Markus Andrews

Almond farmers in California are under pressure to reduce the amount of water they use.Credit: Ed Young /Design Pics Editorial/Universal Images Group/Getty

From the Dust Bowl era of the 1930s in North America to the droughts in Ethiopia in the 1980s, Australia in the early 2000s and Syria, Iraq and Iran in 2020, the spectre of water shortages has long hung over the world’s farmlands. When rain fails to arrive season after season, crops wither and cattle starve, and famine and conflict often follow. Climate change brings a whole new level of unpredictability to the rainfall that farmers rely on, either to water their crops directly, or to feed the rivers, lakes, ground water and snowpack from which they draw water for irrigation. And that means agriculture is having to adapt — quickly.But crops that have been cultivated in the same place or cattle that have occupied the same rangelands for centuries can’t just be packed up and moved to a new area when rain patterns change. Instead, producers — and the governments that rely on them to nourish the populace and the economy — are having to rethink what is really involved in future-proofing an industry that humanity can’t live without. Some of the solutions lie in engineering crops to be more drought tolerant, or choosing crop varieties that are inherently better able to survive in drier conditions. But they also lie in an approach that requires economic and agricultural flexibility.
Nature Spotlight: Agricultural sciences
The Intergovernmental Panel on Climate Change has forecast that the percentage of the world’s population exposed to extreme drought will increase from 3% to 8% by 2100. If global warming exceeds 3 °C above pre-industrial levels, around 170 million people — mostly in low- and middle-income countries — face extreme drought.“I can see that Africa, Latin America and the Mediterranean continues to become drier,” says Hideki Kanamaru, a natural-resources researcher at the United Nations Food and Agriculture Organization in Rome. “These are particular areas of concern [because they] overlap with the historical trend of droughts.”Modelling suggests that rainfall will generally increase at higher latitudes — towards the poles — but decrease over subtropical areas. Over the past century, there has been a trend towards more rainfall in eastern North and South America, northern Europe, and northern and central Asia. However, there has been less rainfall overall in the Sahel, southern Africa, the Mediterranean and southern Asia. Climate change is also likely to alter monsoon rain patterns, which many agricultural regions rely on for predictable rainfall.However, this isn’t the only water-scarcity threat faced by producers. Amal Talbi, a hydrogeologist and water-resources management specialist at the World Bank in Washington DC, says that drought can also arise from economic water scarcity.Whereas physical water scarcity is when there is not enough water to meet the needs of agriculture or other uses that need fresh water, economic water scarcity is when, “you have the water, but you don’t have access to the water because you don’t have the infrastructure,” Talbi says. This distinction is important because the approach to solving these problems is very different.Flexible food strategiesPhysical water scarcity can be tackled in several ways. The first is to use less water overall: “Either you reduce your irrigation area, or you change the crops, so you use crops that use less water,” Talbi says. The second is to boost water sources with methods such as wastewater reuse or desalination plants.Another method is to be flexible with what crops are grown and when, and then use this to make the most of both water and market demand. This is the approach taken by Jordan, one of the most water-scarce nations. Receiving less than 50 millilitres of rainfall per year, the country is facing an even drier future, with its freshwater supplies per person now just 3% of what they were two decades ago, owing in part to climate change. Despite this, agriculture contributes around 30% of the country’s gross domestic product.

Severe droughts in eastern Australia in 2019 meant farmers had to feed cattle with fodder imported from the other side of the country.Credit: David Gray/Getty

