Markus Andrews

Many people had to be evacuated following flooding caused by the destruction of the Kakhovka dam in Ukraine.Credit: ZUMA Press, Inc./Alamy Stock Photo

Oleksandra Shumilova was hundreds of kilometres away from her native Ukraine when, in early 2022, Russian troops invaded and destroyed a water pipeline near her hometown of Mykolaiv. For 24 days, the taps ran dry. When the water finally returned it was contaminated with salt and harmful chemical deposits that rendered it unsafe for drinking.The incident struck a chord with Shumilova, a freshwater ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin — on multiple levels. Not only was Mykolaiv her hometown, but she studies the floodplains of a river in Italy that still bear the scars of the First World War.Pockmarked by craters from decades-old aerial bombardments, the area around the Italian river is scattered with unexploded military ordnance. When discovered, these munitions must undergo controlled detonations, creating blasts that reverberate in Shumilova’s mind during her fieldwork.“Remnants of war can stay in nature for a long time,” Shumilova says. Now, she fears, history is repeating itself in the Russia–Ukraine war.Shumilova took it upon herself to chronicle what is happening in Ukraine. “This research is very personal,” she says. She began poring over government records and media reports, meticulously searching for instances of damage to water resources and infrastructure, resulting from military actions. She and her collaborators cross-checked their findings with information from a diverse array of Ukrainian, Russian and international sources.

Parts of Kherson, Ukraine, were flooded after the Kakhovka dam was destroyed during the Russia–Ukraine war.Credit: Roman Pilipey/Getty Images

Focusing on just the first three months of the conflict, a detailed record of 64 events emerged, each affecting crucial water facilities and often triggering a chain of events1. Missiles striking hydropower dams, for example, might result in power outages that render water-pumping stations and wastewater treatment facilities inoperable — with devastating consequences. Millions of people can be left without access to clean water, agricultural fields might have insufficient irrigation, and the unchecked flow of untreated sewage and contaminated ground water from industrial mines can pollute nearby river basins, causing significant harm to both people and the environment.These incidents highlight just one aspect of the intricate interplay between water and armed conflicts. Water resources can be casualties of violence, but disputes over water control can also act as triggers for unrest — for example, when two communities clash over access to a single water source. Furthermore, water is frequently weaponized, as has happened in Gaza, when Israel responded to the deadly attacks by Hamas on 7 October by restricting access to fresh water and cutting off fuel shipments needed to run desalination and water-treatment plants for local production of potable water.Historical records show that conflicts over water access stretch back millennia, with water systems often being targeted or manipulated as strategic assets on the battlefield. But this water–war nexus seems to be intensifying. Driven by escalating tensions and intensified hostilities in places such as the Middle East, Ukraine, southern Asia and the Sahel region of sub-Saharan Africa, water resources have increasingly become targets or triggers of violence over the past decade, notes environmental scientist Morgan Shimabuku.“What we do see right now is a really large increase in the total number of water-related conflicts around the globe,” says Shimabuku who is at the non-profit organization the Pacific Institute in Oakland, California, and helped to develop an online database of water-related clashes called the Water Conflict Chronology. There were more than 200 incidents in 2022 alone, and with the ongoing conflicts in Ukraine and Gaza, 2023 is projected to surpass that figure (see ‘Warring over water’).

Source: Adapted from O. Shumilova et al. Nature Sustain. 6, 578–586 (2023)

Add in the mounting stresses on water systems from population growth and climate change, and the potential for violence and instability is only expected to grow. Shimabuku thinks that there is an urgent need for comprehensive and cooperative efforts to safeguard water resources and to promote peace, but she also sees reasons to be hopeful.For example, many previously conflicting factions are embracing collaborative water-sharing treaties, which can de-escalate hostilities. “We have a lot more tools in our toolbox now for addressing these challenges,” Shimabuku points out.Troubled watersIn 1995, the then-vice-president of the World Bank, Ismail Serageldin, made a dire warning for the years ahead: “Many of the wars in this century were about oil, but wars of the next century will be over water.”

People queue to refill drinking water in Mykolaiv, Ukraine, after Russian forces cut off the pipeline used to supply water.Credit: AP Photo/Emilio Morenatti/Alamy Stock Photo

It’s an oft-repeated idea that persists to this day, cited frequently in discussions about water scarcity and conflict. But according to Jampel Dell’Angelo, who studies governance and disputes over freshwater resources at the Vrije University Amsterdam Institute for Environmental Studies in the Netherlands, “this is a myth”. At least when thinking about conflicts between nations.In a globalized economy of countries interconnected by trade, including that of agricultural commodities that can be considered a form of virtual water transfer, tensions over cross-border issues of water scarcity are often diffused by forms of interstate cooperation. This typically prevents water from serving as a trigger for major wars, but it doesn’t stop localized violence from arising in water-strapped nations.In Iraq, for example, large-scale protests over tainted water supplies have been met with strong police crackdowns. Frequent droughts in Somalia have prompted herders to abandon their pastoral lifestyles and join militant groups instead. And neighbouring states in India clash every year over their allocated river waters, with demonstrations that often turn violent.What’s more, the very conditions that promote interstate water peace can actually exacerbate water-related instability at the local level. This is particularly evident in water-scarce low-income countries, which frequently export their limited natural resources, amplifying their local water challenges and potentially fuelling smaller-scale conflicts. “There’s a shift in the burden of water scarcity on the heads of people who already suffer from the lack of water,” Dell’Angelo says.Further compounding the problem, foreign agribusiness investors have increasingly acquired vast land holdings in low- and middle-income countries, often at the expense of local communities and appropriating valuable water resources in the process. Termed land or water grabbing, this phenomenon gained momentum after the economic and food crises of 2008 — and, according to Dell’Angelo, the ongoing war in Ukraine is likely to further intensify this trend, owing to reductions in Ukrainian grain exports and heightened concerns about food security. “This will put additional pressure on land and water,” Dell’Angelo says, “with escalating consequences.”One priority of water-security researchers is to prevent similar crises in the future. “We want to put points on a map and show people where water conflict is happening so that we can better apply solutions,” says Samantha Kuzma, a data scientist at the World Resources Institute, an environmental policy and research group in Washington DC.Those efforts begin with prediction. To identify problem hotspots, Kuzma and her colleagues developed an artificial intelligence algorithm called the Global Early Warning Tool. This tool considers a wide range of environmental, economic and social factors. By comparing past trends to present data, it flags regions where water-related issues such as flooding, pollution and scarcity could spell trouble2.Kuzma hopes that the tool will help to inspire localized conflict-mitigation measures when and where they are needed. This could prove instrumental in drought-affected areas such as Ethiopia, where restricted access to potable water might escalate tensions between government forces and armed factions, or in water-scarce regions of Syria that are already marred by conflicts between rival militia groups.Although implementation can be challenging, Kuzma emphasizes that the timely identification of emerging conflicts can streamline the adoption of collective land-use strategies that will ultimately ease tensions. “If we can see water as the security risk that it is — and have more investments going into securing the resource, and managing it sustainably — we should see real impacts on the ground,” she says.Shifting baselinesThe world’s rapidly changing climate could intensify the need for those kinds of water-security intervention. As a team led by Solomon Hsiang, now chief environmental economist at the White House Office of Science and Technology Policy in Washington DC, showed in an influential 2013 analysis, extreme rainfall conditions can be causally linked to an increased risk of violence and civil war3.

