Global warming's impact on Asia's rivers overblown

(Nature 10 June 2010 Richard A. Lovett)

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.

That's important, says Walter Immerzeel, a hydrologist at FutureWater in Wageningen, The Netherlands, and lead author of the study, 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. 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.

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