Water shortages, sudden floods on the climate change menu in South America
Researchers already know that the world’s tropical glaciers are melting fast, but a new study published in The Cryosphere, an European Geosciences Union journal, helps pinpoint some of the potential impacts. The research focused on the Bolivian Andes, where glaciers dwindled by 43 percent in the last 30 years. The melting ice has created lakes that could burst and flood downstream towns, according to lead author Simon Cook a lecturer at the Manchester Metropolitan University in the UK.
The glacier meltdown also threatens regional water supplies. The 2.3 million residents of La Paz and El Alto get about 15 percent of their water supply from glaciers, and double that during the dry season. One lake in the region has already dried up, according to the authors, who said their study is one of the first to look specifically at recent large-scale glacier change in Bolivia.
“The novelty of our study lies in the bigger picture — measuring glacier change over all main glaciated ranges in Bolivia — and in the identification of potentially dangerous lakes for the first time,” Cook said.
Satellite images from the Landsat program, the the U.S. Geological Survey’s and NASA’s Earth observation program show that the area of the Bolivian Andes covered by glaciers decreased from about 530 square kilometres in 1986 to only around 300 square kilometres in 2014, a reduction of 43 percent.
As glaciers recede, they leave behind lakes typically dammed by bedrock or glacial debris. Avalanches, rockfalls or earthquakes can breach these dams, or cause water to overflow them, resulting in catastrophic floods known as glacial lake outburst floods. The team reports that both the number and size of glacier lakes in the study region increased significantly from 1986 to 2014.
“We mapped hundreds of lakes,” Cook said. “Some lakes are very small and pose little risk. Others are very large, but there’s little or no possibility that they would drain catastrophically. Others are both large enough to create a big flood, and sit beneath steep slopes or steep glaciers, and could be dangerous.”
They identified 25 glacial lakes across the Bolivian Andes as potentially dangerous to communities and infrastructure, as they could result in very damaging floods. If the smallest of these 25 lakes was to drain completely, it would yield a flood with a peak discharge of 600 cubic metres per second. The largest could result in a discharge of over 125,000 cubic metres of water, about 50 times the volume of an Olympic swimming pool, in a second.
While measuring glacier area change was a relatively simple task, Cook said “identifying which lakes are dangerous is the million dollar question” as there are various factors to take into account.
“We considered that a lake was dangerous if there were settlements or infrastructure down-valley from the lake, and if the slopes and glaciers around the lake were very steep, meaning that they could shed ice or snow or rock into the lake, which would cause it to overtop and generate a flood – a bit like jumping into a swimming pool, but on a much bigger scale!”
Such catastrophic floods have occurred in the region in the past, said Dirk Hoffmann, a researcher at the Bolivian Mountain Institute and co-author of The Cryosphere study. A 2009 glacial lake outburst flood in the Apolobamba region 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 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 said, calling for a nation-wide risk assessment of potentially dangerous glacial lakes.
The Bolivian glaciers are expected to recede even more. By the end of the century, many mountain communities could face water scarcity, Cook said.
“We predicted in our study that most glaciers will be gone or much diminished by the end of the century – so where will the water come from in the dry season? Big cities like La Paz are partially dependent on meltwater from glaciers. But little is known about potential water resource stress in more remote areas. More work needs to be done on this issue.”