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Hydrological modeling suggests that by 2100 more than 65 percent of the world’s population might, at least sporadically, lack access to clean water.

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It falls from the sky and, in some parts of the world, runs freely from faucets. But accessing clean water is an issue for a significant swath of the population, and the situation is only apt to worsen in coming decades, new modeling work reveals. About 55 percent of the world’s population currently has trouble accessing clean water at least one month out of the year, and by 2100, that number could rise above 65 percent, researchers calculated. Minimizing water scarcity now and into the future will rely on curbing water use, reducing pollution, and mitigating the effects of climate change, the researchers suggest.

From agriculture to manufacturing to cooking and drinking, human existence is inextricably linked to the availability of clean water. That need is reflected in the United Nations’ Sustainable Development Goal 6, one of 17 goals representing targets for global development.

But reliably accessing clean water is, in many parts of the world, a dance: Water availability and demand must be synced not only geographically but also in time, said Edward Jones, a hydrology and water quality modeler at Utrecht University in the Netherlands. “There are strong seasonal variations in availability and quality and, to some extent, demand.”

Quantity and Quality

Jones and his colleagues used a hydrological model to estimate water scarcity worldwide through the year 2100.

One innovative aspect of their research was considering not only water quantity (the traditional focus of water scarcity studies) but also water quality. “We’re going beyond the more traditional look,” Jones said.

“We’re going beyond the more traditional look.”

While not having enough water is an obvious problem — “If we turn on our tap and water doesn’t run, that’s very visible,” Jones said — the issue of water quality is equally important. “It’s always been the invisible brother of water availability,” he continued. “It’s past time to also consider water quality.”

To assess water quantity and quality, Jones and his colleagues considered five global climate models. The outputs of those models — air temperatures, precipitation, and evapotranspiration — in turn fed into a hydrological model that simulated the water cycle and how water moves between surface and subsurface reservoirs. It was important to analyze multiple climate models because there’s a fair bit of uncertainty in each, Jones said. “We try to show the range of what could happen.”

The hydrological model that the team used parameterized water demands across domestic, industrial, livestock, and irrigation sectors. The researchers also relied on a separate surface water quality model that took into account potential water contaminants caused by anthropogenic activity such as agricultural runoff and improper wastewater management. Those contaminants included salts, organic pollution, and bacterial pathogens.

Allowable contaminant levels were permitted to vary depending on how the water was being used, Jones said. “We consider different thresholds associated with the different sectors.” Water bound for domestic use was required to have the lowest levels of contaminants, and water used for irrigation was allowed to be the most contaminated.

To investigate how water scarcity would evolve over time, Jones and his colleagues considered three scenarios combining Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP). These scenarios describe not only the environmental changes in air temperatures, precipitation, and evapotranspiration associated with a changing climate but also societal shifts such as population growth, urbanization, and technological and economic development. The researchers considered monthly outputs from their modeling in roughly 10- × 10-kilometer grid cells.

The Importance of Efficiency

Researchers were surprised to find that the worst-case scenario they considered — RCP 8.5 and SSP 5 — didn’t result in the largest number of people being exposed to clean water scarcity. The researchers attributed that finding to the increased economic development and heightened water use efficiency built into that particular SSP. The scenario defined by RCP 7.0 and SSP 3 yielded the largest population experiencing clean water scarcity, the team reported in Nature Climate Change.

When interpreting findings like these, it’s important to remember their context, said Bridget Scanlon, a hydrologist at the University of Texas at Austin not involved in the research. For instance, the authors’ choice to parameterize clean water scarcity as lacking access to clean water for just one month out of a year is rather conservative, Scanlon said. “You can probably manage one-month-a-year scarcity.” Focusing on prolonged water scarcity — triggered by multiyear droughts, for example — might be a more illustrative way of looking at clean water scarcity, she said. However, other studies have also considered water scarcity on the basis of shortages occurring one month out of a year.

“It could be even worse than what they’re presenting.”

This work also considered only a limited range of water contaminants and didn’t include compounds such as arsenic and nitrates, said Scanlon, who hosts the Water Resources Podcast. “They don’t take naturally occurring contaminants into account.” That omission could have biased the team’s results to be low, she said. “It could be even worse than what they’re presenting.”

Minimizing clean water scarcity going forward will require a concerted effort, Jones and his colleagues acknowledged. There’s a trifecta of challenges to tackle, and each is nothing short of a major undertaking: limiting climate change, reducing water use, and minimizing pollution in the environment. “These three aspects are really key,” Jones said.

This story by Katherine Kornei was originally published by Eos Magazine and is part of Covering Climate Now, a global journalism collaboration strengthening coverage of the climate story.

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