Displaced people in Gaza fetch drinking water outside a school.Credit: MAHMUD HAMS/AFP via Getty Images

Such was the case in Syria, where discontent over the government’s handling of a prolonged drought period contributed to uprisings in 2011. This spiralled into a devastating internal war that persists to this day.Yet, perhaps counter-intuitively, as climate extremes become less anomalous and people become accustomed to more frequent water disasters, it’s possible that the number of water-related conflicts will decline, although the severity of such clashes could intensify.That’s what Marc Müller, a water engineer at the Swiss Federal Institute for Aquatic Science and Technology in Dübendorf, and his colleagues found when they modelled the relationship between climate variability and conflict. Because the reference point for ‘normal’ levels of water availability changes, they concluded, so will the threshold at which people are willing to fight4.Water-related conflicts could therefore become fewer and further between. But when they do arise over ever-shrinking water supplies, Müller warns, the hostilities are likely to escalate quickly. “Conflicts will be less often, but worse,” he says.Such predictions offer little consolation to the people of Gaza, who continue to endure shortages of clean water and a sanitation crisis that is facilitating the spread of waterborne diseases.Water challenges are nothing new in the region. Even before the current conflict, extreme overuse of groundwater resources, a lack of large-scale desalination plants and the discharge of mostly untreated sewage all contributed to water scarcities in Gaza. But the situation is now much worse — and with the world’s attention focused on the water crisis, some researchers are hopeful that more will be done to address the problem once the war ends.“There’s going to be a lot of rebuilding necessary after all the destruction,” says David Katz, a water-policy researcher at the University of Haifa, Israel, “and maybe investing in water infrastructure will be something that can galvanize the global community.”As Katz points out, water cooperation helped to pave the way for a peace agreement between Israel and Jordan nearly 30 years ago, and he’s hopeful that the same might be possible in any future détente between the Israeli and Palestinian governments. “Political change opens up new opportunities,” he says. “It could conceivably happen.”Meanwhile, in Ukraine, the dire impact of Russia’s assaults on the nation’s water infrastructure is consistently and relentlessly worsening. This is starkly evident in the catastrophic destruction of the Kakhovka dam and hydroelectric power plant on 6 June 2023. The massive concrete structure crumbled. Trillions of litres of water gushed out, deluging areas downstream. At least 50 people died and thousands lost their homes.In the intervening months, the extent of the damage to communities, agriculture and the environment has become apparent. The Black Sea, into which the flood waters and accompanying sediment poured, became polluted, with a potentially devastating impact on the local marine ecosystem. Vast tracts of flooded land are no longer amenable to farming, crippling the Ukrainian economy, and areas that remain arable will probably have reduced yields without the reservoir there to provide a steady stream of water for irrigation.This could spell trouble for global food supplies, given the dependence by many low-income countries on Ukrainian grain. “It will touch millions of people, not only those living in Ukraine,” says Viktor Vyshnevskyi, a hydrologist at the National Aviation University in Kyiv, who has studied the myriad water-related repercussions of the dam explosion5.This human-caused flood was not without precedent. During the Second World War, in August 1941, the retreating Soviet army destroyed a dam at the northern end of the same reservoir where the Kakhovka dam was later constructed. The goal was to slow the advance of German forces. But the obliteration of the dam resulted in massive downstream flooding that reportedly claimed the lives of thousands of people. The occupying German forces partially restored the hydrotechnical structure, before they too demolished it as the tide of the war shifted back in favour of the Soviets.With history as a sobering backdrop — in Ukraine and beyond — the persistent use of water as both a casualty and a weapon of war remains a looming threat in a world grappling with the chaos of climate change. However, by shedding light on this issue, water scientists aspire to catalyse efforts that can avert future conflicts and ensure the protection of this invaluable resource for generations to come.“We want to provide a resource for other researchers, policy makers and those who can enact solutions around how to prevent violent conflict related to water,” says Shimabuku.“Could there possibly be water-resource management approaches or governance structures that can be put in place to reduce the potential harm that these conflicts drive?” she asks. In a world in which water is both a source of life and strife, the choice between conflict and cooperation will determine our shared future. More

Illustration: Sam Falconer

There is an old joke, made famous by the writer David Foster Wallace, in which one fish says to another, “How’s the water?” The second fish replies: “What the hell is water?”That’s more or less the question Nature faced when putting together this collection of articles on the ubiquitous substance on which all life on Earth (not just that of fish) depends. Any attempt to cover the full spectrum of scientific and social issues associated with water is surely doomed to neglect something of importance. But Nature likes a challenge. This supplement is a compendium of intersecting stories that showcase how water affects the sustainability of healthy human civilization.For millennia, the most primal concern about water has been having enough of it. Some argue that too much attention is paid to water supply, and that the real priority should be making do with what is available. Indeed, progress on conservation and efficiency has been impressive — the US economy now needs much less water per dollar of output than in previous decades.But conservation can go only so far. Rivers, wells and artificial reservoirs provide ample supplies for much of the world, but arid regions still struggle. Some researchers in these water-starved regions are turning their attention to the wet sponge that is the planet’s atmosphere. New technologies could extract clean fresh water from thin air, and sharply reduce water scarcity.No matter how abundant the supply, of course, water intended for drinking also needs to be clean and free of contaminants. Among the most pernicious are the ‘forever’ chemicals known as PFAS: chains of carbon and fluorine atoms held together by some of the strongest chemical bonds in nature, and impervious to most attempts to break them down. But engineers are devising various methods to crack them apart and purify PFAS-contaminated water.Although water sustains life, it can also be a threat. Flooding can ravage communities. In a live webcast earlier this month, specialists shared their latest thinking about flood resilience. They painted an alarming picture of the way floods disproportionately affect the world’s poorest populations, even in rich countries, such as the United States. Indeed, 1.8 billion people (22% of world’s population) live in areas at risk of severe flooding — and almost 90% of them are in low- and middle-income countries. Some researchers say that much of the infrastructure put in place to tame waterways is proving inadequate, or even counterproductive. They advocate rethinking how water is handled in the built environment, including re-establishing the abandoned practices of ancient cultures.The danger posed by water is of course increased by climate change. Minimizing its effects will require a large-scale push towards renewable power sources, but these tend to be intermittent, and therefore dependent on technologies to store and transport energy. One leading candidate for renewable power is hydrogen, which can be formed by electrolysing water. However, there is an inherent tension: an economy dependent on hydrogen energy will inevitably consume vast quantities of water. Reducing hydrogen’s water footprint is an important focus as renewable sources become a bigger part of the energy picture.Finally, because of water’s central role in life, it is also a major component of many human conflicts. Wars have been fought over water access. Armies have wielded water as a weapon. But most commonly water is a casualty of war, as has been seen with the destruction of water infrastructure in the Russia–Ukraine war. Those who follow these events closely think that there is an urgent need to identify likely areas of water conflict, and promote greater collaboration between nations on water resources. We all need water to live — not just fish.We are pleased to acknowledge the financial support of the FII Institute in producing this Outlook and the associated webcast. As always, Nature retains sole responsibility for all editorial content. More

A break in the levee holding back the Mokelumne River, California, resulted in the flooding of a farm built on the river’s floodplain.Credit: Erica Gies

When water inundated parts of New York City in September 2023, 28 people had to be rescued from their cars and basement apartments. Thankfully, no one died this time. In 2021, flooding in New York killed 11 people. Neighbourhoods in the city also flooded in 2020 and 2022, and it’s not just New York. Floods are becoming increasingly frequent and severe globally, as are droughts. Steve Bowen, chief science officer for reinsurance firm Gallagher Re in London, described the most recent New York floods on X (formerly Twitter) as “the latest example of ageing infrastructure built for a climate that no longer exists”. Such sentiments are common, and frequently followed by calls for more infrastructure: bigger levees and seawalls, larger pipes and stormwater tanks, and more dams, aqueducts and desalination plants.But human-built infrastructure and land-development practices that leave little space for water are actually a big part of the problem. Eric Sanderson, a conservation ecologist and author of Mannahatta: A Natural History of New York City (2009), called this out pithily in a series of posts on X. He captioned a video of water pouring into a subway stop: “Under former salt marsh” and one of a flooded area in Brooklyn, “Former bog”.Engineered structures intended to control water, urban sprawl and industrial forestry and agriculture have drastically altered the natural water cycle, contributing to both increased flooding and water scarcity. Society has dammed and diverted two-thirds of the world’s large rivers, drained as much of 87% of global wetlands and degraded 75% of Earth’s land area. “We need to let nature play its original function,” says Adnan Rajib, an engineer and director of the H2I lab at the University of Texas at Arlington. “Water doesn’t have anywhere to go.”Climate change is also a factor in today’s water extremes, scientists agree, but blunting the impact of floods and droughts will take more than reducing carbon emissions. Decision makers must also change how they manage land and water. “The climate crisis is real,” says Kris Johnson, a conservation biologist and director of agriculture for The Nature Conservancy, an environmental organization in Minneapolis, Minnesota. “But the biodiversity and water crises are also real and interdependent.” Engineered solutions to water problems — such as levees or dams — typically overlook the complex relationships between water, rocks, soil, plants, animals and atmosphere. Failing to account for that complexity often damages the natural systems that support life and the water cycle, contributing to increased flooding and water scarcity.

