With rising global populations and an already unsustainable reliance on depleted groundwater sources, the world is facing major global water supply shortages––and in some places, the effects have already landed.

According to the United Nations' annual World Water Development Report released last month, about 20 percent of the world's groundwater supplies––water that we collect from beneath the Earth's surface (as opposed to surface water)––are "overexploited." But that problem, the report says, will only get worse by 2050, when the global population is expected to reach more than 9 billion, driving demand for water up by more than half. On a shorter timeline, if meaningful changes to how we manage water resources are not enacted soon, the world could be facing a "40 percent global water deficit" within 15 years––left with only 60 percent of what it actually needs, according to the report.

"Unless the balance between demand and finite supplies is restored, the world will face an increasingly severe global water deficit," the report notes.

"[B]usiness as usual," Richard Connor, the lead author of the report told Reuters, is leading to a "collapse in our global socioeconomic system."

What's causing this problem?

The report notes that one big issue is rapid urbanization, citing predictions that by 2050, some two-thirds of the world's population will be living in cities. But in developing countries, where 93 percent of the world's urbanization takes place, fast-paced city building can often outpace efforts to keep water supplies at pace with demand. In sub-Saharan Africa, for example, urbanizing efforts have led to an almost ten percent decrease since 1990 in the proportion of residents with access to piped water on their premises.

"The spontaneous urbanization, which creates slums makes it very difficult, because of the layout of the slums, to provide water," Joan Clos, executive director of the UN Human Settlements Program, told Reuters.

But individuals' access to water isn't the only concern. As global populations increase at an estimated rate of 80 million per annum, water demands for supplying societal infrastructure becomes even more dire. By mid-century, according to the report, food needs will have increased 60 percent, and water needs for industry are expected to shoot up 400 percent. What's more, the predicted 70 percent increase in energy at the same time will no doubt require more water, which is a key factor in many forms of energy production.

What all this could lead to––and indeed has already led to––is a reckless quest for more groundwater, and exacerbated existing problems. According to a recent report by the investigative outfit Reveal, pumping too much ground water is not only causing wells to dry up, but it's also causing oceans to rise as it's pumped back out through towns' and cities' drainage networks. Since 1990, around 4,500 cubic kilometers of groundwater has been pumped back into the oceans––about enough water to fill Lake Tahoe 30 times. Groundwater's small-but-significant impact on sea level––about six or seven percent of overall sea level rise over the last century––is only set to go up. It's also causing soil of some coastal cities to become salinized, or dredged with salt. In fact, the report notes that inefficient use of freshwater has led to salinization of 20 percent of the global irrigated land area so far. But it's important to keep in mind that groundwater's contribution to rising sea levels––and the salinization that leads to––is on top of increases linked to rising global temperatures.

Where we're already seeing an impact.

Experts point to cities they see as being exemplars of good water usage for possible solutions. In Cyprus, for example, farmers can upgrade to higher efficiency irrigation system thanks to subsidies and low-interest loans, but it's unclear how realistic it is to implement such drastic reforms in developing countries where many governments struggle to provide access to clean water to expanding populations, let alone assure responsible use of it.

But some places are already feeling the effects of using too much water.

Recently, California Governor Jerry Brown officiated California's much-publicized water shortage problems by issuing an executive order to cut water usage in his increasingly dry state.

"Today we are standing on dry grass where there should be five feet of snow," Brown said at California's annual snow survey in the Sierra Nevada mountains. "Therefore, I'm issuing an executive order mandating substantial water reductions across our state. As Californians, we must pull together and save water in every way possible," he continued.

The order came after the Los Angeles Times ran a nerve-rattling Op-Ed recently, with a headline warning that "California has about one year of water left. Will you ration now?" Jay Famiglietti, the scientist who wrote the piece, warned that not only are the state's reservoirs running dangerously low, but also that groundwater there is being used at unsustainable levels. And as the state enters its fourth year of drought, according to Famiglietti, there is no current management plan to sustain groundwater reserves.

Brown subsequently announced a $1 billion emergency drought relief plan, but Famiglietti stressed the need for new systems of management. "We're not going to reverse climate change," he told PBS. "I hope that it rains, but you can't make it rain."

Others, like Connor, the lead author of the UN report, pointed to some of the drastic effects groundwater depletion and a water deficit might have. "California is in particular trouble, which means the U.S. is in trouble because California is where a lot of the U.S.'s food is produced," Conner told Vice News.

"In all of these places people rely on ground water for irrigation and other uses," he continued. "And in a lot of cases the ground water isn't used sustainably, they are basically just taking money out of the bank."