The US exports too much of its most valuable resource

The Sulphur Springs Valley is a windswept desert in southeastern Arizona, bounded on three sides by forest-topped mountain ranges known as the sky islands. It can take an hour or more to drive between inhabited places in the valley, but the community there is tight-knit-many of the farmers went to the same high school (as did their grandparents), and today they graze their cattle on the plains and grow corn, soybeans, and grapes.

All of this relies on an aquifer underneath the valley. This layer of rock and soil accumulated its moisture over tens of thousands of years-caught during the monsoon season, or as snow on the nearby mountaintop melted. For generations, farmers-and the many others who have migrated across the country to make this epic landscape their home-have greened their desert by digging wells a few hundred feet into the ground and tapping the groundwater below.

In the past decade, however, these wells have started to run dry. Travel beyond the homesteads and family-run farms you'll see why-thousands of acres of neatly ordered trees bearing pecans and pistachios, vast fields of alfalfa and corn, huge dairy herds, and rows of greenhouses growing tomatoes cover the once-barren desert. This enormous carpet of industrial agriculture, with food grown for export to places around the world, takes deep wells to sustain. For every 100 acres or so, a corporate farm owner will dig a well as deep as 2,000 feet and pull up water from the ancient aquifer at up to 2,000 gallons per second, often 24 hours a day. The drilling rigs often resemble those used for oil.

There are almost no regulations governing the extraction of groundwater in Arizona. As long as the farms pay a permitting fee, they can pump as much as they like.

Added to the over-extraction of water from the aquifer, Arizona (along with the American Southwest in general) is now experiencing one of the worst droughts in hundreds of years, likely driven by global warming. As the region becomes hotter and drier, necessitating more extraction from the aquifer, less water trickles in from monsoons or snowmelt to replenish it.

What we don't getabout the water cycle

In school we teach children about the water cycle, in which water moves from the oceans to the sky to the land to freshwater basins and eventually back to oceans. In this telling, the water we use never really disappears.

But these tales gloss over something important: the water cycle can take decades or hundreds of years to complete a turn. Much of the fresh water we use every day comes from groundwater, which can take hundreds or thousands of years to accumulate. If we use water faster than it can be replenished, or pollute it and dump it into the seas faster than the natural water cycle can clean it, the resource will eventually run out.

If you instead think of water as a finite material being used up in much the same way as oil or gas, you quickly start to see its presence in every part of the economy. More than 70% of the water we use is put into food production, for example. But water is also used to make everything from T-shirts to cars to computer chips.

If they can't find enough water within their own borders, the thinking goes, why not just import it (embedded in food) from somewhere else?

Like its cousin the carbon footprint, a water footprint can be a useful shortcut to understanding a product's environmental impact-or your own. The water footprint of a cup of coffee is around 140 liters, for example. It takes about 15,000 liters to grow a kilogram of beef. A couple of slices of bread can rack up 100 liters. A kilogram of cotton (a pair of jeans and a shirt, say) can have a footprint of anything from 10,000 liters to more than 22,000 liters, depending on where it was grown.

This means that countries and companies, whenever they trade goods, are in effect moving massive amounts of water across borders. But because the water footprint of food or clothes or anything else is never acknowledged in this trade, the movement of water itself cannot be properly regulated.

Partly for this reason, richer countries such as Saudi Arabia and China have begun buying up land in other countries to compensate for their own lack of fresh water. If they can't find enough water within their own borders, the thinking goes, why not just import it (embedded in food) from somewhere else? The problem is that the places they've been shopping are themselves water-stressed, including countries in sub-Saharan Africa and the Sulphur Springs Valley in southwest Arizona.

Why Arizona? Because the land is cheap and well connected to airports, and because water-use regulations are almost nonexistent.

The United States is, in fact, the largest exporter of water on earth, according to Robert Glennon, a law professor at the University of Arizona and one of the country's leading experts on water policy. Glennon calculated that during a recent severe drought, farmers in the American West used more than a hundred billion gallons of water to grow alfalfa that was then shipped mostly to China.

Across the US, groundwater is regulated by the reasonable use" doctrine, which Glennon dismisses as an oxymoron of the first order." That policy permits limitless use of the water so long as it's for a reasonable purpose," he says, and everything is reasonable ... It's just a recipe for exploitive use of the resources."

Faster than you think

You might expect this to be a major international priority, but it's not.

Maggie White, a senior manager of international policy at the Stockholm International Water Institute (SIWI) and a longtime water advocate, says that even though water is everywhere and is needed for everything, it has never been prioritized in regulations because it doesn't have its own formal lobbying voice. The water needs of powerful industries like agriculture and energy get prioritized over the management of global water supplies.

White told me about the pushback she faced while trying to get the water crisis mentioned in the official texts of the Paris climate agreement in 2015. The sticking point for many negotiators was that water resources were seen as a local or national issue. As soon as they were brought into a multilateral agreement, they were perceived to rub up against sovereignty issues. Water has always been a source of contention between countries, so some might feel that there's good reason to keep water out of the conversation-but whatever the reason, any discussion of water was edged aside.

To see just how global the impending crisis is, you might head into space. Since it was launched in 2002, NASA's Gravity Recovery and Climate Experiment (GRACE) mission has measured how water moves around the world. It uses two satellites, each the size of a car, sweeping over the surface of the planet and responding to gravitational tugs from the masses below. When the two satellites move over a snowstorm or floods, the gravitational attraction of that extra water pulls the satellites closer to the surface. Over dry areas the satellites are less affected. By keeping track of the ups and downs of the satellites, scientists can map out regions of the world that are gaining or losing water over time.

Scientists already knew the ice sheets in Greenland and Antarctica were melting, but GRACE showed how much. Since 2002, Greenland has shed around 280 billion metric tons of ice annually, causing global sea level to rise by 0.8 millimeters per year. The Antarctic lost around 150 billion metric tons of ice per year in the same period. The glaciers of the Tibetan plateau and in Alaska and western Canada have retreated as well. GRACE also revealed that more than half of the world's major aquifers were being depleted, including those in California's Central Valley, the northwest Sahara, the Arabian Peninsula, India, Pakistan, and the northern China plain.

The two key causes: human overuse of groundwater supplies, and the extreme droughts brought on by climate change. The climate crisis and the water crisis are therefore interlocked. GRACE showed that human fingerprints on the freshwater landscape are the dominant force changing patterns of water availability around the world, and that the threats to water security are coming faster than you think.

The locals whose families have lived in the Sulphur Springs Valley of Arizona for generations have already figured this out. With the above-ground water sources drying up, and with the aquifer being depleted, many of them have had little choice but to leave their homes and farms behind.

Alok Jha is science correspondent at The Economist in London and author of The Water Book (Headline, 2015).