Most of the traditional eris tanks in Chennai, India, are now only associated with temples.Credit: Erica Gies

Around the world, scientists, farmers, urban planners, landscape architects, and water utilities and flood managers are taking heed and restoring wetlands, floodplains and forests that development has disrupted. Their efforts, which return space to water where it naturally slows or stalls on land, are unique to each place’s geology, ecology and culture. ‘Slow water’ projects work with natural systems rather than trying to control them and they are socially just. They are distributed across the landscape, not centralized, and make the most of local water.Slow it downFloodplains are one phase of slow water that are prone to human disruption. They hold and release water, redistribute sediment and generate food for aquatic life. But around the world, engineers and farmers have built levees along rivers, cutting them off from their floodplains. “Everyone is doing research on how floods impact humans,” says Rajib. However, he adds, “it’s also the humans that are causing the floods”.

Eris tanks were traditionally used to slow the flow of water from the mountains.Credit: Erica Gies

In a study published in July1, Rajib and his colleagues found that, from 1992 to 2019, humans have encroached on 600,000 square kilometres of floodplains — an area about the size of Ukraine. In taking space from water, such development causes rivers to rise and places people living nearby at higher risk of flooding.Reducing that risk requires the engineered infrastructure installed by humans to be altered or undone. Hydrologist Nicholas Pinter at the University of California, Davis, studies how some communities reduce their risk. Along smaller rivers in the Sacramento Valley, California, non-governmental environmental organizations have returned floodplain space to rivers. Pinter says that during the numerous atmospheric river storms of winter 2022–23, “the only portion where the levees broke were where they didn’t set them back”.Sprawling citiesMany cities around the world are built on floodplains, covered-over streams and filled-in wetlands. Urban areas have doubled since 1992, exacerbating flood risk — for every 1% increase in paved area, annual flood magnitude in nearby rivers increases by 3.3% from run-off. When cities flood, municipal leaders attempt to disperse the water as fast as possible, rather than retain it for dry seasons. Then when water is in short supply, they drill deeper wells to reach ground water, bring in distant water through aqueducts or desalinate seawater to meet the needs of the community.In the wake of increasingly frequent and expensive disasters, some cities are changing tack and making places for water to soak into the ground again. These include stormwater ditches lined with native plants, permeable pavement, green roofs, planted medians, tree wells, and parks on reclaimed industrial areas in river floodplains. These strategies go by different names: low-impact design in the United States, for example, and sponge cities in China — where creating them is a national policy.Chennai, India, is one place that is returning space to water. Nearly every summer, the city runs out of water. The painful irony, however, is that the annual monsoon brings 1.5 times the water that Chennai’s residents use each year. Flooding is also frequent, and starts soon after even moderate rains. The city’s area is now nine times larger than it was in 1980, hemming in three rivers, as well as covering over backwaters, coastal estuaries, mangrove forests and ancient human-built lakes. In 2015, a disastrous flood killed at least 470 people and pushed city managers to alter their course.Today, Chennai deploys slow-water techniques across the city, including protecting remaining wetlands, restoring them where possible and reviving the region’s 2,000-year-old water infiltration system that was once used to provide water year-round. Made up of structures called eris, the system ran from the mountains down to the Bay of Bengal. Eris (Tamil for tanks) are open on the higher side to catch water flowing downhill and closed by an earthen wall on the lower side. A divet in the wall on the low side allows water to flow downhill to the next eri. By slowing the flow of water, the eris reduce flooding, prevent soil erosion and give water time to seep underground — where it is filtered and kept within reach of wells.Despite being impressed by the estimated 53,000 or so eris across southern India, the British introduced centralized management in the nineteenth century, destroying the communal system by which local people maintained their eris and shared water resources. The eris that remain in cities are often connected with temples. Chennai is home to 54 temple eris, and water managers are restoring pathways for storm water to flow to them — and to link them with remaining and restored natural water bodies. The managers expect this to reduce both flooding and scarcity by absorbing and storing local rain.The eris system is unique to southern India. But as Yu Kongjian, co-founder of landscape architecture firm Turenscape, Beijing, and proponent of sponge cities in China, says in the Chinese edition of Water Always Wins (2023), “Each nation has a ‘slow water’ cultural heritage.” Part of the solution in a given place is to include the strategies of earlier inhabitants to work with natural systems to manage water.Forests and farmsSlow-water approaches can also reduce fire severity. Canadian wildfires burnt almost 19 million hectares during the summer of 2023, choking cities across North America. Climate change and misguided policies of stamping out all fires have played a large part in extreme blazes, but commercial forestry shares some of the blame because of how it alters the natural water cycle.

A levee was broken for the Cosumnes River near Sacramento, California, to allow water from the river access to part of the floodplain.Credit: Erica Gies

Tree roots create pathways for water to move underground, storing rainfall locally. The ground water that trees transpire into the air forms clouds and, along with evaporation from soil, becomes the source of 10–80% of rain over continents, depending on location2. Losing forests can, therefore, increase run-off and decrease rainfall.Francina Dominguez, a hydroclimatologist at the University of Illinois Urbana–Champaign, has found another way that deforestation reduces rain. The surface roughness of mixed-species forests makes them better than tree plantations or crops at slowing wind, and thereby makes it more likely that vapour will condense into rain3.Natural forests are much more efficient at regulating water and climate than are commercially logged forests. A mature native forest transpires more water than younger tree plantations, and it contains understorey plants, rich soil and decomposing wood that create a spongy, moist environment. Clear-cutting desiccates this system. Anastassia Makarieva, an atmospheric physicist at the Technical University of Munich in Germany, says that for greater water-cycle stability, remaining old growth should be conserved and some altered areas restored. This should start at the edge of wetter areas, she says, to cumulatively increase water-vapour density and restart the local rain cycle.Replacing perennial vegetation, either forests or grasslands, with annual crops also reduces the amount of evaporation and transpiration, says Dominguez. Agriculture changes the water cycle in more obvious ways too, such as accounting for 70% of water use, and, in wetter places, such as the US Midwest, through draining of swamps by farmers to create crop land.Other standard agricultural practices tend to work against a sustainable water cycle. The higher the percentage of organic matter in soil, the more water it can hold, and the better it can absorb flood water and retain the water until plants need it, reducing the need for irrigation. But ploughing dries out and compacts soil, and pesticide treatments kill animals that help to keep water and biochemical cycles healthy.Returning some marginal cropland — land with degraded agricultural value — to wetland or grassland “could actually reduce the flood peak for the system overall”, says Johnson, who is a co-author on Rajib’s floodplain encroachment study. That wouldn’t have to mean a reduction in the quantity of food produced. Globally, people are pulling back from marginal farmland, leaving that land available for restoration. Some agricultural lands that flood routinely should be fallowed, says Johnson, rather than insured against flood damage. “We want to make sure that we’re not incentivizing behaviour that is likely to fail.” In places such as California and the Netherlands, some floodplains have been partially returned to rivers as relief valves for high flows. Farmers who grow on the land are compensated when they lose their crops.In drought-prone areas, agricultural and urban expansion, and unsustainable groundwater pumping are exacerbating water scarcity, says Johnson. Shifting thirsty crops away from water-stressed places makes sense, he says. California, for example, has introduced a funded programme that could take as much as 400,000 hectares out of use by 2040, because agriculture in the area has expanded beyond what the available water can support.A draining experienceSome water stress is caused by what biologist and hydrologist Brock Dolman calls the “age of drainage”. According to Dolman, who is co-founder of the non-profit organization Occidental Arts & Ecology Center in California, European settlers and their descendants dried out land by killing beavers that created wetlands across 10% of North America, overgrazing animals they brought with them, and overpumping ground water so that plant roots could no longer reach it. But various efforts are starting to turn that around, including supporting the recovery of beaver populations.

