Published online 10 June 2010 | Nature | doi:10.1038/news.2010.292

News

Global warming's impact on Asia's rivers overblown

Freshwater flow dominated by monsoon rains rather than glacier run-off.

Pakistan,Sind Region,Hyderabad,buffalos in the Indus river Date: 10.06.2008Meltwater from glaciers makes a large contribution to the Indus river but not to all Asian rivers.World Pictures/Photoshot

Although global warming is expected to shrink glaciers in the Himalayas and other high mountains in Central Asia, the declining ice will have less overall impact on the region's water supplies than previously believed, a study concludes.

It's an important finding, says Richard Armstrong, a climatologist at the US National Snow and Ice Data Center in Boulder, Colorado, who notes that the Intergovernmental Panel on Climate Change had previously predicted dire restrictions on water supplies in Asia. "There clearly were some misunderstandings," he says.

The researchers behind the latest study began by calculating the importance of meltwater in the overall hydrology of five rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow River and the Yangtze in China1. The authors found that meltwater is most important to the Indus, with a contribution roughly 1.5 times that from lowland rains. In the Brahmaputra, meltwater flow is equivalent to only one-quarter of the volume supplied by lowland rainfall, and, in the other rivers, it forms no more than one-tenth of the input.

Furthermore, the study found that in the Indus and Ganges basins, glacial ice contributes only about 40% of the total meltwater, with the rest coming from seasonal snows. In the other three rivers its contribution is even lower.

High and dry?

That's important, says Walter Immerzeel, a hydrologist at FutureWater in Wageningen, The Netherlands, and lead author of the study1, because Asian rivers are fed by three sources: rain, snow melt and melting glaciers.

The first two are driven by current weather patterns, because rains fall either as water or as snow that will later melt. The last is a carry-over from the build-up of glaciers in prior centuries. As the glaciers shrink, their contribution will also decline until the glaciers have either melted entirely, or stabilized at smaller sizes.

“The glaciers are tiny, compared with the monsoon.”


Climate change will therefore have two effects, Immerzeel says. One will be to reduce the contribution of glaciers to total run-off. The other will be to change weather patterns, including rain and snowfall. Combining these and looking at averages from five climate models, Immerzeel and colleagues concluded that the change in upstream water inputs will range from a decrease of 19.6% for the Brahmaputra to a 9.5% increase for the Yellow River. The latter, he notes, is due to increased winter rains. "The Yellow River depends only marginally on meltwater," he says, "and, on average, the models project an increase in winter precipitation in the Yellow River basin."

What this means, Armstrong says, is that river flows are dominated by seasonal rains. "The glaciers are tiny, compared with the monsoon," he says.

Nevertheless, the study concludes that climate change will reduce water supplies enough that by 2050, declines in irrigation water are likely to reduce the number of people the region's agriculture can support by about 60 million — 4.5% of the region's present population.

Model uncertainty

One caveat, Immerzeel notes, is that climate models don't fare well at simulating the effect of warming on Asian rainfall. "There's still a lot of research going into the effect of climate change on the behaviour of the monsoon," he says.

ADVERTISEMENT

Further refinements will also come from improved mapping of the area's glaciers, something that Armstrong's team has recently started, using remote-sensing data from satellites. That's an important next step, he says, although he adds, "I don't think we'll have a substantially different result."

The findings are important for policy-makers, says Jeffrey Kargel, a glaciologist at the University of Arizona in Tucson. "This paper adds to mounting evidence that the Indus Basin [between India and Pakistan] is particularly vulnerable to climate change," says Kargel. "This is a matter that obviously concerns India and Pakistan very much."

"The two nations must talk to one another and see that it is in their mutual best interests to arrive at an equitable means of sharing and utilizing water," he adds. 

  • References

    1. Immerzeel, W. W., van Beek, L. P. H. & Bierkens, M. F. P. Science 328, 1382-1385 (2010). | Article | ChemPort |
Commenting is now closed.