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People gather around a roadside pipeline to collect drinking water in Bangladesh.Credit: Mamunur Rashid/NurPhoto/Getty

Approximately 4.4 billion people drink unsafe water — double the previous estimate — according to a study published today in Science1. The finding, which suggests that more than half of the world’s population is without clean and accessible water, puts a spotlight on gaps in basic health data and raises questions about which estimate better reflects reality.That this many people don’t have access is “unacceptable”, says Esther Greenwood, a water researcher at the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and an author on the Science paper. “There’s an urgent need for the situation to change.”The United Nations has been tracking access to safely managed drinking water, recognized as a human right, since 2015. Before this, the UN reported only whether global drinking-water sources were ‘improved’, meaning they were probably protected from outside contamination with infrastructure such as backyard wells, connected pipes and rainwater-collection systems. According to this benchmark, it seemed that 90% of the global population had its drinking water in order. But there was little information on whether the water itself was clean, and, almost a decade later, statisticians are still relying on incomplete data.“We really lack data on drinking-water quality,” Greenwood says. Today, water-quality data exist for only about half of the global population. That makes calculating the exact scale of the problem difficult, Greenwood adds.Crunching numbersIn 2015, the UN created its Sustainable Development Goals to improve human welfare. One of them is to “achieve universal and equitable access to safe and affordable drinking water for all” by 2030. The organization updated its criteria for safely managed drinking-water sources: they must be improved, consistently available, accessible where a person lives and free from contamination.
The world faces a water crisis — 4 powerful charts show how
Using this framework, the Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP), a research collaboration between the World Health Organization (WHO) and the UN children’s agency UNICEF, estimated in 2020 that there are 2.2 billion people without access to safe drinking water. To arrive at this figure, the programme aggregated data from national censuses, reports from regulatory agencies and service providers and household surveys.But it assessed drinking-water availability differently from the method used by Greenwood and her colleagues. The JMP examined at least three of the four criteria in a given location, and then used the lowest value to represent that area’s overall drinking-water quality. For instance, if a city had no data on whether its water-source was consistently available, but 40% of the population had uncontaminated water, 50% had improved water sources and 20% had water access at home, then the JMP estimated that 20% of that city’s population had access to safely managed drinking water. The programme then scaled this figure across a nation’s population using a simple mathematical extrapolation.By contrast, the Science paper used survey responses about the four criteria from 64,723 households across 27 low- and middle-income countries between 2016 and 2020. If a household failed to meet any of the four criteria, it was categorized as not having safe drinking water. From this, the team trained a machine-learning algorithm and included global geospatial data — including factors such as regional average temperature, hydrology, topography and population density — to estimate that 4.4 billion people lack access to safe drinking water, of which half are accessing sources tainted with the pathogenic bacteria Escherichia coli.The model also suggested that almost half of the 4.4 billion live in south Asia and sub-Saharan Africa (see ‘Water woes’).

Source: Ref 1.

‘A long way to go’It’s “difficult” to say which estimate — the JMP’s or the new figure — is more accurate, says Robert Bain, a statistician at UNICEF’s Middle East and North Africa Regional Office, based in Amman, Jordan, who contributed to the calculation of both numbers. The JMP brings together many data sources but has limitations in its aggregation approach, whereas the new estimation takes a small data set and scales it up with a sophisticated model, he says.The study by Greenwood and colleagues really highlights “the need to pay closer attention to water quality”, says Chengcheng Zhai, a data scientist at the University of Notre Dame in Indiana. Although the machine-learning technique used by the team is “very innovative and clever”, she says, water access is dynamic, so the estimation might still not be quite right. Wells can be clean of E. coli one day and become contaminated the next, and the household surveys don’t capture that, Zhai suggests.“Whichever number you run with — two billion or four billion — the world has a long way to go” towards ensuring that people’s basic rights are fulfilled, Bain says. More

High levels of sodium in drinking water have been linked to increased rates of pre-eclampsia in Bangladesh.Credit: Paula Bronstein/Getty