Compared with Rock Creek (left), which has no natural infrastructure, the slow-water approach at Turkey creek (right) extends water availability into the dry season.Credit: Laura Norman

Where beavers aren’t present, land managers are also attempting to slow water in degraded streams. When Valer Clark and Josiah Austin moved to their ranch south of Tucson, Arizona, in the 1980s, they found a land denuded of trees and grazed to the bone. Monsoon rains roared through stream channels, called washes, eroding them. The water then quickly disappeared. Clark and Austin hand-built small rock dams in the headwaters of the often-dry Turkey Creek, following local Indigenous methods. Within a few monsoon seasons, the structures caught sediment, held water and became a series of wetland sponges that seeped water year-round. Downstream landowners were worried that Clark and Austin were holding onto ‘their’ water.But physical scientist Laura Norman at the US Geological Survey in Tucson found that this was not the case. Intrigued by the transformation, she compared Turkey Creek with neighbouring Rock Creek. She found that the rock dams slowed flash floods and extended water availability into the dry season. And most surprisingly, the structures actually increased the stream’s flow by 28%4. That’s because, in Rock Creek, some of the water flowing over the bare bedrock evaporates immediately, she explains. By contrast, the water-slowing approach taken at Turkey Creek allows the water to sink underground. The US Forest Service and the state of Arizona are now authorizing the building of these structures on their land.A growing body of evidence is showing that floods and droughts are caused, in part, by people’s land-use choices. And researchers are documenting the multiple benefits of restoring slow-water systems in cities, forests, agricultural lands and grasslands. Bringing the natural water cycle back into balance, researchers say, will require a decentralized mindset, with a focus on developing thousands of small projects throughout water’s path. “When you look at one particular storage space for water in one particular location, maybe that is insignificant,” says Rajib. “But when you look at their connectivity across the basin, continent or the world, the cumulative impact is substantial.” More

Illustration: Sam Falconer

In late summer, Death Valley National Park earns its name. The heat in this region of California and Nevada is relentless. Record temperatures are set, and the air is often bone dry. The 22 August 2022 was no exception, with an average temperature during daytime of 51.6 °C and humidity of just 14% in the location aptly known as Furnace Creek.Despite the heat and aridity, there was a slow but steady drip of water into the collection vial of Omar Yaghi’s device, an assembly of components loosely resembling a telescope. By the end of the day, this system had collected only a few millilitres of water — barely enough for a refreshing sip. But these results, published in July1, nevertheless represent a landmark in the field of atmospheric water harvesting (AWH).Given the extremity of the testing conditions, the results suggest that the key ingredient in this device — a water-absorbing compound called MOF-303 — has the potential to deliver life-sustaining volumes of clean water to regions that currently struggle to access it. “The vision there is to have something like a village-scale device,” says Yaghi, a chemist at the University of California, Berkeley. “If you’ve got a tonne of MOF-303, you could deliver about 500 litres of water a day, every day for five to six years.”By current estimates, roughly two billion people lack access to clean drinking water. Desalinated seawater can meet some of this need, but the technology required remains costly and is limited to communities with coastal access. This explains the growing enthusiasm for alternative solutions that extract clean water from the air. The US Geological Survey estimates that Earth’s atmosphere contains nearly 13,000 cubic kilometres of water — more than six times the volume of the world’s rivers. “You cannot deplete it — it’s always replenished by natural evaporation from a larger water body,” says Tian Li, a materials scientist at Purdue University in West Lafayette, Indiana. And although many of the most promising AWH technologies are still at the stage of lab demonstrations or proof-of-concept devices, the field is quickly building momentum towards real-world systems that produce plentiful amounts of water at low cost.Searching for suitable sorbentsThere are already several commercially available AWH systems. In mountainous, foggy regions, it is possible to literally cast a net to collect water from ever-shifting cloud masses. Such installations are producing water from the air in South America, India and parts of Africa, according to Thomas Schutzius, a mechanical engineer at the University of California, Berkeley. There are also systems for collecting the water that accumulates overnight as dew. But both fog and dew harvesting are limited to high-humidity areas. And for dew, only modest volumes of water can be produced even under optimal conditions.

Dry (left) super-moisture-absorbent gels swell as they absorb atmospheric water (right).Credit: Guihua Yu, University of Texas at Austin

Systems that condense water from ambient air offer a more generally useful solution. Several companies have already developed electrically powered ‘active’ AWH machines for this purpose. In most cases, these use fans to draw warm, moisture-bearing air into an apparatus that directly cools the air and collects the resulting water condensate; in some cases, this water is also subject to filtration and additional treatment. These systems can produce considerable volumes. The Maximus system from the firm SkyH2O in Irvine, California, for example, can produce more than 10,000 litres of purified water per day. But this system is complex and massive — weighing around 13 tonnes — and requires continuous external power to run. It is also priced at a costly US$395,000. Such systems could be a solution in wealthy water-deprived regions — the southwestern United States, for example, or Saudi Arabia — but they are a non-starter in locations with limited budgets or unreliable electrical infrastructure.The need for more affordable options has spurred interest in ‘passive’ AWH systems that use moisture-hungry sorbent compounds to collect water. The small amounts of power that such systems require could, ideally, be supplied by the Sun. Typically, these sorbents are exposed to the air overnight, when temperatures are cooler and moisture is more abundant. They collect the airborne moisture as liquid in a process known as adsorption. When day breaks, the sorbents are transferred to a device that uses solar energy to drive the release of water. This water is then condensed and collected. These passive systems are tricky, however, because they require sorbents that bind water strongly — but not so strongly that they refuse to yield their bounty without a fight. “That’s an energy penalty that you need to pay,” says Guihua Yu, a materials scientist at the University of Texas at Austin.The field got a big boost in 2017 when Yaghi, along with engineer Evelyn Wang at the Massachusetts Institute of Technology in Cambridge and their colleagues, described a solar-powered system that could extract nearly 3 litres of water per day per kilogram of sorbent — an unprecedented feat at the time2. “I was inspired by that paper,” says Peng Wang, an environmental scientist at Sun Yat-sen University in Guangzhou, China. “This is how I got into this field.”The leap in performance was thanks to the use of a different kind of sorbent — a metal–organic framework or MOF. These porous compounds, developed in Yaghi’s lab, offer a vast surface area for water to bind to, and can be readily chemically modified to further enhance their capacity and water affinity. “It takes up water even at as little as 5% relative humidity,” says Yaghi about his current sorbent of choice, MOF-303. Equally important is that little heat is needed to drive the water back out, with temperatures of 40–45 °C typically proving sufficient. Moreover, Yaghi says, MOFs remain stable throughout years of continuous use.Other promising sorbents are also emerging. Polymers known as hydrogels are a low-cost and highly customizable class of materials that can potentially achieve even greater capacity for moisture capture than MOFs. This is especially true if these gels are loaded with water-absorbing salts such as lithium chloride. Hydrogel-based AWH systems are not yet as efficient as their MOF-based counterparts at capturing and releasing water — particularly under ultra-dry conditions — but they are steadily improving. In September, Yu’s team described a microgel formulation that offers a much larger water-binding surface area than other hydrogel designs, and incorporates a heat-sensitive component to induce water release at lower temperatures3. This allows water to be cleared from the gel in about 20–30 minutes — three to four times faster than previous iterations of his team’s hydrogel-based system, Yu says. This is still about ten times slower than the release from MOF-303, however.Even simpler materials are also being explored. Li and her colleagues have been developing specialized fabrics based on cellulose, a plant-derived fibrous molecule that can absorb water4. In addition to being abundant and inexpensive, says Li, cellulose “has the nanoscale features already there without you doing anything”. Her group is exploring ways to extend the capabilities of cellulose. Impregnating the fabric with lithium salts, for example, has been shown to boost its water-harvesting capacity by more than five-fold relative to the salt-free version5.But cellulose-based systems yield a substantial amount of water only when the relative humidity is at least 60%. By comparison, the MOF-303-based system operates effectively at relative humidity of 20% or less, as shown in the Death Valley field test. And Yu’s microgels could achieve reasonably fast uptake of meaningful volumes of water at 30% relative humidity — although, of course, the water yield will always be lower in such conditions owing to the limited moisture available.Preparing for the harvestA good sorbent is only a starting point. Wang says that most passive AWH systems that have been described so far have the capacity for only one round of water absorption and release every 24 hours. This single-cycle operation can squander the potential output of a material that saturates quickly.

