This story has been updated.
In a new study in the journal The Cryosphere, Simon Cook of Manchester Metropolitan University in the U.K. and British and Bolivian colleagues examine the mountain glaciers of the Bolivian Andes in particular, which are typically classified as “tropical glaciers.” That’s no oxymoron — the Andes feature such high elevations that even tropical or equatorial countries can have glaciers. The average elevation in the Andes is around 13,000 feet, and according to Cook, Bolivia alone contains about a fifth of the world’s tropical glaciers.
“There are three glaciated ranges that we looked at, and we actually classified glacier change across that spine of mountains that run through Bolivia,” said Cook, the study’s lead author. “And we found that sadly, the glaciers there have shrunk by more than 40 percent since the 1980s.”
He and his colleagues used satellite data to investigate the way glaciers had changed in the Bolivian Andes between 1986 and 2014. They found that about 88 square miles of glacier cover disappeared during the study period — around 43 percent of the total area that had existed in 1986.
The warming climate is almost certainly to blame. Previous reports have suggested that temperatures in the region have risen by about 0.15 degrees Celsius each decade between 1950 and 1994 and may be warming at an even higher rate now.
The researchers have a few major concerns about the losses. First, millions of citizens in Bolivia rely on these glaciers for fresh water as they melt in the spring. Smaller glaciers means less meltwater will be available to feed the water systems these residents rely on each year.
“Bolivia experiences marked seasonality in its precipitation — there’s a distinct wet season, where water is plentiful, and there’s a distinct dry season, where water is scarce,” Cook said. “And during the dry season, Bolivia’s glaciers are an important water resource.”
For instance, the high-altitude cities of La Paz and El Alto and their surrounding areas, collectively home to more than two million people, receive about 15 percent of their water supply from glaciers throughout the year, Cook pointed out. And during the dry season, this percentage may actually double.
The scientists have another concern as well. As the glaciers shrink, they tend to leave behind large pools of meltwater, constrained by walls of soil, rock and other debris that built up as the ice eroded the landscape.
“Glaciers are the most effective erosional force on the planet,” said Cook. “The fact that they cut deep valleys and leave them behind, it means they’re basically priming the landscape to fail.”
The largest of these lakes are believed to hold more than 16 billion gallons of water — that’s equivalent to about 25,000 Olympic swimming pools. Because these piles of debris are essentially the only thing holding the water back, these pools can pose a serious danger to nearby communities.
“Those lakes can burst and wash away villages or infrastructure downstream,” Cook said. “And we actually identified 25 such lakes that could prove a risk.”
Cook and his colleagues examined the landscape and made note of any pools that could be cause for concern. These included large lakes that were upstream of villages or human infrastructure and were located near steep slopes or glaciers where rockslides or avalanches could occur.
A few glacial lake floods have already been documented around the world. In 2009, one of these lakes burst in the Apolobamba region of the Bolivian Andes, killing farm animals and destroying fields. Cook said similar floods have already occurred not only in Bolivia but in nearby Peru, the Himalayas, and New Zealand.
The paper highlights what may be an under-recognized, but growing, crisis for the world’s tropical glaciers, which will likely only continue to shrink. Cook noted the research suggests most of the Bolivian glaciers will probably be gone by the end of the century — and that’s a big problem for the human communities nearby.
“Not only have they got reduced meltwater, but they’re going to be directly in the path of some of these potential floods, if they occur,” he said.
Chris Mooney contributed to this report.
Correction: A previous version of this article stated that the climate in the Bolivian Andes had been warming at a rate of about 0.15 degrees Celsius each year between 1950 and 1994. Reports suggest that the climate has, in fact, been warming at a rate of about 0.15 degrees per decade.