From the Andes to the Himalayas, scientists are starting to question exactly how much glaciers contribute to river water used downstream for drinking and irrigation. The answers could turn the conventional wisdom about glacier melt on its head.
A growing number of studies based on satellite data and stream chemistry analyses have found that far less surface water comes from glacier melt than previously assumed. In Peru's Rio Santa, which drains the Cordilleras Blanca mountain range, glacier contribution appears to be between 10 and 20 percent. In the eastern Himalayas, it is less than 5 percent.
"If anything, that's probably fairly large," said Richard Armstrong, a senior research scientist at the Boulder, Colo.-based Cooperative Institute for Research in Environmental Sciences (CIRES), who studies melt impact in the Himalayas.
"Most of the people downstream, they get the water from the monsoon," Armstrong said. "It doesn't take away from the importance [of glacier melt], but we need to get the science right for future planning and water resource assessments."
The Himalayan glaciers feed into Asia's biggest rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow and Yangtze rivers in China. Early studies pegged the amount of meltwater in these river basins as high as 60 or 70 percent. But reliable data on how much water the glaciers release or where that water goes have been difficult to develop. Satellite images can't provide such regular hydrometeorological observations, and expeditions take significant time, money and physical exertion.
New methods, though, are refining the ability to study this and other remote glacial mountain ranges. Increasingly, scientists are finding that the numbers vary depending on the river, and even in different parts of the same river.
"There has been a lot of misinformation and confusion about it," said Peter Gleick, co-director of the California-based Pacific Institute for Studies in Development, Environment and Security. "About 1.3 billion people live in the watersheds that get some glacier runoff, but not all of those people depend only on the water from those watersheds, and not all the water in those watersheds comes from glaciers. Most of it comes from rainwater," he said.
A key step forward came last year when scientists at Utrecht University in the Netherlands, using remote sensing equipment, found that snow and glacier melt is extremely important to the Indus and Brahmaputra basins, but less critical to others. In the Indus, they found, the meltwater contribution is 151 percent compared to the total runoff generated at low elevations. It makes up about 27 percent of the Brahmaputra -- but only between 8 and 10 percent for the Ganges, Yangtze and Yellow rivers. Rainfall makes up the rest.
That in itself is significant, and could reduce food security for 4.5 percent of the population in an already-struggling region. Yet, scientists complain, data are often inaccurately incorporated in dire predictions of Himalayan glacial melt impacts.
"Hyperbole has a way of creeping in here," said Bryan Mark, an assistant professor of geography at Ohio State University and a researcher at the Byrd Polar Research Center.
Mark, who focuses on the Andes region, developed a method of determining how much of a community's water supply is glacier-fed by analyzing the hydrogen and oxygen isotopes in water samples. He recently took that experience to Nepal, where he collected water samples from the Himalayan glacier-fed Kosi River as part of an expedition led by the Mountain Institute.
Based on his experience in the Rio Santa -- where it was once assumed that 80 percent of water in the basin came from glacier melt -- Mark said he expects to find that the impact of monsoon water is greatly underestimated in the Himalayas.
Jeff La Frenierre, a graduate student at Ohio State University, is studying Ecuador's Chimborazo glacier, which forms the headwaters of three different watershed systems, serving as a water source for thousands of people. About 35 percent of the glacier coverage has disappeared since the 1970s.
La Frenierre first came to Ecuador as part of Engineers Without Borders to help build a water system, and soon started to ask what changes in the mountain's glacier coverage would mean for the irrigation and drinking needs of the 200,000 people living downstream. Working with Mark and analyzing water streams, he said, is upending many of his assumptions.
Doomsday descriptions don't fit
"The easy hypothesis is that it's going to be a disaster here. I don't know if that's the case," La Frenierre said. He agreed that overstatements about the impacts are rampant in the Himalayas as well, saying, "The idea that 1.4 billion people are going to be without water when the glaciers melt is just not the case. It's a local problem; it's a local question. There are places that are going to be more impacted than other places."
Those aren't messages that environmental activists will likely find easy to hear. Armstrong recalled giving a presentation in Kathmandu on his early findings to a less-than-appreciative audience.
"I didn't agree with the doomsday predictions, and I didn't have anything that was anywhere near spectacular," Armstrong said. But, he added, "At the same time, it's just basic Earth science, and we want to do a better job than we have been."
The more modest numbers, they and other scientists stressed, don't mean that glacier melt is unimportant to river basins. Rather, they said, they mean that the understanding of water systems throughout the Himalayan region must improve and water management decisions will need to be made at very local levels.
"We need to know at least where the water comes from," Armstrong said. "How can we project into the future if we don't know where the water comes from now?"
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