A water harvester containing MOF-303 can collect water from desert air with high efficiency and without power.Credit: Yaghi Laboratory, UC Berkeley

To address this, many researchers are using batch-process systems, which require swapping the sorbent beds between an air-exposed state for water absorption and an enclosed state for Sun-assisted water release. Most of these are active systems that require external sources of electrical power. That’s not necessarily a deal breaker, however — such systems could prove cost-effective. “If you just have a battery that can open a door and close it, you can triple your delivery because now you can do more than one cycle a day,” says Yaghi. In a 2019 study, his group demonstrated a compact device6 that used batteries to power multiple cycles of atmospheric water collection throughout the day. These batteries could be fully recharged by solar power during daylight hours, allowing the system to function off grid.Cost is a crucial consideration, especially given that passive AWH will — at least initially — be targeted at resource-limited populations. Fortunately, many of the sorbents now under development should be affordable. Yaghi says that MOF manufacture is already being done at an industrial scale, and that the cost is largely determined by the metal involved. For MOF-303, that means aluminium, which he says costs just $1–2 per kilogram. Some hydrogel polymers can be expensive to produce, but others can be made more cheaply. Yu’s team is even exploring whether hydrogel ingredients can be directly extracted from biomass. The opportunity for low-cost production from easily accessible materials is a key asset of Li’s cellulose fabrics. Her group is working on deploying its system in coastal communities in Senegal where fresh water is scarce. “The burden of getting fresh drinkable water there falls onto the teenage girls,” she says. “We’re trying to educate the girls, and developed a curriculum so that they can build a set-up themselves with locally available cellulose sources.”Li’s system simply requires a textile drape that can be wrung out by hand. Other sorbent-based systems depend on more sophisticated apparatus for the harvesting process — but even those do not need to be expensive. For example, Wang recalls a prototype hydrogel-sorbent-based device that he developed about five years ago7. Apart from the sorbent itself, Wang says, all the materials for the system were purchased from a local supermarket. For just $3.20, Wang and his colleagues estimated that they could construct a device that would supply roughly 3 litres — the minimum amount of water needed daily by a typical adult.Of course, there is also the issue of ensuring that the water pulled from the air is free of dangerous substances. Yaghi says that his experiences in field testing in US deserts have been reassuring. “We tested the water for metal and organics, and it was like the purest water you could find,” he says. But this is not a certainty in every environment, particularly near sources of industrial pollution. Careful assessments will be needed to ensure that collected water is separated efficiently from contaminants.Pollution has been a particular concern when harvesting fog, Schutzius says. In August, his group described a fog-harvesting net enhanced with a titanium dioxide coating, which efficiently breaks down organic pollutants such as diesel after being activated by ultraviolet light from the Sun8. He thinks that researchers should take similar considerations into account for other domains of AWH. “The whole point of adsorption is you can concentrate a lot of stuff that’s otherwise dilute,” he says.Opening the tapSome passive AWH systems are already moving into commercial development. Yaghi’s lab, for example, has spun off a start-up firm in Irvine, California, called Atoco, which aims to roll out first-generation MOF-based harvesters in the next year or so. Different water-harvesting technologies will find different applications. The robust performance of MOFs in extremely arid conditions will make them a versatile choice, whereas systems based on cellulose or hydrogels might be restricted to more humid environments.

A fog-collector park (left) in the mountains of Morocco traps water vapour on nets (right).Credit: aqualonis.com

These technologies are unlikely to fully replace existing systems such as seawater desalination, which has a proven track record of high-volume water production. But AWH could greatly reduce dependency on centralized water processing, making it accessible at the village or even single-household scale. Yaghi sees a future in which any house with electricity could reliably address its drinking-water needs with an appliance roughly the size of a microwave oven.And there are abundant opportunities beyond simply producing drinking water. For example, Wang’s group has described a harvesting system that piggybacks on existing photovoltaic solar panels, using the waste heat and energy from these panels to power water production9; the resulting water helps to cool the panels and therefore improves their efficiency. Similar approaches have been described for managing — and exploiting — waste heat in industrial settings. AWH also has agricultural applications; Yu’s group, for example, is working on using hydrogel-based materials to produce self-watering soils that directly draw moisture from the air10.It is indisputable that, as the ongoing climate catastrophe worsens, society will need to leverage every solution at its disposal to meet the planet’s water needs. “I worked in Saudi Arabia, and people there say water security is national security — that’s 100% true,” says Wang. “It’s getting more serious, and we need to do things more effectively.” More

Floods, droughts, pollution, water scarcity and conflict — humanity’s relationship with water is deteriorating, and it is threatening our health and well-being, as well as that of the environment that sustains us. The good news is that a transition from the water policies and technologies of past centuries to more effective and equitable ways of using and preserving this vital resource is not only possible, but under way. The challenge is to accelerate and broaden the transition.Water policies have typically fostered a reliance on centralized, often massive infrastructure, such as big dams for flood and drought protection, and aqueducts and pipelines to move water long distances. Governments have also created narrow institutions focused on water, to the detriment of the interconnected issues of food security, climate, energy and ecosystem health. The key assumption of these ‘hard path’ strategies is that society must find more and more supply to meet what was assumed to be never-ending increases in demand.That focus on supply has brought great benefits to many people, but it has also had unintended and increasingly negative consequences. Among these are the failure to provide safe water and sanitation to all; unsustainable overdraft of ground water to produce the food and fibre that the world’s 8 billion people need; inadequate regulation of water pollutants; massive ecological disruption of aquatic ecosystems; political and violent conflict over water resources; and now, accelerating climate disruption to water systems1.A shift away from the supply-oriented hard path is possible — and necessary. Central to this change will be a transition to a focus on demand, efficiency and reuse, and on protecting and restoring ecosystems harmed by centuries of abuse. Society must move away from thinking about how to take more water from already over-tapped rivers, lakes and aquifers, and instead find ways to do the things we want with less water. These include, water technologies to transform industries and allow people to grow more food; appliances to reduce the amount of water used to flush toilets, and wash clothes and dishes; finding and plugging leaks in water-distribution systems and homes; and collecting, treating and reusing waste water.Remarkably, and unbeknown to most people, the transition to a more efficient and sustainable future is already under way.Singapore and Israel, two highly water-stressed regions, use much less water per person than do other high-income countries, and they recycle, treat and reuse more than 80% of their waste water2. New technologies, including precision irrigation, real-time soil-moisture monitoring and highly localized weather-forecasting models, allow farmers to boost yields and crop quality while cutting water use. Damaging, costly and dangerous dams are being removed, helping to restore rivers and fisheries.

Source: US Geological Survey

In the United States, total water use is decreasing even though the population and the economy are expanding. Water withdrawals are much less today than they were 50 years ago (see ‘A dip in use’) — evidence that an efficiency revolution is under way. And the United States is indeed doing more with less, because during this time, there has been a marked increase in the economic productivity of water use, measured as units of gross domestic product per unit of water used (see ‘Doing more with less’). Similar trends are evident in many other countries.

Source: US Geological Survey/US Department of Commerce.

