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I contend that calls to govern ‘green’ water (in air, biomass and soils) on a global scale through markets are unrealistic (see J. Rockström et al. Nature 615, 794–797; 2023). They could also distract from addressing the everyday challenges of ensuring that access to ‘blue’ water from rivers, lakes and reservoirs is sustainable and equitable.
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The author declares no competing interests. More

Last month, an extended drought in Uruguay forced the water-treatment plant that supplies 60% of the population to start processing and distributing brackish, non-potable water. Even though the authorities are used to highly variable precipitation and are aware of the social and political consequences of water shortages, they were completely unprepared for the crisis.
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Children collect drinking water from a well in Murkata, India.Credit: Biju Boro/AFP via Getty

The Earth has lost enough groundwater to thirsty humans to measurably tilt the planet’s axis of rotation1.The net water lost from underground reservoirs between 1993 and 2010 is estimated to be more than 2 trillion tons. That has caused the geographic North Pole to shift at a speed of 4.36 centimetres per year, researchers have calculated. The results appeared on 15 June in Geophysical Research Letters.A wobbling of the EarthThe tilt of the axis on which any celestial object spins tends to be stable. But small changes can occur when large masses shift location inside a planet and on its surface. “Every mass moving around on the surface of the Earth can change the rotation axis,” says Ki-Weon Seo, a geophysicist at Seoul National University.Astronomers can track such motions in the Earth’s axis by observing quasars, the bright centres of distant galaxies that constitute practically immobile points of reference. The largest axis change is seasonal and is triggered by the motion of atmospheric masses as the weather and seasons change. This effect causes the Earth’s geographic poles to wobble by up to several metres every year.
The world faces a water crisis — 4 powerful charts show how
Shifts in water masses can cause smaller but still measurable changes in the tilt of Earth’s axis. Until recently, researchers thought that these water-driven effects would be caused mainly by the melting of glaciers and ice caps. But when Seo and his collaborators tried to model the Earth’s water content to account for how much the axis has tilted, they could not fully explain the data. Adding the effects of changes in surface reservoirs did not help, says Seo, “so I just scratched my head and said, ‘probably one effect is groundwater’”.Gravitational surveys have measured the depletion of underground reservoirs, which is caused in large part by irrigation, especially in northwestern India and western North America. These surveys show that groundwater pumping shifted enough mass into the oceans to cause 6.24 millimetres of global sea-level rise between 1993 and 2010.By including these changes in their model, the authors calculated that they should have a substantial impact on the Earth’s rotation axis. They predicted that the displacement of groundwater alone causes a shift in the North Pole of 4.36 centimetres per year, roughly in the direction of Russia’s Novaya Zemlya islands. More

The Chilean government plans to nationalize 20 salt lakes in the unique Altiplano ecosystem and exploit them for lithium extraction. As well as creating a huge and long-lasting trail of environmental damage, intensive mining activity will disrupt the region’s delicate ecological and hydrological balance.
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CORRESPONDENCE
06 June 2023

Mohsen Maghrebi

University of Tehran, Tehran, Iran.

Roohollah Noori

University of Tehran, Tehran, Iran.

Amir AghaKouchak

University of California, Irvine, Irvine, California, USA.

To relieve Iran’s water crisis, the government invested about US$1.5 billion in modernizing the country’s irrigation systems, aiming to reduce agricultural water demand (Islamic Parliament Research Center 18724, https://rc.majlis.ir/fa/report/show/1756716; 2023). However, technology alone could not resolve the situation.

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Nature 618, 238 (2023)
doi: https://doi.org/10.1038/d41586-023-01851-y

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Visitors to Lake Mead, a reservoir in Nevada and Arizona that is fed by the Colorado River, see a bleached bathtub-like ring on its banks, indicating past water levels.Credit: Justin Sullivan/Getty

Seven US states this week finally agreed on a plan to cut their use of water from the Colorado River, an icon in the nation’s Southwest that supplies water to more than 40 million people, including in Mexico. The river, battered by overuse, drought and climate change, has been drying up. Although scientists welcome the plan, they say the agreement is only a temporary fix to a much thornier problem.
The world faces a water crisis — 4 powerful charts show how
“It does not change the fundamental problem of the overallocation of the Colorado River,” says Kathryn Sorensen, an economist at Arizona State University in Phoenix. “But it helps.” Combined with an unexpectedly wet winter in the western United States, which is now filling the river with snowmelt, the agreement buys time for officials to negotiate a more sustainable solution.From its headwaters in the Rocky Mountains to where it empties into the Gulf of California, the 2,300-kilometre-long river serves farmers, homeowners, businesses and many others, including people in 30 tribal nations and in cities including Los Angeles. The 22 May announcement is the latest in a decades-long dispute over who should cut back on their water use to preserve the river. The ‘lower basin’ states of California, Arizona and Nevada — the three that are farthest downstream as Colorado flows to the sea, with California the heaviest user of the river’s water — have agreed to conserve 3 million acre-feet (3.7 trillion litres) of water between now and 2026. That amount is enough to supply around 6 million households for a year.