Jordan’s answer to worsening water shortages is to focus on growing high-value, water-intensive crops for export, such as strawberries and tomatoes, in the central and northern Jordan Valley region. Although this area gets some rainfall, farmers also have access to the Jordan River and the King Abdullah Canal, an irrigation project that provides water to the Jordan Valley.It might seem illogical to grow water-hungry crops in a water-deprived landscape, but Talbi says it makes more sense than growing a crop such as wheat. “For the same land, what you would get in terms of these foods — exporting them, getting that money and then buying wheat — you will have much more than if you were using wheat in that area,” she says. Jordan also has another advantage: its climate means that those high-value seasonal products ripen earlier than they do in regions such as Spain and Portugal, so Jordan gets them to European markets ahead of other producers. “In a way, it is among the best countries in the region in terms of managing the water scarcity, given that they have so little options,” Talbi says.Morocco has a more complex water scenario to negotiate because different parts of the country experience different rainfall. Its largest crop is wheat, followed by barley, but it also produces high-value, water-hungry crops such as tomatoes, potatoes, citrus fruits and watermelon. Farmers and businesses there, like those in Jordan, grow high-value crops in irrigated areas where the water supply can be more carefully controlled and is therefore reliable, and save the less water-hungry crops for the rain-fed regions. “Roughly 50% of the time Morocco has a low rainfall, 50% it has good rainfall, so it has high variability,” Talbi says. When rainfall is good, they plant wheat and grains, and when it isn’t they maximize their irrigated high-value crops and use this money to buy wheat and to compensate the grain farmers.Change in the timesAnother factor that influences physical water availability is changes to the timing of previously predictable climatic patterns. In the northwest United States — Oregon, Washington and Idaho — wheat, tree fruit such as apples and cherries, and potatoes are the dominant crops. These are watered by a combination of rain and irrigation, the latter of which relies on the annual snowpack melting and delivering a flush of water to rivers and lakes in the Columbia River basin.But rainfall patterns are changing, says Georgine Yorgey, the associate director of the Center for Sustaining Agriculture and Natural Resources at Washington State University in Mount Vernon. “We’re going to hold less water in snowpack, more precipitation falling as rain at shoulder times of year and in shoulder elevations, and then also earlier snowmelt,” Yorgey says. And that has implications for planting and harvesting. “We have more of a mismatch between when the water comes and when the water is needed.”The timing of a crop’s sensitivity to water stress — when it is likely to have the greatest impact — varies between crops, Kanamaru says. “The last stage — ripening through harvest stage — they are not so sensitive to water stress,” he says. “The next critical stage is planting to early vegetation and the most critical stage is during reproduction.” If rainfall patterns change, it could mean that the timing of planting and harvesting of crops has to change. It’s not a new strategy in agriculture, but one that is being considered much more broadly in the face of shifting temperatures and rainfall patterns. One study has found evidence that the sowing of spring crops such as maize (corn), rice, sorghum and soya bean can shift by 10–30 days across different regions (S. Minoli et al. Nature Commun. 13, 7079; 2022). Another project in Australia found that moving the planting window for sorghum forward by four weeks reduced the risk of high summer temperatures causing heat stress during flowering (see go.nature.com/3vp3dt3).However, being flexible and tailoring each year’s agricultural focus to rainfall works only with crops that are planted and harvested in yearly cycles. It’s less viable for longer-lived crops, such as tree nuts, as California’s almond industry is discovering. The almond sector has quadrupled in size in the past 20 years, and is now the fourth largest agricultural commodity in the state, supplying around 80% of the world’s almonds. This expansion comes at a water cost: in 2021, the crop consumed 520 billion gallons more water than it did in 2017.

Strawberry farmers in Morocco grow the crop in irrigated areas where the water supply is carefully controlled but reliable.Credit: Youssef Boudlal/Reuters