Overcoming barriersThe challenge is how to accelerate this transition and overcome barriers to more sustainable and equitable water systems. One important obstacle is the lack of adequate financing and investment in expanding, upgrading and maintaining water systems. Others are institutional resistance in the form of weak or misdirected regulations, antiquated water-rights laws, and inadequate training of water managers with outdated ideas and tools. Another is blind adherence by authorities to old-fashioned ideas or simple ignorance about both the risks of the hard path and the potential of alternatives.Funding for the modernization of water systems must be increased. In the United States, President Biden’s Infrastructure Investment and Jobs Act provides US$82.5 billion for water-related programmes, including removing toxic lead pipes and providing water services to long-neglected front-line communities. These communities include those dependent on unregulated rural water systems, farm-worker communities in California’s Central Valley, Indigenous populations and those in low-income urban centres with deteriorating infrastructure. That’s a good start. But more public- and private-investments are needed, especially to provide modern water and sanitation systems globally for those who still lack them, and to improve efficiency and reuse.Regulations have been helpful in setting standards to cut waste and improve water quality, but further standards — and stronger enforcement — are needed to protect against new pollutants. Providing information on how to cut food waste on farms and in food processing, and how to shift diets to less water-intensive food choices can help producers and consumers to reduce their water footprints3. Corporations must expand water stewardship efforts in their operations and supply chains. Water institutions must be reformed and integrated with those that deal with energy and climate challenges. And we must return water to the environment to restore ecological systems that, in turn, protect human health and well-being.In short, the status quo is not acceptable. Efforts must be made at all levels to accelerate the shift from simply supplying more water to meeting human and ecological water needs as carefully and efficiently as possible. No new technologies need to be invented for this to happen, and the economic costs of the transition are much less than the costs of failing to do so. Individuals, communities, corporations and governments all have a part to play. A sustainable water future is possible if we choose the right path. More

Floods are becoming more intense and frequent around the world.Credit: Thianchai Sitthikongsak/Getty Images

In the past six months alone, floods have hit countries including Italy, Libya and the United States. Beyond the immediate tragedy of lost lives and livelihoods, floods also have longer-term effects, such as causing damage to urban infrastructure and agriculture. In this webcast, a panel of researchers discuss how to improve resilience to floods and the need for expertise from fields such as climate modelling, urban studies and behavioural science. More

Solar power is the cheapest source of renewable energy available to produce hydrogen.Credit: Timothy Hearsum/Getty Images

To steer economies away from fossil fuels and to cut carbon emissions, hydrogen needs to be produced from low-carbon energy sources, such as wind and solar power. This green hydrogen is particularly attractive to certain big-energy users such as the shipping, aviation and steel industries, which would struggle to run on batteries or plug into renewable energy directly through power grids. And producing hydrogen is straightforward: electricity zapped into water splits the H2O molecules into hydrogen and oxygen, which bubble off as gaseous H2 and O2. The hydrogen can then be piped to where energy is needed.In a world of growing water stress, however, the process’s reliance on water is raising alarm (see ‘Watering the power plant’). Earlier this year, the non-governmental organization Food & Water Watch in Washington DC warned that, by 2050, hydrogen production could gulp down as much water as is used by 34 million US citizens each year. A spate of studies over the past few years provides a more positive picture, however, presenting evidence that scaling up hydrogen production need not threaten water supplies. “Water withdrawals for hydrogen production are negligible compared to total water withdrawals,” says Lorenzo Rosa, an environmental engineer who specializes in links between water, energy and food at the Carnegie Institution for Science in Palo Alto, California.

Source: E. Grubert. Clean. Prod. Lett. 4, 100037 (2023).

Still, Rosa says that water considerations should shape how and where hydrogen is produced, especially because heat and altered precipitation driven by climate change are tightening pressure on water supplies. “Hydrogen’s water consumption is small compared to what’s currently used in fossil-energy conversion and inconsequential compared to agricultural water use,” says Jack Brouwer, director of the Clean Energy Institute at the University of California, Irvine. “But there are serious water availability and delivery challenges at the local and regional levels that will need to be considered.”Brouwer and Rosa say that expanding hydrogen-production technologies and water-treatment options could enable hydrogen producers to tap into a range of non-potable water resources — including seawater — or to slash their water consumption. At the same time, geospatial analyses that maps water and renewable-energy resources against projected hydrogen production and demand can pinpoint where investing in water-saving technology — or deciding to import rather than produce hydrogen — will be key to minimizing tension between the push for hydrogen and the need to preserve water resources.A thirsty processThe water requirements for producing hydrogen by electrolysis begin with a simple calculation: every kilogram of H2 molecules requires 9 litres of H2O. Treatment to purify that water — eliminating minerals that would gum up the works — consumes another 15 litres of water per kilogram of H21.That’s not the end of the story, however. There’s a lot more water use to be counted if the renewable energy that powers the process is included. The operation of solar panels and wind turbines might not consume much water, but manufacturing them does. All told, manufacturing a wind turbine adds 11 litres to green hydrogen’s water footprint. And the manufacture of today’s leading variety of solar power adds a huge 124 litres, mostly from the fabrication of silicon photovoltaic wafers.Still, the water requirements of hydrogen production seem to be manageable. As part of an analysis co-led by Rosa, one scenario for a net-zero global economy in 2050 that consumes 400 megatonnes of green hydrogen per year would use a meagre 0.13% of the world’s available water supply if the energy came entirely from wind power, and about 0.56% if it all came from solar power 2.Countries that already face water scarcity, Rosa and his colleagues argue, could choose to import hydrogen to meet domestic needs rather than ramping up production. In fact, many of the countries that their study identified as land-limited are already working to foster a hydrogen trade, including Japan and parts of Europe. Other regions and nations — including areas of sub-Saharan Africa, South America, Canada and Australia — have sufficient land and water to become major hydrogen exporters, and many are already gearing up to fulfil this role.Multiple factors suggest that the impacts of green hydrogen on regional water could be even more limited than Rosa’s findings suggest. For example, over the past few years analysts and energy planners have lowered their estimates of how much green hydrogen will be produced over the long term. In its September prediction for limiting global temperature rise to 1.5 °C, the International Energy Agency (IEA) projects there will be one-fifth less hydrogen use in 2050 than it had predicted in its 2021 report.Another important consideration when computing hydrogen’s hydrological footprint is the capacity of hydrogen to be a substitute for fossil fuels, which are themselves water-intensive sources of energy. Most studies so far do not account for the effect of such a substitution. Rosa and his colleagues, for example, assume a fivefold reduction in petroleum refining between 2020 and 2050, as the use of gasoline and diesel is phased down. But they do not work in a corresponding reduction in water use, which could further improve hydrogen’s hydrological footprint. And if green hydrogen replaces natural gas, the substantial water consumption by gas producers that use hydraulic fracturing will be reduced. For example, many jurisdictions in the United States and Europe are considering using green hydrogen to fuel gas-fired power plants to cover gaps in supply from solar and wind farms.Love that dirty waterIn many cases, hydrogen producers might be able to avoid adding strain to potable water supplies by tapping polluted or salty water, instead of potable water. Options include municipal waste water, waste water from oil and gas production and even seawater. Water treatment and desalination plants are expensive to build, but the investment is comparatively small relative to the overall cost of hydrogen production.

Jack Brouwer, director of the Clean Energy Institute at the University of California, Irvine, demonstrates the electrolysis stack in a system used to make renewable hydrogen.Credit: Steve Zylius/UCI

