For most of history, thirsty humans made do with what moisture fell from above: The sun warmed the salty seas, pure water evaporated into the air and then cooled and fell to the earth as precipitation. There it clung to glaciers, froze and thawed in lakes, was absorbed by plant roots, coursed through fractured bedrock, and seeped slowly through soil, into aquifers. Most of it returned to sea and sky all over again. There is as much of that water on the planet today as when the first amphibian flopped ashore; as much as when the ancient Greeks divined the future in the babble of brooks.

So why do experts in science, economics, and development warn that a "global water crisis" threatens the stability of nations and the health of billions?

From space, the idea of a global water crisis may seem perplexing: 75 percent of the planet's surface is blue. But usable fresh water is a tiny fraction of what we see – only 2.5 percent of the water on Earth. And two-thirds of that fresh water is locked away in glaciers, icecaps, and permanent snow. Of the stock of accessible fresh water, 99 percent is in underground aquifers – some are nonrenewable; and in some that are replenishable, ground water is slurped up faster by a growing population than it can be replaced.

But even so, say experts, the problem is perhaps more an issue of recognizing water's true value, using it efficiently and planning for the lean times, than it is a lack of overall supply.

The ongoing historic American drought, with its cascade effect on food and utility prices at home and food costs abroad, is an example of scarcity's effect.

But superstorm Sandy's deluge and flooding, says Geoff Dabelko, an environmental expert at Ohio University in Athens, is an example of how the term "global water crisis" can be misleading. It tends to imply that there's just one kind of crisis – a water shortage.

"The kind of dead-cow-carcass-in-the-desert image that global 'water crisis' evokes is very real for some people," Professor Dabelko says. "But there are so many dimensions." Too much water – whether from flooding, sea level rise, or more extreme storms – can be just as deadly as too little.

While the balance between water supplies and the demands of a burgeoning population are further complicated by the effect of climate change on delicate hydrological margins, there are those who say there is enough water, if nations learn to plan for a different future – one in which past abundance is no guide.

The growing thirst for water

Water is a part of everything we do: It feeds crops, powers cities, cools computer servers, and is key to the manufacturing of everything from clothes to cars. The billion more people expected on the planet by 2025 will increase water demand for all of those functions. And just to feed those people, water withdrawals for agriculture are expected to increase by about half.

But it's not only about the additional mouths to feed; it's also the growth of new appetites. Much of the growth in demand will emerge from the swelling sprawl of bustling, slum-pocked metropolises across the developing world. For the first time in history, the share of the global population living in cities recently surpassed 50 percent – on its way to 75 percent expected by 2050.

With each step up the economic ladder, people demand more water for sanitation, industry, hydroelectric power, and water-intensive diets – such as preferring beef to wheat, a shift that requires 10 times as much water per kilogram to produce. Urban-rural competition for water has already pushed countries to import grains – "virtual water" – or, in the case of wealthier countries like China, South Korea, and Saudi Arabia, to lease land in developing countries.

By 2030, the Water Resources Group forecasts, global water requirements may outstrip sustainable use by 40 percent. And almost half the world's people will be living under severe water stress, predicts the Organization for Economic Cooperation and Development (OECD).

Already, water stress – where the reliable water supply is being used up more quickly than it can be replenished – is widespread and is expected to increase significantly in the years ahead, particularly in North Africa, the Middle East, and Asia. By 2050, according to the UN's Food and Agriculture Organization, 1 in 5 developing countries will face water shortages.

Too many straws in the glass

In 2009, twin NASA satellites – orbiting 300 miles above Earth, measuring changes in the mass of underground water in northern India – yielded disturbing data: Excessive irrigation practices were sucking the region dry. Even though rainfall had been slightly above average, millions of tube wells – like too many straws in the glass – were draining ground-water levels by as much as a foot per year, threatening farm output in the country's fertile breadbasket and raising the risk of a major water crisis. Over the past seven years, an amount equivalent to nearly three times the water in Lake Mead, America's largest water reservoir, had been lost.

"If measures are not taken to ensure sustainable groundwater usage," NASA scientists concluded, "the consequences for the 114 million residents of the region may include a collapse of agricultural output and severe shortages of potable water."

