Declining snow packs put many nations' water supplies at risk
hemisphere, combining multiple climate models with present water-use patterns and demographics. From these data, they identified ninety-seven basins currently serving some two billion people that depend on snowmelt, and run at least a two-thirds chance of declines, given present water demands.
The researchers further zeroed in on thirty-two of those basins, with the largest populations — collectively nearly 1.45 billion people — where present snowmelt meets a substantial proportion of human demand. These are the most sensitive to any changes, and have the most at stake. Among them are the basins of northern and central California, where much of U.S. produce is grown; the basins of the Colorado and Rio Grande rivers, which serve much of the American West and northern Mexico; the Atlas basin of Morocco; the Ebro-Duero basin, which feeds water to Portugal and much of Spain and southern France; and a series of basins covering eastern Italy, the southern Balkans, several Caucasus nations, and northern Turkey.
Also on the list is the volatile Shatt al Arab basin, which channels meltwater from the Zagros Mountains to Iraq, Syria, eastern Turkey, northern Saudi Arabia, and eastern Iran. Studies suggest that warming climate has already decreased rainfall and increased evaporation across much of the eastern Mediterranean and Mideast, and a study earlier this year suggested that the spreading civil war now engulfing the region was sparked in part by a record 2006-10 drought.
The study concentrates only on human water supplies, but lack of snowpack may have wider ecological consequences. In the American West, land managers fear that forest fires undamped by snow might start earlier in the year, and food sources for nesting birds might start running out in late summer. Native trout depend on a steady flow of cold water during summer, so some species could face extinction.
The Earth Institute notes that the news is not uniformly bad. Across most of North America, northern Europe, Russia, China and southeast Asia, rainfall alone is projected to keep meeting human demand. Many changeable factors can affect water supply and demand, so the projections contain large uncertainties; in large regions, the models show that changing climate is about as likely to increase water supply as to decrease it, in the form of greater seasonal rains. All things considered, supplies could stay about the same in India’s Indus and Ganges basins, home to about one billion people. This is true also of the Huai, a sub-basin of China’s Yellow River, with a current population of more than 130 million. And at least temporarily, accelerated melting of glaciers in the Himalayas and outlying ranges may actually increase water supplies to wide areas, including central Asian countries like Uzbekistan and Kazakhstan.
“Managers need to be prepared for the possibility of multi-decadal decreases in snow water supply,” said Mankin. “But at the same time, they could have large multi-decadal increases. Both of those outcomes are entirely consistent with a world with global warming.”
Yoshihide Wada, a researcher at the Netherlands’ Utrecht University who studies global water resources, said the study makes a “convincing argument” that “climate change will add another stressor over many regions” where water scarcity is already an issue. He said this would be especially so in semi-arid areas where irrigation is supplemented by shrinking supplies of groundwater. If anything, the study could underestimate some risks, he suggests, because it does not account for projected increases in population.
What Mankin calls the “irreducible near-term uncertainties” in many projections will leave water managers faced with whether to make big infrastructure investments or not. Some areas may be able to compensate for shrinking snowmelt by expanding storage reservoirs or pumping more groundwater. But Wada says reservoirs are so expensive, many areas may not be able to afford them. And groundwater has limits; in parts of California, farmers are pumping up so much water, the land surface is visibly sinking — a trend that probably cannot be maintained for more than a few decades. Wada says more efficient irrigation methods, water recycling, and switches to less water-intensive crops may be more realistic solutions.
— Read more in Justin S Mankin et al., “The potential for snow to supply human water demand in the present and future,” Environmental Research Letters 10, no. 11 (12 November 2015): 114016 (DOI: 10.1088/1748-9326/10/11/114016)
