Cape Town is withering. If current projections hold, the South African city of 4 million will run out of water on May 11, known as Day Zero. It’s been three long years of drought—we’re talking a once every 1,000 years kind of problem that Cape Town’s water infrastructure just wasn’t built for.

The irony is that a whole sea of water laps at the shores of the coastal city. But if you wanted to drink it, you’d have to build an expensive, energy-intensive desalination facility. Cape Town is indeed rushing to bring such projects online, at least on a temporary basis, and in so doing is exposing a dire reality: Pockets of humanity around the world may have to rely on the sea to survive drought in the very near future. Because it’s likely that climate change is exacerbating this drought.

Models show that for certain parts of the world, things are going to get real hot and real dry. The American South, for instance, could see a tripling of 95-degree-plus days per year by 2050. “Cape Town is a warning shot for us,” says Michael Kiparsky, director of the Wheeler Water Institute at UC Berkeley. “What we can see is that it’s very possible for water crises—which emerge all the time around the world—to get close to the point of real, massive human disaster.”

The key to managing water is diversifying. Think of it like stocks—if you’re all in on Enron and Enron implodes, so does your money. But invest in a range of companies and you can hedge against uncertainty. Same goes for water sources. Dams, however ecologically terrible their impacts, let you save up a stock of water. You may even decide to treat wastewater to boost your supply. And you’ll of course want to convince your populace to save water, even in times of plenty.

Cape Town does not have a stellar portfolio. “The diversification of our water sources would have helped a whole lot earlier,” says environmental scientist Kevin Winter of the University of Cape Town. “It’s difficult to do that because you need sometimes these triggers to be able to change the budgetary system and be able to think differently about a long-term strategy.”

The city is certainly triggered now. (The local government was unable to provide comment before this story published.) And what it’s been able to do in recent months is pretty remarkable, at least from a public education perspective: A city that once consumed 290 million gallons of water a day now uses 160 million gallons. But that’s still a whole lot of water in a region where rain just refuses to fall.

So Cape Town is turning to desalination to tackle the shortfall. Specifically, temporary reverse osmosis plants that’ll spin up in the coming months and provide fresh water. Not much of it in the grand scheme of things—4 million gallons a day—but still a start.

Desalination is not a new idea. For decades now, researchers have been doggedly exploring the technology, which comes in two flavors. The first you can do at home if you like, just boiling water to collect steam and leave salt behind. The second is reverse osmosis, and involves forcing water through a permeable membrane to filter out the salt. Problem is, boiling water takes a ton of energy, as does pumping water.

The technology is improving. Fancy new materials, like membranes just an atom thick, are making reverse osmosis more efficient. (That is, making it easier to push that water through.) “Desalination technology is going to change considerably in the coming years,” says Winter. “I think what the city is currently doing right now is to go slowly with its experiments and it will start to ratchet those up in time.”

Which has some scientists crying foul. Late last year, a group of researchers published a paper detailing how desalinated water could theoretically be tainted by sewage piping into the waters off Cape Town. In their samples of seawater they found 15 pharmaceutical and household chemicals, as well as nasty microbes like E. coli. These are not things you’d want to suck into a desalination plant and turn into drinking water without some serious testing and purification if necessary.

On top of the potential pollutants coming out of a desalination plant, there’s also the byproduct of brine (lots and lots of salt), which is pumped back out to sea, potentially disrupting ecosystems. That and desalination plants can kill sea critters by hoovering them up. “It doesn’t make sense to me to solve one ecological problem by creating a whole lot more,” says the University of Cape Town’s Lesley Green, co-author of the paper, “which is saltier seawater and not managing the discharge of medicinal compounds and persistent organic pollutants.”

Desalination may also present unexpected social costs in Cape Town, because not every citizen would benefit from it. “At home I have water, it flows out of the tap,” says the University of Cape Town’s Tom Sanya, an architect who specializes in sustainable design. “But we have a significant number of people in the informal settlements of Cape Town who don’t have water flowing in their homes. If the city has up to now failed to supply each individual resident in Cape Town with water, then I can’t be convinced that after investing heavily in technologies we’ll have enough money left to invest in distribution.”

Still, in Cape Town, the ecological and social costs of desalination may pale in comparison to the consequences of not turning to the sea for help. The energy costs of the technology are still huge, but Israel has proven it can be done on a massive scale: The nation now makes more freshwater than it needs. And as certain parts of the world descend into a new era of heat and dryness, desalination is going to look like a mighty tempting solution.

“It kind of depends on how bad you need the water,” says engineer Amy Childress of the University of Southern California. “And that’s exactly where South Africa is, and it’s where California would have been if we didn’t have a rainy last year. It really is pure and simple—how bad you need the water and how unlucky you are with the drought.”

Cape Town has been very, very unlucky. But it’s taking steps to diversify its water portfolio, and the rest of the world would be wise to follow. Otherwise it’ll be Enron for the lot of us.