Source: US Bureau of Reclamation

Researchers say that the Colorado River is overtaxed by serving more people than it can handle, in a part of the country that doesn’t get a lot of rainfall. A drought that began in 2000 has caused the river’s water levels to drop steadily — culminating last year when the massive Lake Mead reservoir in Nevada and Arizona shrank (see ‘Drying up’) to the point that long-lost human bodies emerged on the shoreline. If the levels in Lake Mead and in Lake Powell in Arizona and Utah — both fed by the river — drop low enough, water will no longer pass through their dams to generate the thousands of megawatts of hydropower that are used by people across the west. Both reservoirs are currently only about 30% full, down from 95% full in 2000.The inevitability of climate changeOne bright spot in the Colorado River outlook is this winter’s heavy snows, which are now melting from the mountains and feeding the river. As a result, lakes Powell and Mead are starting to rise faster than they have in recent years. But scientists say that it is a rare event, and not something to be counted on in the future. “This one year doesn’t get us out of it,” says Jack Schmidt, a geomorphologist at Utah State University in Logan.
What the science says about California’s record–setting snow
Climate change, however, is plodding and relentless — and it’s expected to increase the frequency of droughts in the Colorado River basin. Every temperature increase of 1 ºC in the upper part of the river basin leads to a 9.3% drop in the river’s flow, the US Geological Survey has estimated1. Long-term strategies are needed to account for the inevitability of climate change, as well as the uncertainty of wet and dry seasons, says Kevin Wheeler, a water resource engineer at the University of Oxford, UK.Reducing water use is a linchpin to all such plans. In California and Arizona, for instance, water management officials are ramping up plans to recycle wastewater into drinking water. Some are experimenting with encouraging changes in residents’ behaviour by, for instance, charging more for water in the summer than in the winter. Other efforts focus on cutting back on agricultural uses of water — which is key, because farmers account for 75–80% of the water consumption from the Colorado River, much of which is used to grow hay and other fodder for livestock. Technologies such as drip irrigation, which delivers water directly to plants rather than flooding an entire field, can help — although their effectiveness is debated.A temporary fixThe four states that are upstream along the river — New Mexico, Utah, Colorado and Wyoming — have agreed with the lower-basin states. Together, they are asking the federal government, which oversees water releases from lakes Powell and Mead, to consider the new plan as a way forward between now and 2026. The US Department of the Interior has the power to modify, reject or accept the plan. The proposed cutback is only around half of what the department had been asking of the group of seven, but it still agreed to pay US$1.2 billion to support the proposal, for instance by paying farmers to let their fields lay fallow.
Mexico is seeding clouds to make rain — scientists aren’t sure it works
That’s a lot of money to support what is only a temporary fix, says Jennifer Gimbel, a water policy expert at Colorado State University in Fort Collins. “It concerns me that we’re spending that kind of money when we should be focused on a more permanent solution,” she says. Any permanent solution will involve a complex stew of government-backed incentives to conserve water across the entire river basin, all trying to balance the needs of many users.The agreement pushes off any further decision-making until 2026, when interim water-management guidelines are due to expire. Between now and then, the United States, Mexico and tribal nations must come up with an approach for how they will conserve the river’s limited resources in future.There’s no time to waste, Sorensen says. “Everyone understands that time is of the essence.” More

In their call for protection of the global water cycle, Johan Rockström and his colleagues propose putting an economic value on ‘green’ water, which is held in the air, biomass and soils (Nature 615, 794–797; 2023). In my view, it’s even more urgent to fix the underpricing of ‘blue’ water in rivers, lakes, reservoirs and aquifers if we are to reduce the risk of water-scarcity crises.
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In our view, assessing the “social cost of water” is unrealistic (see J. Rockström et al. Nature 615, 794–797; 2023) because it risks oversimplifying a range of complex water issues. The social value of water varies across space, time and cultures, so it is not “akin to” the social cost of carbon, the global economic costs resulting from emitting one extra tonne of carbon dioxide, which is consistent across different contexts and countries. Such a flawed, catch-all concept could result in policies and investments that target the wrong challenges.
Competing Interests
The authors declare no competing interests. More