece of glacier ice crashed into Lake Palcacocha in Peru, causing it to wash over its borders and down into the valley below, killing 5,000-7,000 people in the city of Huaraz. Cook told Seeker that there are very few published records of similar such events in Bolivia, although "one village elder said that his father had witnessed a flood from a glacier when he was working as a miner in a neighboring valley." In addition, Dirk Hoffmann, a researcher at the Bolivian Mountain Institute and co-author of The Cryosphere study, recently documented a glacial lake outburst flood in the Apolobamba region that happened in 2009 and killed farm animals, destroyed cultivated fields and washed away a road that left a village isolated for months.

"As those locations are very remote and far away from the cities, authorities at the national level and the wider public are often not even aware of the new dangers that mountain dwellers are facing due to the impacts of climate change, and no appropriate measures are being taken," Hoffmann says.

Cook and his co-authors identified 25 glacial lakes across the Bolivian Andes as potentially dangerous to communities and infrastructure. If the smallest of these 25 lakes were to drain completely, it would yield a flood with a peak discharge of 21,000 cubic feet per second. The largest could result in a discharge of almost 4.5 million cubic feet of water, about 50 times the volume of an Olympic swimming pool, in a second.

"The next phase of our work is one of my Ph.D. students is going to be doing numerical modeling, to see just what kind of damage floods from those lakes would do," Cook explained.

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