Scientific Method —

Himalayan water supply gets an improved outlook

Glacial melt unlikely to cause water availability concerns this century.

Meltwater pond on Lirung Glacier.
Evan Miles

Glaciers are very sensitive to changes in climate, making them clear (even visually striking) indicators of the recent warming trend. Apart from raising sea levels and threatening to turn the name of Glacier National Park in the US into a sad irony, the loss of glacial ice can also endanger water availability.

The seasonal melting of mountain glaciers (and snowpack) provides an important source of water in most places where they can be found. The Indus, Brahmaputra, and Ganges Rivers in India, and the Yellow and Yangtze Rivers in China, are particularly noteworthy examples of rivers with headwaters that are fed by glacial melt high in the Himalayas.

Those rivers supply water to a staggeringly large number of people, so the impact of climate change on Himalayan glaciers has understandably been the focus of study. As a glacier shrinks, the amount of meltwater it produces increases for a while before the diminishing volume of ice wins out, causing melt to dwindle.

A 2010 study, for example, showed reason for concern, indicating meltwater reductions within just a few decades—although increasing precipitation minimized the impact on the Ganges, Yellow, and Yangtze Rivers. Two of the researchers behind that study have attacked the question again with updated tools. This time they got a different answer, which could be good news for the many people who rely on those rivers.

The new study relies on the latest generation of climate models and a much more sophisticated model of the glaciers themselves. In order to model the glaciers with more confidence, they had to zoom in and focus on small areas with just a few glaciers. They picked two locations with different characteristics, one in the Indus River watershed, and one in the Ganges River watershed.

A number of climate models from the Coupled Model Intercomparison Project (the ones utilized by IPCC reports) were used to simulate the rest of this century for a range of emissions scenarios. The simulated temperature and precipitation trends were then used to drive a model of the glaciers themselves, along with the rest of the local hydrological cycle—including precipitation runoff into streams or infiltration into groundwater. The primary focus was the amount of surface water flowing out of the model, which would eventually make its way into the Indus or Ganges Rivers. Would it dwindle in the near future?

Contrary to their earlier results, the researchers found no decrease in total water supplied to either of the rivers before the end of the 21st century. The glaciers retreated plenty (up to 60 percent of ice volume was lost by 2100), but the amount of glacial meltwater didn’t peak until roughly 2060 or so, after which it began a slow decline. Increasing precipitation, however, made up for some of that drop, preventing the total contribution to the rivers from decreasing.

The greater sophistication of their glacial model is partly responsible for the change in results, as it suggested the glaciers would not be melting as quickly, postponing the peak in meltwater. The other factor was the latest generation of climate models, which are projecting greater increases in precipitation for this region. Not only does that additional precipitation result in more water flowing downstream, it also slows glacial retreat in the same way that a pay raise bolsters an otherwise-troubled bank account. (In other words, it dumped more snow on the retreating glaciers.) The precipitation change also carries the biggest uncertainty, however, as the latest climate models are giving a wide range of projections in this region. Still, it appears that worrisome reductions in water availability are less likely to come to pass, at least for many years.

While the study only attempts to characterize the Indus and Ganges Rivers, there’s a good chance that its conclusions could extend to the Yellow and Yangtze Rivers as well. Although glacial melt provides a smaller portion of the rivers’ flow, precipitation is projected to increase over those glaciers, as well. Walter Immerzeel, a researcher at Utrecht University in the Netherlands involved in this study, told Ars that “a conclusive answer would require a similar modeling approach as [our study], as the runoff change also depends on intra-annual changes in precipitation and temperature and on changes in evapotranspiration.”

Increasing precipitation (especially monsoon rains) could present problems in other ways, but it looks like it will help avert water availability problems in a region where demand is increasing.

Nature Geoscience, 2013. DOI: 10.1038/NGEO1896  (About DOIs).

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