If renewable water supplies – rainfall in lakes, streams, and rivers – are like an annually replenished checking account, then ground water and deep aquifers are the savings. A few thousand years ago, when civilizations first branched out from rivers, they populated areas where they could draw from that savings in the form of ground water 20 to 30 feet below the surface. Globally, this was the norm until the 1950s, when fossil fuel energy became widely available to allow pumping water from ever-deeper depths. Ever since, humanity has increasingly lived beyond the margins of its renewable water supply.

In ancient fossil aquifers – in the Great Plains of the United States, the North China Plain, or Saudi Arabia – water levels are not recharged by rainfall. Elsewhere, as in northern India, ground water is used faster than it can be replenished. According to the United Nations, ground-water extraction globally has tripled in the past 50 years, during which time India and China's ground-water use has risen 10-fold.

As a result, half of the global population lives in countries where water tables are rapidly falling. These supply problems are compounded by new land use patterns, like deforestation and soil grading, as well as leakage from poorly maintained infrastructure in cities.

Climate complications

To make things worse, climate change is expected to cause water shortages in many parts of the world, making ground water all the more important as a buffer.

The spike in global grain prices caused by the US drought last summer, on the heels of an epic winter drought in Spain and summer heat waves in southern Europe, showed the cascade effect of the sort of droughts that the Intergovernmental Panel on Climate Change expects will multiply in the decades ahead.

Not surprisingly, the greatest impact is on the poor. While American households spend, on average, 13 percent of their budgets on food, that expenditure is often 50 percent or more in the developing world. So a spike in food prices can trigger explosive riots like those that erupted there in the past five years.

According to Richard Seager, a drought specialist at Columbia University in New York, the recent US drought was mostly a result of naturally occurring weather patterns. But it's probably influenced by a background of unprecedented record-high temperatures that reflect an already warming environment. A significant recent development in climate research is that scientists have begun linking climate change to the probability of individual weather events. Professor Seager's own research predicts that, owing to climate change, the aridity levels experienced in parts of America during the Dust Bowl of the 1930s and again in the 1950s will be the new normal in the American Southwest by midcentury.

Seager says that climate change exacerbated the impact of superstorm Sandy, as well – contributing to higher sea levels: "We've known forever that hurricanes of this intensity can get up to New York City. Nothing there that couldn't be due to just natural variability. But it's happening with sea levels higher, and sea levels are still going up. So when these things happen, they can do extra damage because it's easier to breach the sea wall protections."

He says Gov. Andrew Cuomo and New York City Mayor Michael Bloomberg have accurately gauged the threat climate change poses to the city. "The sea levels are going to continue to go up. So, boy, do we have a problem. There's no reason to believe this won't happen again in the next two decades."

Climate change increases variability, says Dabelko: "That's the challenge that we have to [face] as individuals, as societies, as governments, as businesses ... to understand that it's going to change. It's going to be bigger swings. And some people may benefit ... and many are not going to be well adapted..."

There's also another misconception about the global water crisis, he says, which is the assumption that "it's somebody else's problem, on the presumption that we're wealthy enough to just deal.

"[S]o climate change, it's Bangladesh. Global water crisis, it's the Horn of Africa. It's somebody else's problem, and those [who] can't afford something are going to be the ones that suffer. Rather than understanding, 'Yes, it's global. Yes, it's a crisis. But it's also very meaningful for us even if we can insulate ourselves from the changes in food prices.' "

Consequences don't respect borders

"During the next 10 years, many countries important to the United States will almost certainly experience water problems – shortages, poor water quality, or floods – that will contribute to the risk of instability and state failure, and increased regional tensions," predicted the federal government's National Intelligence Council in an assessment of global water security earlier this year.

Annual river runoff and water availability in some high altitudes and in some wet tropical areas will actually increase 20 to 40 percent by midcentury, while it will decrease 10 to 30 percent in some already water-stressed dry regions in the mid-latitudes and in the dry tropics, according to the Intergovernmental Panel on Climate Change (IPCC). This may affect water resources in many arid and semiarid areas in the Mediterranean Basin, western US, Southern Africa, northeast Brazil, and much of Australia.