In 2022, an analysis 3 by a team at Yale University in New Haven, Connecticut, concluded that even to treat seawater — the toughest water source to prepare as input to such a facility — would require only 0.3% of an electrolysis plant’s total energy budget. “The bottom line is that “we do not need to consume freshwater resources used for drinking” to produce hydrogen, says Lea Winter, a chemical and environmental engineer at Yale and the study’s lead author. The International Renewable Energy Agency agrees, concluding in 2020 that: “Even in places with water stress, seawater desalination can be used with limited penalties on cost or efficiency.”In California, electrolyser manufacturer Plug Power in New York plans to build a water-treatment plant and hand it over to the local municipality, in return for a source of water for hydrogen production. Mendota, where the plant is to be built, is currently depleting ground water to meet demand for potable water. City officials say that the new plant will clean up sewage to increase the city’s water supply, so that it can reduce its use of ground water and sell water to Plug Power.Using seawater presents almost limitless potential, but also troubling environmental impacts. Some desalination plants release heated brines laden with treatment chemicals back into the sea; they can also suck in and destroy marine creatures. The most significant ecosystem impact of these plants, according to a 2020 review4, is lethal osmotic shock to marine organisms, including fish, plankton and algae when super-salty brines cause their cells to dehydrate. Most at risk are organisms in semi-closed seas such as the Red Sea, the Mediterranean and the Persian Gulf. Nearly half of the world’s desalination capacity is concentrated in the Persian Gulf.Some observers, however, foresee potential environmental dividends if hydrogen producers tap seawater and waste water. Thomas Adisorn, a political scientist at Germany’s Wuppertal Institute for Climate, Environment and Energy, sees potential for projects such as that of Plug Power to improve the environment by supporting international development. “Putting more effort into using recycled waste water in developing countries that are exporting hydrogen could raise their capacity to build wastewater infrastructure,” says Adisorn, who organized a meeting in 2022 to help officials from water-scarce Jordan who were planning its hydrogen economy.New technology and engineering integrations promise to trim the cost of non-conventional water use even further, while capturing other valuable benefits that pay for the extra water treatment. One active area of research and development, for example, would monetize green hydrogen’s oxygen by-product. Aerobic treatment tanks at wastewater plants rely on pumped air to sustain their waste-eating microbes. According to Brouwer, some large wastewater plants pump in pure oxygen instead of air to spur faster digestion. With an electrolyser they could get that oxygen for free, says Brouwer, rather than operating pricey air-separation units.Researchers with the municipal water authority Sydney Water and the University of Sydney, Australia, estimated in 2022 that integrating electrolysers into wastewater treatment plants could save the city about US$1.5 million per year 5. They calculate that the city’s 13.7 gigalitres per year of unused effluent could yield 0.88 megatonnes of green hydrogen per year — one-tenth of the amount Australia and New Zealand are expected to produce in 2030, according to analysts S&P Global in New York. Sydney Water says that its unpublished research confirms the viability of hydrogen and oxygen production using its treated water, following further purification.Another innovation that could prove a major benefit to using seawater for hydrogen production is the ability to operate electrolysers offshore. Over the past year, teams in China and Europe have deployed platforms combining desalination equipment and electrolysers. The hope is that the floating electrolysis plants — if they can operate reliably amid storms and other assaults to offshore hardware — will cut the cost of offshore wind energy. Shipping hydrogen through pipelines is generally cheaper than moving the equivalent amount of energy through electrical transmission lines, and hydrogen proponents are betting that this rule will hold for passing energy from offshore wind farms back to land.The hydrogen producer Lhyfe’s 1-megawatt pilot platform operated offshore for 5 months this year using desalinated seawater, and a 10-megawatt platform is planned for Belgian waters in 2026. Lhyfe in Nantes, France, wants to mitigate the impact of desalination by eschewing chemical additives in its treatment process, and by diluting brine with extra seawater, says Stéphane Le Berre, Lhyfe’s offshore project manager.Lhyfe is now exploring whether the oxygen from offshore electrolysis could counteract declining levels of dissolved oxygen in the ocean — conditions that are stunting marine ecosystems in some regions. In July, researchers projected that artificial oxygenation from global deployment of offshore wind farms and electrolysers could reduce the volume of severely hypoxic zones by 1.1–2.4%6. But they also reported some counterintuitive regional impacts. For example, their simulation projected that oxygen injection might enlarge an existing hypoxic zone in the Indian Ocean’s Bay of Bengal.Known unknownsTechnological wild cards, meanwhile, could alter water consumption calculations around hydrogen production. In a review, the IEA identified 40 companies that are exploring a potential hydrogen source that might be cleaner than electrolysis: natural pockets of the gas, some of which might be tapped using little water. But, as with injecting oxygen in to the sea, seemingly water-saving technologies could have perverse effects. Accessing ‘geological’ hydrogen might require fracturing of rock layers akin to hydraulic fracturing or ‘fracking’ used to recover oil and gas. And some of the hydrogen prospectors plan to stimulate hydrogen production in situ by injecting water into iron-rich rock formations.Green-energy company Eden GeoPower in Somerville, Massachusetts, plans to test hydrogen stimulation in the peridotite rock formations of water-scarce Oman, in collaboration with the country’s Ministry of Energy and Minerals. The company hopes to increase underground permeability using its water-free electrical fracturing technology. Chief executive Paris Smalls, says “back of the envelope calculations” suggest that net water consumption will be comparable with that of electrolysis per kilogram of hydrogen delivered.Eden GeoPower’s attention to water resources is the exception to the rule among hydrogen producers. Water supply is not mentioned in the IEA report or in a 2022 “critical” review of hydrogen-production technology 7.Given that hydrogen production and water use are inextricably bound, it is unlikely that water supply will continue to be omitted. The inconvenient truth, say both Brouwer and Rosa, is that solar energy is the cheapest source of low-carbon power available to produce hydrogen, but the regions with the best solar resources are also some of the most parched.Brouwer is one of the main team members behind California’s hydrogen-development programme, which picked up $1.2-billion of the $7 billion in US federal funds awarded in October to regional ‘hydrogen hubs’ that link producers and consumers. He says that hydrogen plants can tap into conventional water supplies or clean up waste water, but that reaching net-zero carbon emissions in California will ultimately require a lot more solar panels in the desert.Converting that solar energy to hydrogen will force the state to build more infrastructure, and to make an important choice. As Brouwer puts it: “We’re going to eventually have to figure out whether we want to run big wires from the solar resource to where the water is, or big pipes sending water to the desert.” More

Water-treatment firm Aquagga’s ‘forever’ chemical destruction unit in Fairbanks, Alaska, uses a technique called hydrothermal alkaline treatment.Credit: Gus Millevolte

Selma Thagard watched in astonishment as the indestructible chemicals did the one thing that they shouldn’t do — fall apart.A chemical engineer at Clarkson University in Potsdam, New York, Thagard was developing a plasma reactor for water treatment in 2016 when an environmental-engineer colleague suggested she add chemicals known as PFAS to the water she was testing. These per- and polyfluoroalkyl substances, also commonly referred to as forever chemicals, are made up of chains of carbon and fluorine atoms held together by some of the strongest chemical bonds in nature. They don’t break down naturally, and many decontamination techniques can’t touch them either. The PFAS wouldn’t be destroyed by Thagard’s plasma reactor, her colleague told her, but might act as a useful reference sample.But it didn’t play out that way. In just a few minutes, the chemicals were no more. “When plasma degraded PFAS so rapidly, within minutes, he told me: ‘That’s not right. Nothing can degrade PFAS,’” Thagard says. She ran the test seven or eight more times, and each time the chemicals disappeared.Thousands of variations of PFAS chemicals have been used for decades in a wide variety of products, including food packaging, stain-resistant textiles and firefighting foam. Their widespread use, combined with their inability to break down naturally, means that they have spread to water, soil and wildlife. Thagard’s colleague was studying the accumulation of the chemicals in fish in North America’s Great Lakes, but they are present all around the globe.The substances also accumulate in people, and are thought to contribute to reproductive issues, impaired immune function and even cancer. Over the past two decades, concern about these forever chemicals has grown, leading to the imposition or proposal of regulations to cap their presence in water in the United States, the European Union and the United Kingdom.But ‘forever’ might be a shorter time than previously thought. Scientists, including Thagard, are developing methods to break down PFAS into fluoride and carbon dioxide, which are not dangerous in the small amounts produced. These approaches to degrading the molecules have arisen in the past few years and could become widely available in just a few more. The big questions are where in the water cycle to deploy them, and which method makes the most economic sense.Treatment technologies by themselves won’t completely solve the problem of PFAS pollution. For one thing, the number of possible molecules based on the carbon–fluorine bond is vast, making it difficult to know for certain whether a particular method can tackle each one. “There are new ones being put on the market each year,” says Timothy Strathmann, a civil and environmental engineer at the Colorado School of Mines in Golden. It can also be difficult to measure some of these molecules, especially at low concentrations. The sheer number of possible molecules, plus their stealthiness, are an ongoing challenge, Strathmann says. “This is why we need to also keep up with our ability to detect and sense these chemicals. Because if you don’t know what you’re looking for, you don’t find it.”Electrical zappingThagard’s water-treatment technique relies on electrical discharge plasma1. She puts water contaminated with PFAS in a reactor and pushes bubbles of argon gas through it. The PFAS is attracted to the interface between the water and the bubbles, and rides them to the surface of the water. The atmosphere above the water is also argon — chosen because it has a high density of electrons. High-voltage pulses of electricity flow between electrodes near the surface of the water, knocking electrons loose from the argon atoms and turning the insulating gas into a conducting plasma.

