One of the big unknowns of climate change predictions -- and one that has led to considerable contention -- lies in knowing the future of water runoff from the Himalayas. The snow- and ice-rich region supplies water for billions of people in Asia and is sometimes referred to as the Earth's "Third Pole."
For years, scientists struggled to understand how precipitation will change in these mountains (ClimateWire, Oct. 24, 2011). They have also had difficulty determining how much glacier melt from the mountains contributes to water supply.
A study out yesterday in Nature Geoscience by Walter Immerzeel, a physical geographer at Utrecht University, suggests that, in at least two major Himalayan watersheds, river flows and runoff should rise until 2100.
"We show that the peak in meltwater is later than we previously thought, which in combination with a projected increase in precipitation results in an increase in water availability until the end of the century," he said.
The two watersheds Immerzeel reports on in the paper are those of the Baltoro and Langtang glaciers, which feed the Indus and Ganges rivers, respectively. In the Baltoro watershed, this is largely due to more glacier runoff from melt. In the Langtang, increased precipitation drives the extra runoff.
Immerzeel and his co-authors used the output of the latest global climate models from the Coupled Model Intercomparison Project (CMIP5) to look at temperature and precipitation projections. They combined that data with a hydrologic model of glacier responses to climate change.
They found that in both watersheds, runoff from glaciers should increase until the 2040s or 2060s, later than previous estimates, depending on which climate scenarios are applied.
'Scanty measurements on the ground'
Richard Armstrong, a senior research scientist at the National Snow and Ice Data Center, called the paper "excellent."
"I think this assessment will be greatly appreciated by the scientific community and the interested community at large [that is] concerned with future water resources in High Asia," Armstrong said.
There is still considerable uncertainty in the finding that runoff will increase through 2100, Immerzeel noted, because global climate models vary widely in their projections for future precipitation in the region.
J. Graham Cogley, an emeritus professor of geography at Canada's Trent University who reviewed the paper, said "plausible" is probably a good way to describe the results.
"Their study makes just about the best use of very scanty measurements on the ground. It relies on the latest generation of climate models, so it is almost certainly an improvement on the other choice -- which is to wait and see what happens," Cogley said.
Immerzeel, an intrepid cataloger of glacier behavior in the Himalayas, has worked to improve that input data.
He's been on expeditions to install weather stations that measure precipitation and temperature stations in the Himalayas, and trekked across rugged glaciers and snowfields to put in snow gauges.
'This is good news'
The hydrologic model of glaciers used in the paper was one he and others developed recently, in order to better represent how Himalayan glaciers will change with the climate.
In the paper, Immerzeel points out that his new finding contradicts previous work he has published, suggesting that runoff in the Indus and Ganges basin would decrease. At least for now, this is good news for people and farmers who rely on that water, he said.
"Strong increases in water demand are projected in the Indus as the food production needs to grow to feed the quickly rising population," Immerzeel said. "An increased water availability from the mountains may help to sustain this growing demand."
On the other hand, Trent University's Cogley noted, this finding, which will be welcome news to those in the region, could have a downside.
"[There is] the risk that people, and particularly planners, will relax when in truth there is no cause for complacency," he said.
Immerzeel agreed, highlighting that climate change could still affect the region's water resources in many other ways.
He plans to research how the timing of water availability might shift in his own future research. The researcher also pointed to the need to examine how extremes, such as floods, landslides and glacier lake outburst floods might change along with the climate.
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