Over the next century, climate change will reduce the runoff from melting glaciers that feed major rivers, affecting water availability in important regions. More than one-sixth of the world's population relies on the meltwater from receding glaciers and snowpacks, including tens of millions of people in the Andes and hundreds of millions who depend on melt-water from the Hindu Kush and the Himalayas.

These supply problems will be exacerbated by bad water management, including the over-pumping of ground-water supplies, wasteful irrigation practices, deforestation, soil grading, leaking urban infrastructure, and faulty economic models that don't account for the true value of water, according to the National Intelligence Council.

In light of these changes, the OECD predicts that, by 2030, nearly half of the world's population will be living under severe water stress.

The problem is more than just water shortages. The risk of drought and flood would increase by the end of the century, according to the IPCC, and rising sea levels and deteriorating coastal buffers will increase vulnerability to coastal storms over the next few decades. While vulnerability will be greatest in urban areas of the developing world, where flood control structures are often poorly maintained, "at times water flows will be severe enough to overwhelm the water control infrastructures of even developed countries, including the United States," predicts the National Intelligence Council. Developing countries without the resources or ability to solve their water problems risk destabilizing social disruptions, even state failure, the report concludes, especially if the population believes the government is responsible.

While interstate conflict over water is unlikely within 10 years, beyond that time frame, water will increasingly be used as economic and political leverage between states and could even become a weapon, using dams to choke off water supplies to downstream neighbors or to flood them. Dams, desalinization facilities, canals, and pipelines may also make appealing targets for terrorist attacks.

If these problems are not managed, food supplies could decline, the risk of waterborne diseases could increase, and energy shortages might hamper growth. (More than 15 countries rely on hydropower to generate at least 80 percent of their electricity. And, in the US, nuclear, hydroelectric, coal-fired, and gas-fired power plants account for half of water withdrawals.)

Pessimistic scenarios may be averted

But the picture may not be as bad as it seems. While the projections about the growing global water crisis drastically underestimate how bad things really are, says Upmanu Lall, director of the Water Center at Columbia University, they also underestimate the scale of waste and the water efficiency improvements that could make adaptation easier.

"Things could actually be worse than what these guys are putting out," says Professor Lall. "They are too optimistic about the current situation compared to what it actually is. And they're too pessimistic about the situation for the future ... I do see a way to get there."

That's what he's learned from much of his work on water issues in India, which he calls "a basket case for water." He adds: "You could actually eliminate water stress in India if you were just a little bit smarter about which places you were procuring which crops from."

Science, he says, is part of the solution: Agricultural efficiency can be drastically improved with a better mix of what is grown where, accounting for geography, water constraints, and income; governments will have a role to play in setting economic signals to promote conservation and the right mix of crops, and regulation to ensure access in urban and rural areas; cheap soil-moisture sensors could improve agricultural water efficiency by 10 to 15 percent by reducing waste in irrigation systems; recycled waste water could save in the billions of dollars that the US spends purifying water up to drinking quality even though only 10 percent is used for drinking and cooking; flood-control systems can be repurposed to store water.

But most important, says Lall, "the economics of it has to be sorted out." Water allocations for personal consumption and ecological preservation should be protected, he said, but about 75 percent of water consumed globally should be subject to more competitive pricing. In a sense, he argues, water should be treated like oil, allowing developers a guaranteed allocation as an incentive to develop it. About a quarter of water supplies should be protected to ensure people have water for drinking and to preserve ecology, he says. But everyone – from the home-owner watering the lawn to big industry and agriculture – should pay more for water.

Instability, conflict, and economic stagnation may be the prod societies need before they adapt, says Lall.

He deems the US system for allocating water rights as "not too bad." Where those rights were not tradable, he says, "things are a mess."

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Some states – Arizona, California, Idaho, and Texas – have water banks that facilitate leases between rights-holders and users. But since these water banks don't incorporate forecasting, they fail to make deals until a drought begins. What the US needs, says Lall, is a national water policy that incorporates forecasts, trading mechanisms, options, and the coordinated use of both surface and ground-water resources.

While the tools and strategies exist to cope with the impending pressures of a warmer and more populous planet, Lall says, "the question is, will we do it right?"