Environmental biochemist Susie Dai is using a fungus to help break down ‘forever’ chemicals.Credit: Michael Miller, Texas A&M AgriLife

The process delivers enough energy to break the carbon–fluorine bonds. If any PFAS is left, it’s at concentrations too low to detect, below the parts-per-billion level. The fluoride and carbon dioxide that are produced from the disintegration of PFAS are absorbed by the water, but in amounts that Thagard says are too small to be concerning. However, the mechanism that causes the bonds to break — something that was never expected to happen — is still unclear. “The science is largely unknown,” she says. “We are doing extensive research from the fundamental side.”Thagard and her colleagues carried out a field test on PFAS-polluted water at Wright–Patterson Air Force Base, outside Dayton, Ohio, in 2019 and showed that they could treat 4 litres of water and reduce the amount of PFAS to below the health-advisory level of the US Environmental Protection Agency in a couple of minutes2. That was using a crude system, she says; an optimized reactor could treat about 40 litres per minute. The US military has been funding research, including Thagard’s work, into cleaning up PFAS because the long-time use of firefighting foams has contaminated many bases.Thagard is chief executive of DMAX Plasma, a start-up firm she founded in Potsdam to commercialize the technology. The start-up has sold small systems to military and industrial customers. Its standard treatment unit, the company says, requires less electricity than most household electric ovens. With some engineering work, Thagard says that the systems could be scaled up to meet the needs of water-treatment plants.Under pressureAnother effort to destroy PFAS is being led by Aquagga, a water-treatment start-up company in Tacoma, Washington, in collaboration with Strathmann. It is using a technique called hydrothermal alkaline treatment (HALT), which involves adding an alkaline substance such as sodium hydroxide to the PFAS and heating it to 350 °C under high pressure (roughly 160 times atmospheric pressure)3. Under these conditions, the hydroxide draws the fluorine to itself and destabilizes the PFAS molecules. Using high-resolution mass spectrometry, the researchers found that after treating a sample of water containing PFAS they had extracted as much fluoride as should have been bound up in the PFAS to begin with — suggesting it had all been broken down.In the absence of destructive technologies, PFAS in water systems has been filtered out and sent to landfill or an incinerator. But even burning doesn’t destroy all the PFAS, which can be spread by smoke or ash from the incinerator or leach out of landfill. Neither process results in the substances being removed from the environment permanently, the way that the destructive approach does.Some sort of filtration or separation process to increase the pollutant-to-water ratio will probably be a step in any PFAS-destructive technology. “You’re not going to treat a million gallons with the destructive process,” Strathmann says. Indeed, the HALT method that Aquagga is developing works with PFAS that has been caught by an activated-carbon filter. So far, the pilot versions can treat only around 4–8 litres of concentrated PFAS per hour, but the company is working to scale that up. It’s taking orders for systems that can treat up to 75 litres per hour and developing ones that will treat nearly 600 litres per hour.Some attempts to destroy PFAS have only succeeded in breaking long-chain molecules into smaller ones with fewer than six carbon atoms. The HALT method seems to be more versatile. “This process applies across the full spectrum, from the very shortest chains, with only one carbon, all the way to the longest chain we’ve tested”, with ten carbon atoms, Strathmann says. That means it should destroy any PFAS, even those that regulatory agencies have not yet listed as of concern4,5.A sound techniqueIn addition to heated chemicals or bright plasma, high-frequency sound waves might also provide the energy needed to break up the molecular chains, by knocking the fluorine atoms loose. Jay Meegoda, a civil and environmental engineer at the New Jersey Institute of Technology in Newark, is among those working on this approach — known as sonolysis. He sends sound waves at a frequency of about 1 megahertz into a concentrated solution of PFAS6. This ultrasound creates bubbles in the water that are only a few nanometres across.Meegoda keeps pouring acoustic energy into the solution until the bubbles become unstable and implode. That releases a burst of energy, raising the temperature of the water that immediately surrounds the bubbles to 5,000 °C for about 10 nanoseconds. Although brief, the heating “is good enough to break all the molecular bonds”, Meegoda says. Everything in the immediate vicinity of the bubble gets broken down to individual atoms, even the water. Hydrogen and oxygen atoms quickly recombine as water. Carbon atoms from the PFAS join with oxygen to become carbon monoxide, and then carbon dioxide. Fluorine atoms form fluoride ions.Meegoda is working with Tetra Tech, an engineering services company in Pasadena, California, and hopes to run a pilot project with his technology in 2024. He expects to see some sort of PFAS degradation technology, whether his own or another, on the market in about two years.Meegoda, Strathmann and Thagard, along with many other researchers, are focused on degrading PFAS at existing water-treatment facilities, where the chemicals would have to be concentrated before destruction. But Michelle Crimi, a civil and environmental engineer and a colleague of Thagard’s at Clarkson, is taking the attack closer to the source. She wants to use a version of sonolysis to handle polluted ground water. Her idea is to build horizontal wells at contaminated areas, such as air-force bases or industrial sites, where there is already a high concentration of PFAS. “We don’t want to treat extremely low concentrations and huge volumes of drinking water indefinitely,” Crimi says. “That’s super expensive.” As the ground water slowly flows through the well — it could take two days to traverse a 46-centimetre well — an ultrasound system hits it with sound waves at frequencies in the mid-kilohertz range7. In the same way as Meegoda’s sonolysis system, the sound waves deliver enough energy to create bubbles in the water and break apart the PFAS molecules. The water would then continue on its natural course, into rivers, lakes or the aquifers that feed more-familiar vertical wells. “Our goal is to stop the contaminated water from reaching the drinking-water wells,” Crimi says.Crimi has co-founded a start-up company — RemWell in Potsdam — to commercialize her technology. She launched a field test in late October at Peterson Space Force Base in Colorado Springs, Colorado, to gather data on how well the system works.Let it rotLeaning towards a more naturalistic approach, Susie Dai, an environmental biochemist at Texas A&M University in College Station, is working on a technique using bioremediation, which relies on living organisms to break down the PFAS. “Bioremediation is typically cheaper than any other chemical or mechanical process, because you have an organism that’s growing by themselves do the work,” she says.Dai starts with maize (corn) stover — the leaves, stalks and cobs that remain after the maize is harvested. She separates its two main components: the cellulose that makes up plant cell walls and fibres, and the lignin that gives the stalks their stiffness. She then modifies the lignin by treating it with polyethylenimine to add functional groups, then mixes it back together with the cellulose to form a fibrous, organic filter material that can catch and hold the PFAS molecules8. Finally, Dai adds a fungus called Irpex lacteus, or white-rot, that commonly grows on fallen trees. The fungus devours the PFAS, using enzymes to break it down into more benign molecules. It also eats the filter material.Dai still needs to measure whether the fungus fully breaks down all of the contaminant and produces pure fluoride, or leaves behind some chains. “I think it’s pretty promising if PFAS are disappearing from the environment,” she says. “It is still important for us to know what the degraded products are, but it’s less important than the removal of the parent molecule.” She is looking for a site where she can test her technology under real-world conditions.Crimi would like to see the producers of PFAS pollution take further steps to shoulder the costs of cleaning up the problem, which tends to disproportionately affect lower-income communities. “It’s tricky with PFAS, because the solutions are really just emerging. There’s still a lot of work to do to really inform what is the most sustainable and cost-effective way to address the big problem,” she says.Still, she’s optimistic that the world won’t be stuck with forever chemicals eternally. “I always say, ‘Forever no more.’” Scaling up the various techniques now under development would turn that hope into a cleaner-water reality. More