In the past two years, drought has forced a reckoning, and there are now calls for the almond industry to reduce in size to preserve the state’s water supply in times of shortage. An almond tree can take around seven years to become fully productive, so it’s an industry that can’t just turn on a dime. As a result, producers are facing some tough decisions about its future viability in a drier, hotter climate.Cattle are a lot more mobile than an almond tree, but even in a country with grazing lands as vast and expansive as Australia’s, droughts have had devastating effects on this agricultural sector. “There were genuine shortages of feed for livestock. We had farmers in the eastern side of Australia with very hungry livestock, having to pay very high prices to ship grain and fodder from the other side of the country because there was none in eastern Australia,” says Neal Hughes in Geelong, Australia, who is an economist at the Australian Bureau of Agricultural and Resource Economics and Sciences — a national government research agency.Australia is usually a significant exporter of grain around the world, accounting for around 13% of all global wheat exports. But during the last devastating drought, which culminated in the Black Summer bush fires of 2019 and 2020, Australia’s contribution to wheat exports dropped drastically, and the nation even ended up having to import small amounts of grain to meet domestic needs, Hughes says. It was a shot across the bow of a country with an economy that is heavily dependent on its natural resources, warning that climate change could threaten a long-cherished status quo.An issue of accessEconomic water insecurity is a very different challenge, because solutions require a cross-disciplinary approach. A big issue is that the water exists, but requires efficient and affordable irrigation to enable farmers to get to it. In regions such as West Africa and the Sahel, the cost of irrigation is astronomical compared with that in other nations, Talbi says. For example, to irrigate one hectare in the Sahel can cost up to US$20,000, whereas doing the same in China might be around $600–700 per hectare, she says.One reason is that the supply chain for irrigation equipment is not yet established in Africa, so these products must be imported. Getting irrigation set up not only where it’s needed, but how it’s needed is also a challenge. Pumps and infrastructure can’t simply be parachuted in for free, Talbi says. Those systems have to be built from the ground-up if they are to be sustainable in the long term.Water isn’t the only challenge that agriculture faces in a climate-changed future, but historically it has been the most devastating, accounting for at least half of agricultural losses, Kanamaru says. And that’s only going to get worse. “Climate change is an additional amplifier to the long-standing problems of managing water,” he says.Finding solutions will require a holistic approach. “There are many parameters: variables we can modulate in this complex balance between demand and supply of water,” he says. “But I think we need to take a step back and look at the water budget of the whole hydrological cycle.” More

More than two billion people worldwide lack access to reliable, safe drinking water. Challenges around managing water resources are complex and wide-ranging. They are interlinked with those affecting land and food systems and are exacerbated by the climate crisis. Four scholars propose ways to prompt progress in water governance — and highlight just how crucial it is for local communities to be involved.

Farhana Sultana approaches research on environmental harms and social inequities in tandem.Credit: Wainwright Photos

FARHANA SULTANA: Collaborate to advance water justice Throughout my childhood in Dhaka, Bangladesh, the frantic call ‘Pani chole jaitese!’ (‘The water is running out!’) prompted my family, along with the entire neighbourhood, to scramble to fill pots and buckets with water before the taps ran dry. I witnessed women and girls walk long distances to secure this basic necessity for their families, long before water governance became central to my academic career. Amid water insecurity, the opposite extreme was just as familiar — going to school through devastating floods and experiencing the fall-out from disastrous cyclones and storm surges.Municipal water services in Dhaka also struggled to meet the growing demands of a rapidly urbanizing and unequal megacity. Access to electricity — needed to run water pumps — was sporadic, and there weren’t enough treatment plants to ensure clean water for millions of residents.These early experiences fuelled my dedication to tackling water injustices. Today, as an interdisciplinary human geographer with expertise in Earth sciences, and with policy experience gained at the United Nations, I approach environmental harms and social inequities in tandem — the root causes that connect both must be addressed for a just and sustainable future. My research also encompasses climate justice, which is inextricably linked with water justice. Climate change intensifies water-security concerns by worsening the unpredictability and severity of hazards, from floods and droughts to sea-level rise and water pollution.Such events hit marginalized communities the hardest, yet these groups are often excluded from planning and policymaking processes. This is true at the international level — in which a legacy of colonialism shapes geopolitics and limits the influence of many countries in the global south on water and climate issues — and at the national level.However, collaborative work between affected communities, activists, scholars, journalists and policymakers can change this, as demonstrated by the international loss-and-damage fund set up last year to help vulnerable countries respond to the most serious effects of climate-related disasters. The product of decades of globally concerted efforts, this fund prioritizes compensation for low-income countries, which contribute the least to climate change but often bear the brunt of the disasters.I also witnessed the value of collaboration and partnership in my research in Dhaka. Community-based groups, non-profit organizations and activists worked with the Dhaka Water Supply and Sewerage Authority to bring supplies of drinking water at subsidized prices to marginalized neighbourhoods, such as Korail, where public infrastructure was missing.Globally, safe water access for all can be achieved only by involving Indigenous and local communities in water governance and climate planning. People are not voiceless, they simply remain unheard. The way forward is through listening.

