The gradual melting of winter snow helps deliver water to both farmland and cities, but new research suggests declines in snowpacks will severely deplete water supply to many regions of the globe.

A team of scientists looked at snow-dependent drainage basins in the northern hemisphere, which currently provide water to over two billion people, the Earth Institute at Columbia University reported. Snowpack water is an important resource for people across American West, southern Europe, the Mideast and central Asia. Water from snowpacks is especially relied on in mountainous regions, where the snowmelt slowly runs down the mountain during the growing seasons. Global warming appears to be disrupting this process, causing more winter precipitation to fall as rain instead of snow, washing away immediately.

"Snow is important because it forms its own reservoir. But the consequences of reduced snowpack are not the same for all places--it is also a function of where and when people demand water," said lead author Justin Mankin, a postdoctoral fellow at Columbia University's Earth Institute based jointly at the institute's Lamont-Doherty Earth Observatory and its affiliated NASA Goddard Institute for Space Studies. "Water managers in a lot of places may need to prepare for a world where the snow reservoir no longer exists."

As the world has been warming, once-permanent snowfields have been vanishing in the Rocky Mountains from Colorado to northern Montana and even the Himalayas. Snowpack in California has reached its lowest point in about 500 years as a result of the devastating drought.

To make their findings, the researchers looked at 421 drainage basins spanning the northern hemisphere and combined this data with multiple climate models. The team identified 97 basins serving about two billion people that rely on snowmelt and have at least a two-thirds chance of declines. The most sensitive and heavily relied on basins were found to exist in: northern and central California, which is a major provider of U.S. produce; the basins of the Colorado and Rio Grande rivers, which provide for the American west and Mexico; the Atlas basin of Morocco; the Ebro-Duero basin, which provides water for Portugal, Spain, and France; and a series of basins across Italy and Turkey.

The researchers noted across most of North America, northern Europe, Russia, China and southeast Asia, rainfall is expected to continue to meet human demands for the foreseeable future, but reduced snowpacks could lead to forest fires and loss of valuable ecosystems including bird nesting habitats. Accelerated melting of glaciers in the Himalayas could also cause increases in water supplies to regions such as Uzbekistan and Kazakhstan.

"Managers need to be prepared for the possibility of multi-decadal decreases in snow water supply," Mankin said. "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."

The findings were published in a recent edition of the journal Environmental Research Letters.

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