People have been separating salt from water for millennia, harvesting both salt and fresh drinking water from salty seawater. But there are limits to what can be done — sometimes with drastic consequences. When people in ancient Mesopotamia couldn’t work out how to desalinate their irrigation water and prevent salts from accumulating in their soils, their society collapsed. “It’s kind of the world’s oldest, most boring, but serious problem,” says Sujay Kaushal, a hydrologist at the University of Maryland in College Park.This problem is now growing more pressing, as salinity levels creep up in fresh waters for a slew of reasons. Rising sea levels are pushing salt into coastal groundwaters, while excessive groundwater extraction in other places is drawing deeper, saltier waters up into aquifers. And human activities — from deicing roads to washing clothes and fertilizing fields — are polluting surface waters with many kinds of salt. Last October, Kaushal and his colleagues reported that salt levels in major streams and rivers around the world are booming; some bodies of water are now several times saltier than they were a few decades ago1. Freshwater salinization is a massive global problem, not just a regional one, he says.A second, related issue is the growing burden of problematic waste brines. A variety of industries — from oil and gas extraction to the desalination plants that produce drinking water — create salty waste waters that are costly to dispose of. “We need to do something with the brine,” says Menachem Elimelech, an environmental engineer at Yale University in New Haven, Connecticut.
New desalination technique yields more drinkable water
On the flip side of these problems is the salt-mining industry. Hundreds of millions of tonnes of mineral salts are extracted every year from rock or briny waters. The Dead Sea is a major source of potassium; the Great Salt Lake in Utah, magnesium. Mining companies seeking supplies of lithium, a metal crucial for batteries and green technologies, are turning to brines around the world.Researchers who sense opportunities in this field are hoping that they can extract salts from waste brines, turning a problem into a profit while squeezing out more fresh water.To do all that, scientists are now exploring techniques to separate salt from water more efficiently, using electricity, new materials and solvents. With a wide range of brine chemistries to tackle and a host of different goals, there isn’t one “killer” technology, says Shihong Lin, an environmental engineer at Vanderbilt University in Nashville, Tennessee. “This is like a thousand different problems,” says Lin.Super saltyIt’s clear that extremely salty water isn’t healthy for humans, animals or plants. Drinking seawater, which contains about 3.5% salt, most of it sodium chloride, can shut down our kidneys and be fatal. Brackish groundwater, which contains around 0.1–1% salt, is typically desalinated before drinking. But the health impacts from less-salty water are murky. “Whether you can drink it for 30 years without any problem, not increasing any risk of disease, nobody knows,” says Lin.There’s no agreed limit for a safe level of salt in drinking water. The World Health Organization suggests that drinking water should have sodium levels below 200 milligrams per litre (0.02%) and chloride levels below 250 mg/L (0.025%), but these guidelines are based mainly on taste. Kaushal points to a study that has linked drinking water with sodium levels of more than 0.03% to increased rates of pre-eclampsia and gestational hypertension among pregnant people in Bangladesh2. Salty water could also pose indirect harms, for instance by helping heavy metals to leach out of soils or plumbing as these metals swap places with salt in the water, says Allison Lassiter, a social scientist at the University of Pennsylvania in Philadelphia.With salinity on the rise, specialists agree that the top priorities should be to stem salt-polluting human activities, conserve fresh water and reuse waste waters. But many researchers, including Lassiter and Kaushal, expect desalination to be increasingly needed as a buffer against freshwater scarcity.

Lithium mines in Chile spread brines over vast areas to evaporate the water. Researchers are seeking alternative, more compact methods of extracting lithium salts.Credit: Cristobal Olivares/Bloomberg/Getty