Tara McAllister is exploring the interface between Mātauranga Māori (Māori Knowledge) and non-Indigenous science.Credit: Royal Society of New Zealand

TARA MCALLISTER: Let Māori people manage New Zealand’s water I have always been fascinated by wai (water) and all the creatures that live in it. Similar to many Indigenous peoples around the world, Māori people have a close relationship with nature. Our connection is governed by geneaology and a concept more akin to stewardship rights than to ownership. This enables us to interact with our environment in a sustainable manner, maintaining or improving its state for future generations.I was privileged to go to university, where I studied marine biology. I then moved to the tribal lands of Ngāi Tahu on Te Waipounamu, the South Island of New Zealand, which triggered my passion for freshwater ecosystems. Intensive agriculture is placing undue pressure on the whenua (land) and rivers there. Urgent work was required. Undertaking a PhD in freshwater ecology, I studied the causes of toxic benthic algal blooms in rivers. For me, there is no better way to work than spending my days outside, with my feet in the water.

A worker fills people’s water containers from a tanker in Kolkata, India.Credit: Rupak De Chowdhuri/Reuters

Having just started a research position at Te Wānanga o Aotearoa, a Māori-led tertiary educational institution, I am now exploring the interface between Mātauranga Māori (Māori Knowledge) and non-Indigenous science, and how these two systems can be used alongside each other in water research. I have also been working on nurturing relationships with mana whenua, the community that has genealogical links to the area where I live, so that I can eventually work in the community’s rivers and help to answer scientific questions that its members are interested in.Despite a perception that Aotearoa (New Zealand) is ‘clean and green’, many of its freshwater ecosystems are in a dire state. Only about 10% of wetlands remain, and only about half of rivers are suitable for swimming. Water resource management is challenging, because of a change this year to a more right-wing government. The current government seems intent on revoking the National Policy Statement for Freshwater Management, established in 2020.This policy has been crucial in improving the country’s management of freshwater resources. Although not perfect, it does include Te Mana o te Wai — a concept that posits that the health and well-being of water bodies and ecosystems must be the first priority in such management. It is now in danger of being repealed.I think that, ultimately, our government’s inability to divulge control and power to Māori people to manage our own whenua and wai is what limits water resource management. More than any change in policy, I would like to see our stolen lands and waters returned.

Suparana Katyaini calls for more policy support for Indigenous-led water management.Credit: Milan George Jacob

SUPARANA KATYAINI: Consider water, food and land together Growing up in New Delhi, I always had easy access to drinking water — until the summer of 2004, when a weak monsoon triggered a water crisis and the city had to rely on water tankers. I realized then that good management of water resources supports our daily lives in ways we take for granted until we experience scarcity.My professional journey in research and teaching has been motivated by this experience. During my environmental studies of water poverty in India, I noticed that the field relied largely on quantitative data over qualitative insights — the degree of water-resources availability, access and use are typically assessed through metrics such as the water-availability index or the water-demand index. But in many places, Indigenous and local communities, including farmers and women in any occupation, have collectively developed skills to weather periods of water scarcity. Paying attention to these skills would lead to better water management. For example, the issue of food and nutritional insecurity in water-scarce areas in the state of Odisha, India, is being solved by Bonda people through revival of the crop millet, using varieties that are nutritious, water-efficient and climate-resilient.But these efforts need more policy support. My current work at the Council on Energy, Environment and Water explores how water, food and land systems are interlinked in India, and how better understanding of these relationships can inform policies. I am looking to identify similarities and differences in objectives of national and regional policies in each sector, as well as exploring whom they affect and their intended impacts. The aim is to move towards unifying water, food and land governance.

Michael Blackstock examines climate change from a water-centred perspective.Credit: Mike Bednar

