Snowfields once considered permanent are shrinking from the Rocky Mountains to the Himalayas.
Nearly 100 drainage basins across the northern hemisphere that currently supply water to 2 billion people in the American West, southern Europe, the Mideast and central Asia are at considerable risk over the next century.
Dwindling water supplies will not be a universal impact of declining snowpacks due to climate change, however.
Scientists have observed declining snowpack accumulation across the globe, with snowfields that were once considered permanent shrinking from the Rocky Mountains to the Himalayas. Snowpack in California’s Sierra Nevadas was reported earlier this year to be at just 5 percent of the historical average — the lowest it’s been in five centuries.
A number of studies have shown that there will be declines to snowpack across the globe by mid-century thanks to rising global temperatures, but less attention has been paid to how that will impact human water supplies.
A new study published in the journal Environmental Research Letters aims to address this gap in our knowledge by examining how reduced snowmelt runoff will affect different parts of the world.
The results are significant: Nearly 100 drainage basins across the northern hemisphere that currently supply water to 2 billion people in the American West, southern Europe, the Mideast and central Asia were found to be at considerable risk over the next century.
Snow is an important seasonal water source, especially in areas surrounding large mountain chains, where it accumulates at high elevations in the winter. In the spring and summer, snow melts and the runoff works its way down to lowlands just in time to nourish annual crops.
But global warming is disrupting this cycle, according to Justin Mankin, a postdoctoral fellow at Columbia University and lead author of the report. In some regions, more winter precipitation is falling as rain instead of snow, and when snow does fall it’s at progressively higher elevations, where it melts earlier.
Mankin led a team of researchers in the US and Europe who consulted multiple climate models and looked at water usage and demographics data from 421 drainage basins in the northern hemisphere. They identified 97 snow-sensitive basins that have a 67 percent chance of declines in snow supply if emissions of greenhouse gas emissions continue unabated.
“Snow is important because it forms its own reservoir,” Mankin said in a statement. “Water managers in a lot of places may need to prepare for a world where the snow reservoir no longer exists.”
Mankin and team say that 32 of the 97 at-risk basins currently rely on snowmelt to supply a substantial portion of the water required by the 1.45 billion people who live there, among them basins in the American West and Southwest that are collectively responsible for much of the US’s agricultural production and several million people’s drinking water; the Atlas basin of Morocco; the Ebro-Duero basin, which supplies water to Portugal, Spain and southern France; and a number of basins in eastern Italy, the southern Balkans, several Caucasus nations, and northern Turkey.
The Shatt al Arab basin, which channels snowmelt from the Zagros Mountains to areas in Iraq, Syria, Turkey, Saudi Arabia and Iran, is also on the list of the 32 most at-risk basins — which is certainly not welcome news for an already unstable part of the world. A previous study suggested that the Syrian civil war was sparked in part by a record drought that lasted from 2006 to 2010.
The study concentrates only on human water supplies, but the authors add that declining snowpack could have broader ecological consequences, from forest fires starting earlier in the year to food for nesting birds becoming more scarce.
The authors also say that dwindling water supplies will not be a universal impact of declining snowpacks due to climate change. Southeast Asia, northern Europe, much of North America and Russia are all projected to continue receiving enough rainfall to meet human demand, for instance. Meanwhile, India’s Indus and Ganges basins, home to about 1 billion people, will probably continue to receive about the same amount of water.
Some regions, the researchers say, could even see an increase in their water supply, at least temporarily, as accelerated melting of glaciers in the Himalayas provides more water to central Asian countries like Uzbekistan and Kazakhstan.
Given the wide range of possible impacts, Mankin says it’s all the more important for decisionmakers to have a plan for how they’ll ensure water availability in a warming world.
“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.”
- Belmecheri, S., Babst, F., Stahle, D.W., Trouet, V., & Wahl, E.R. (2015). Multi-century evaluation of Sierra Nevada snowpack. Nature Climate Change. doi:10.1038/nclimate2809
- Kelley, C. P., Mohtadi, S., Cane, M. A., Seager, R., & Kushnir, Y. (2015). Climate change in the Fertile Crescent and implications of the recent Syrian drought. Proceedings of the National Academy of Sciences, 112(11), 3241-3246.
- Mankin, J. S., Diffenbaugh, N., Hoekstra, A.Y., Singh, D., Viviroli, D. (2015). The potential for snow to supply human water demand in the present and future. Environmental Research Letters, 10(11).