A standard method for desalination involves heating seawater to evaporate the water, then condensing the vapour; this basic principle is used today in a large number of the world’s desalination plants, especially those that dot the coasts of the Gulf in the Middle East. But this method consumes a lot of energy.A more energy-efficient technique emerged in the 1960s, using physical pressure to force water molecules through tiny pores of a thin membrane while leaving dissolved salt ions behind. This process, called reverse osmosis, is the gold standard for desalination plants today.The trouble is that reverse osmosis has a limit. As fresh water is extracted, the source waters get ever-saltier, making it harder and harder to continue the separation process. This is an “inescapable problem”, says Christopher Fellows, a chemist at the Saline Water Conversion Corporation (SWCC) in Jubail, Saudi Arabia. All forms of desalination leave a waste brine that needs to be managed.Mine the brineSome waste brines are simply put into the ocean; in California, a pipe system called the Brine Line carries waste brines that are produced more than 100 kilometres inland out to sea. In other places, the most economical solution is to inject waste brines underground, in a spot far enough away from utilized groundwater. This technique has been criticized for its potential to cause micro-earthquakes.
Sustainable implementation of innovative technologies for water purification
Alternatively, brine can be spread out in ponds to evaporate under the Sun, and the leftover salts collected — a land- and time-consuming strategy that also demands an amenable climate. A faster, more compact way to concentrate brine entails heating it and compressing the water vapour to accelerate evaporation. But this requires a staggering amount of energy, says environmental engineer Ngai Yin Yip at Columbia University in New York City, as well as expensive alloys that can withstand corrosive hot brine.Paying for the safe disposal of brines can be exorbitant. Communities that have brackish groundwater, for example, sometimes can’t afford desalination because of the costs of brine disposal, and so must find fresh water elsewhere. Researchers who have suggested desalinating California’s Salton Sea — which is growing so salty that it threatens the wildlife living in it — are also contending with high brine-management costs.Rather than throwing it away, some researchers are thinking about mining waste brines for minerals. For environmental engineer Jason Ren at Princeton University in New Jersey, this idea aligns better with his opinion that clean drinking water should be a human right: desalination companies, he says, should profit from selling salts, not clean water. “For many years, we’ve missed the point,” says Ren. “We focused on the water as the product; in my view, water should be a by-product of the other resources.”Ren and others have their sights set on one particularly valuable mineral: lithium. Today, a large chunk of the world’s lithium supplies comes from natural brines in arid South America. The brine is spread out in sprawling ponds, evaporating under the Sun for many months. As researchers identify other lithium-rich brines — including waste waters from the oil and gas industry — they are realizing they need new techniques to suit places where there isn’t enough land or the right climate for evaporation ponds.

Researchers at Princeton University in New Jersey use the principle of chromatography to separate lithium from water and other salts along vertical strings.Credit: Bumper DeJesus/Princeton University

Other brines could provide an unconventional source of sodium chloride. Today, ironically, some companies use fresh water to mine salt: they puncture underground salt deposits with pipes carrying fresh water to dissolve sodium chloride. This highly pure brine is then pumped up and piped or transported to chlor-alkali plants — more than half of all chemicals produced rely on these chemical refineries.Even if waste brines don’t contain any particularly valuable salts, water researchers have other reasons to champion the idea of mining waste brines: removing salt from brine liberates more fresh water, and the cost of disposal drops when volumes of waste brine are smaller.Fresh solutionsIn Saudi Arabia, recognizing the opportunity to bring in extra revenue while producing more fresh water, the government-owned SWCC is now building a demonstration plant to harvest sodium chloride, among other salts, from seawater desalination wastes.The plant, scheduled to break ground at the end of this year in Haql, Saudi Arabia, uses an emerging salt-sorting technique called nanofiltration as part of a long string of processes, says Fellows. Like reverse osmosis, nanofiltration works by pushing water molecules across a membrane. But the membrane has larger pores that also allow some salt ions through: dissolved salt ions carrying only one electrical charge, such as sodium, potassium and chloride, can cross the barrier, whereas those with two or more charges, such as magnesium and calcium, stay behind. The SWCC’s key challenge is to produce sodium chloride that is pure enough for the chlor-alkali market.The final stage at the SWCC plant entails boiling the hot brine until pure sodium chloride crystallizes. This energy-intensive step is far from ideal, says Fellows. His team has begun exploring other strategies for this stage, including freezing desalination. This approach is inspired by the fact that sea ice is composed of fresh water, even though seawater is salty. It is alluring, Fellows says, because it takes one-seventh of the energy to freeze ice-cold water as it does to evaporate boiling water. “I don’t know what’s the winning [strategy] at the moment, but it will be different for the different separations we want to do,” says Fellows.Many groups are focusing on an alternative strategy that uses electricity, rather than pressure, to do the work of separation. In this technique, an electric current is used to pull dissolved salt ions across specialized ion-exchange membranes, which permit the movement of ions in one direction only. As the ions pass through these membranes, the brine they started out in becomes more dilute, or fresher. Researchers expect the technique to be useful for pre-diluting extremely salty brines so that conventional reverse osmosis can then be used to squeeze out more fresh water.In one variant of these electricity-based techniques, Lin’s team tried to let the concentrations of salt ions that have crossed the ion-exchange membranes build until they form solid crystals3. This attempt to crystallize salts without evaporating the water worked well for certain salts — such as sodium sulfate, which is commonly found in power-plant waste water, Lin says — but not for the most abundant salt in waste brines, sodium chloride. Sodium and chloride ions hold on to water molecules so tightly that they also drag the water across the membrane, Lin says.