MICHAEL BLACKSTOCK: Shift attitudes towards water In 2000, I conducted an ethnographic interview with Indigenous Elder Millie Michell from the Siska Nation in British Columbia, Canada, that transformed my interest in water from intellectual curiosity to passion. She passed a torch to me that fateful day. During our conversation for my research about the Indigenous spiritual and ecological perspective on water, she asked me: “Now that I shared my teachings and worries about water, what are you going to do about it?” She died of a stroke a few hours later.As an independent Indigenous scholar, I went on to examine climate change from a water-centred perspective — drying rivers, downpours, floods and melting ice caps are all water. This approach, for which I coined the term ‘blue ecology’, interweaves Indigenous and non-Indigenous ways of thinking. It acknowledges water’s essential role in generating, sustaining, receiving and, ultimately, unifying life on Mother Earth. This means changing our collective attitude towards water.In 2021, I co-founded the Blue Ecology Institute Foundation in Pavilion Lake, Canada, which teaches young people in particular to acknowledge the spiritual role of water in nature and in our lives, instead of taking it for granted as a commodity or ecosystem service. Giving back to nature with gratitude is also crucial. Such restrained consumption — taking only what is needed — would give abused ecosystems time to heal.A focus on keeping water healthy can help to guide societies towards more sustainable environmental policies and climate-change resilience — and ensure that future generations will survive with dignity. Critics say, ‘Blue ecology is kinda out there.’ In my view, however, ‘here’ is not working. More

Striped by its rings’ shadows, Saturn (light blue; artificially coloured) looms behind its moon Mimas (grey sphere), which conceals a liquid ocean underneath its surface.Credit: NASA via Alamy

There’s a newfound ocean in the outer Solar System, and it’s in a very surprising place1. Mimas, a mid-sized moon of Saturn, turns out to have an ocean beneath its icy surface — despite looking too geologically inert to have water sloshing inside.Mimas joins a growing list of icy moons that are also ocean worlds. The fact that boring-looking Mimas has an ocean means that “you could have liquid water almost anywhere”, says Valéry Lainey, an astronomer at the Paris Observatory.That’s important because interactions between ocean water and rock, which would occur where a buried ocean meets a moon’s rocky core, can generate enough chemical energy to sustain living organisms. If there are more stealth ocean worlds out there similar to Mimas, there are greater chances of extraterrestrial life.Peek-a-boo oceanThe discovery, reported today in Nature by Lainey and his colleagues, largely resolves the long-standing question of whether Mimas has an ocean. Many researchers hadn’t expected it to: Mimas’s geology does not display signs of a possible buried ocean, such as the icy rafts that jostle on Jupiter’s moon Europa or the geysers that spew from Enceladus, another icy moon of Saturn.
Pluto’s dark side spills its secrets — including hints of a hidden ocean
But in 2014, a team that included Lainey and that was led by Radwan Tajeddine, an astronomer then at the Paris Observatory, analysed images taken by NASA’s Cassini spacecraft, which explored Saturn and its moons between 2004 and 2017. By studying how the 400-kilometre-wide Mimas wobbled in its orbit around Saturn, the researchers concluded that it had either a buried ocean or a rugby-ball-shaped core2. As more scientists studied how an ocean could have formed and evolved, it became harder to explain the geology of Mimas without invoking an ocean3.In the 2024 study, Lainey and his colleagues seem to have nailed the case. They went further than they had in 2014, by analysing not just the orbit’s wobble but also how Mimas’s rotation around Saturn changed over time. The team combined Cassini observations with simulations of Mimas’s interior and its orbit to conclude that there must be an ocean 20–30 kilometres below Mimas’s surface.Solid evidenceThe work is the best evidence yet for an ocean in Mimas, says Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who will report similar conclusions at a conference next month in Texas. “I am happy to move Mimas from the ‘maybe possibly an ocean world’ category to the ‘yeah it really could be an ocean moon’ category,” she says.
Cassini’s 13 years of stunning Saturn science — in pictures
But it seems to be a young ocean — having formed in the last 25 million years, compared with almost 4 billion years ago for Earth’s first ocean. If the ocean had been around for longer, it would have begun to exert its influence on Mimas’s icy surface by now, for example by fracturing it. At some point in the recent past, Lainey says, Mimas was probably travelling on a stretched-out orbit that caused it to gravitationally interact with other Saturnian moons. That tidal interaction would have heated up Mimas, melting its interior and creating the ocean.Ultimately, the pockmarked Mimas could evolve to look similar to smooth Enceladus, which is coated in ice created by water spraying through cracks in its shell. And beyond Saturn, the discovery suggests that several moons of Uranus could also be hiding oceans of their own, despite looking static and frozen on their surfaces.“There are no boring moons,” Rhoden says. More

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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

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

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