World water supply under serious threat from melting winter snow, study
The world’s water supply, which is highly dependent on gradual melting of winter snow, could soon be critically imperiled because of climate change, a team of scientists from the US, Switzerland and the Netherlands has warned. Cities, farms and ecosystems across the world get much of their water supply from melting winter snow.
The scientists, who published their findings in the academic journal “Environmental Research Letters,” identified drainage basins across the northern hemisphere that currently serve 2 billion people and are dependent on melting snow.
They warn that the two billion humans that inhabit these regions face significantly declining supplies in the coming century.
Snowpack at an altitude of 2,700 feet in the Lesser Caucasus mountains of northeastern Turkey, late April 2012. The lowlands below are heavily dependent on seasonal snowmelt, which is projected to decline in this region and others in decades to come, due to global warming. (Image: earth.columbia.edu. Credit: Dario Martin-Benito)
The basins take in large parts of central Asia, the Mideast, southern Europe and the American West. They range from war-torn regions where people already face severe long-term water shortages, to productive US agricultural land.
Snow is a vital seasonal source of water, especially for regions around large mountain chains. Snowmelt runs down from higher elevations into the lowlands during the spring and summer growing seasons – a period when human demand for water peaks.
Snowmelt supply vs. human demand balance disrupted by global warming
This convenient balance is now being upset by global warming. According to previous studies in many areas, more winter precipitation is falling as rain rather than snow, and washing the snow away directly.
Even the snow that settles – at progressively higher altitudes – is melting earlier today than it used to.
In this latest study, the researchers estimated snow’s potential to supply current human needs in today’s and future climates. The scientists factored in weather trends and population when making their estimates.
Predictions of decreased potential for snow to supply spring and summer human water demand by 2050 (reddest areas are highest risk; yellow, lowest). In blue/white areas, future rainfall should be enough to supply present human needs; gray regions represent basins where rainfall might be insufficient, potentially making them dependent on snow or other sources. (Image: earth.columbia.edu).
Lead author, Dr. Justin Mankin, a postdoctoral fellow at Columbia University’s Earth Institute based jointly at the institute’s Lamont-Doherty Earth Observatory and NASA’s Goddard Institute for Space Studies, said:
“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. Water managers in a lot of places may need to prepare for a world where the snow reservoir no longer exists.”
Snow accumulations declining
As our planet gets warmer, researchers have been observing less accumulation of snow in several regions, a trend that is forecast to continue.
Snowfields – that used to be permanent – are disappearing in the Rocky Mountains from Montana to Colorado, as well as in the Himalayas and several other regions.
A recent study demonstrated that in 2015, the snowpack in California, which is experiencing an ongoing severe drought as well as long-term warming, reached its lowest point since Christopher Columbus set foot on the New World.
Dr. Mankin and his colleagues calculated the possible future effects, based on how many humans consume snowmelt-sourced water.
They studied 421 drainage basins across the northern hemisphere, combining multiple climate models with present water-usage patterns and demographics.
After gathering and analyzing these data, they identified 97 basins that currently serve about 2 billion people that depend on snowmelt. They run at least a two-thirds chance of declines, given current water demands.
The team then focused on 32 of those basins more closely – which collectively represent almost 1.45 billion people – where current snowmelt meets a considerable portion of human demand.
1.45bn people live in regions very susceptible to climate change
These regions are the most sensitive to any changes, and have the most to lose. Among them are basins of central and northern California, where much of US food is grown; the basin of the Rio Grande and Colorado rivers, which serve a large portion of the American West and northern Mexico; Morocco’s Atlas mountains; the Ebro-Duero basin, where water is consumed by millions of people in Portugal, Spain and southern France; several basins in eastern Italy, some Caucasus nations, northern Turkey, and southern Balkan states.
The list also includes the volatile Shatt al Arab basin, where meltwater from the Zagros Mountains supply millions of people in Iraq, Syria, eastern Iran, northern Saudi Arabia, and eastern Turkey.
Studies suggest that global warming has already reduced rainfall and raised evaporation rates across large parts of the eastern Mediterranean and Mideast. A study earlier this year suggested that the civil war that currently engulfs the region was partly triggered by a record 2006-2010 drought.
This study only focused on human water supplies. However, lack of snowpack may have consequences that affect ecosystems. Land managers in the American west believe forest fires undamped by snow could start earlier in the year. There is also a greater risk that food sources for nesting birds may become depleted in late summer.
Several species of native trout, which depend on a steady flow of cold water in the summer, could be facing extinction.
Not all bad news
Across most of China, southeast Asia, Russia, northern Europe and most regions of North America, rainfall alone is forecast to continue meeting human demand.
The authors stress that their projections contain large uncertainties, given that several changeable factors can impact water supply as well as demand. In the larger regions, the models show that climate change is about as likely to increase water supply as to reduce it, in the form of greater seasonal downpours.
After taking everything into account, the authors believe that supplies could stay around the same in the Ganges and Indus basins in India, where approximately 1 billion people live.
This is also true of China’s Yellow Rive sub-basin – the Huai – with over 130 million inhabitants.
For a while at least, accelerated glacier melting in the Himalayas and outlying ranges may, in fact, raise water supplies to wide areas, including central Asian nations like Kazakhstan and Uzbekistan.
Dr. Mankin said:
“Managers need to be prepared for the possibility of multi-decadal decreases in snow water supply. 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.”
Water supply in regions alread with shortages will diminish
Dr. Yoshihide Wada, a research scientist at the Department of Physical Geography, Faculty of Geosciences, Utrecht University, the Netherlands, who studies global water resources, said this study makes a ‘convincing argument’ that ‘climate change will add another stressor over many regions,’ where water scarcity is already a problem.
He added that this would be especially so in semi-arid regions where irrigation is supplemented by diminishing supplies of groundwater. In fact, he suggests, if anything, this study may have underestimated some of the risks, because it does not take into account population growth in the future.
Many water managers will wonder whether or when to make big infrastructure investments, in the face of what Dr. Mankin calls the ‘irreducible near-term uncertainties’ in many forecasts.
Some areas may be able to make up for the reduced water coming from snowmelt by pumping more groundwater or expanding storage reservoirs.
However, reservoirs are extremely costly, Dr. Wada says, and many areas across the world will not be able to afford them.
Groundwater is not an infinite resource. In some parts of California, farmers are pumping up water at such a rate that the land surface is visibly sinking – a trend that they will not be able to maintain forever.
Dr. Wada believes that switching to less water-intensive crops and adopting more efficient irrigation methods is the best way forward.
Citation: “The potential for snow to supply human water demand in the present and future,” Justin S Mankin, Daniel Viviroli, Deepti Singh, Arjen Y Hoekstra and Noah S Diffenbaugh. Environmental Research Letters. 12 November, 2015. DOI:10.1088/1748-9326/10/11/114016.