To avoid both evaporation and the use of membranes, Yip’s team members at Columbia are instead looking to chemical solvents4 (see video). One promising candidate is an off-the-shelf solvent called diisopropylamine5. The solvent floats on top of a salty brine and — at low temperatures — selectively sucks water molecules into itself, leaving most of the salt ions behind. At higher temperatures, diisopropylamine switches to repelling water and spontaneously expels the water that it has absorbed, so the water can be recovered and the solvent reused.Yip says his team has used this method to recover fresh water from brine samples that are up to ten times as salty as seawater — an impossible task for standard reverse osmosis. The fresh water portion might not be potable until further steps are taken to remove contaminant solvent and salt, the researchers say. But the technique could aid industries that are seeking to recycle water from their waste brines. The researchers are currently participating in a prize challenge organized by the US Department of Energy to build a small pilot that would use solar heat for the water-expelling step.Ren and his colleagues have taken an entirely different approach6, inspired by trees. Trees can draw water up several metres against gravity, emitting clean water vapour from their leaves while trapping dissolved compounds in their tissues. His team’s approach mimics trees by using long strings of fibres with one end soaking up salt water. As the brine travels upwards, the salts are separated leveraging the common principle of chromatography — different compounds move at different speeds through a medium.Ren’s main target, lithium chloride, is extremely soluble and small, so its ions move quickly up the string, ahead of larger sodium ions. Ren has successfully used this method to recover lithium from natural brine samples from Chile, using less energy and space than conventional evaporation. The team is designing an enclosed module incorporating stacks of these strings. The researchers aim to extract lithium from waste brines produced from oil and gas operations, while recovering the evaporated water.Yet more inspiration could be found in nature: highly-selective channel proteins embedded in cell membranes. One type of ion channel allows just one sodium ion to pass through for every thousand potassium ions, says Elimelech. His team is currently working on membranes that mimic these channel proteins, although for now they are in the earliest stages of development.Price barrierWhether any of these ideas will take hold depends on economics. If the SWCC mined all the available sodium chloride from Saudi Arabia’s seawater desalination brines, Fellows notes, it would be enough to supply one-third of the world’s market. Meanwhile, waste brines left over from brackish-water desalination could offer the plentiful mineral gypsum, but it’s unlikely that unconventional brine mining could compete economically with conventional quarrying from rock.New markets, such as the advent of salt-fuelled technologies including zinc–bromine batteries, could create fresh demand for certain salts, says Fellows. Regulations could also play a part, either by making it more expensive to dispose of waste brines or by incentivizing the use of brine-sourced salts in various applications, for instance brine-sourced gypsum in road salts.One thing is clear: freshwater needs are increasing. Addressing the current limits of desalination with new technologies is important, researchers say. But it isn’t an alternative to the still-essential step of conserving fresh water. It will always take energy, time or land space to separate salt from water, so there will always be a price to pay for desalination. “There’s no magic there,” says Elimelech. More

“In this photo, I’m visiting a once-abandoned well, known as the Three Trees Well, north of Bengaluru, India. In the past, this type of well would have been filled with concrete and the land used to build properties, but thanks to community outreach efforts it has been protected and cleaned by local community members. Their work has been rewarded by the well reconnecting with the below-ground aquifer and bringing water to the community again. It now contains around 3 metres of water during the peak summer months.This is a success story, and we need more of those. People on the city’s outskirts are struggling, and the state’s government has responded by arranging tankers to bring water from the Kaveri River, 100 kilometres away. Unfortunately, this kind of event is taking place across much of urban India. Around 50% of the supply in Indian cities comes from groundwater and, without it, life grinds to a halt.I work as an adviser for the Biome Environmental Trust in Bengaluru and develop policies related to sustainable water management. I am also an adjunct faculty member at Azim Premji University in Sarjapura, where I teach courses on water conservation and management.Here in Bengaluru, I’ve been working with the traditional well-digging community, known as the Mannu Vaddar or Bhovi, to complete works in the city and surrounding rural areas. If the culture of the well can be revived in India, it might help people to find a path to sustainability in the face of climate change.Since our success at the Three Trees Well, the government has funded a further programme to rejuvenate wells and shallow aquifers in 10 cities across India. Now, the government wants to replicate that programme across a further 5,100 urban areas in the country. I am a small part of this journey, and it all began with Bengaluru’s wells.” More

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