What pushes a major city to the brink of running out of water?

In early 2018, people around the world prayed for rain in South Africa. Outside Cape Town, the city’s reservoirs were down to 26 percent capacity. Inside the city, officials begged people to use just 13 gallons of water a day – just 20 percent of what the average American uses daily. Cape Town’s government said it would have to shut off drinking water to homes across the city. The police worried about riots.

By April, it became clear the city would just avoid reaching “Day Zero,” the date when Cape Town was forecast to run out of water. The world breathed a cautious sigh of relief. Cape Town’s predicament received a lot of attention as people across the globe grappled with the idea that a major city could run out of water — and wondered whether they could be next. Experts say Africa’s four largest cities — Cairo, Kinshasa, Lagos and Johannesburg — could all see their water supplies similarly strained soon, as could Karachi, Lahore, Mumbai and Kathmandu in Asia. Delhi is already struggling, and wealthier cities like Beijing and even Los Angeles will also face long-term supply problems if global warming continues to reduce rainfall in their traditional water supply areas.

“Cape Town’s situation really should be a wake-up call for many cities,” said Betsy Otto, director of global water programs at the non-profit World Resources Institute. “There are things cities can do right now, and should be doing, to prevent and insulate themselves against a similar fate.”

Otto and other experts say cities need to carefully consider their water supplies both from the demand side (how much water people use and waste) and the supply side (how much water the city physically has access to at any given time). Making water supplies more resilient requires foresight, political willpower and — particularly on the supply side — financial investment.

“If something hasn’t got political capital immediately then it really slips off the political agenda,” says civil engineer Holger Maier, who researches urban infrastructure at Australia’s University of Adelaide. Maier laments how easy it is for governments to put off big water projects until threatened by crisis.

Two factors will determine how vulnerable cities will be to water scarcity in the future: population growth and climate change.

Today, more than half of the world’s 7.6 billion people live in cities, and by 2050, the United Nations predicts there will be 9 billion people —  two-thirds living in cities, with the largest growth expected in Asia and Africa.

Climate change, meanwhile, is expected to shift rainfall patterns, making wet areas wetter and dry areas even drier, according to the UN’s Intergovernmental Panel on Climate Change (IPCC). This means relatively dry areas like the southwestern United States, southern Africa, and western Australia will likely see even less rain fill their lakes and rivers. Droughts will probably intensify as well, according to climate scientists in the U.S.

“There’s enough water in the world for everyone to have enough to eat and drink,” says Ian Makin, interim deputy director general of the International Water Management Institute, a nonprofit that develops sustainable water use strategies for communities in developing countries. The problem, he says, is that where and when that water is available “doesn’t match where people want to be.”

Many cities in dry regions rely on snow as well as rain. Snowmelt from the Sierra Nevada Mountains in central California, for example, is pumped all the way to Los Angeles. But over the last two decades, the amount of spring snow has generally decreased in the Northern Hemisphere, according to the IPCC. The western United States is already seeing less snowpack and earlier snow melt in the spring. By the end of the century, scientists predict 40 percent less snow will accumulate each winter in the western U.S.

Fast-growing megacities in the developing world are likely to have problems first, as urban infrastructure struggles to keep up with booming and “unprecedented” water demand, according to Timothy Williams, director in Africa for the International Water Management Institute.

These extreme water shortages can have damaging ripple effects. In Cape Town, Day Zero would have meant people waiting in long lines to receive their daily water rations, which officials worried may have sparked rioting. The ongoing drought in Sri Lanka could be a prelude to severe food shortages. There are major public health concerns, too: if a city can’t deliver water, people may turn to unsafe water contaminated with diseases such as cholera, typhoid or dysentery. Sanitation systems could break down and cases of dehydration and heat stroke could soar.

Cities can adapt to these strains, says Maier, the civil engineer. The first step is to start utilizing water more efficiently on the demand side — installing low-flush toilets, low-flow showerheads and fixing leaky pipes, says Allan Frei, a climatologist and deputy director of the Institute for Sustainable Cities at CUNY Hunter College. New York City reduced its water consumption from 1.5 billion gallons a day to 1 billion gallons just by plugging leaks and installing low-flow devices, says Frei. “That’s a huge savings.”

But cutting down on wasted water only goes so far. Cape Town won an award from an international environmental nonprofit for its water conservation program in 2015. These efforts, while important, did little to help Cape Town in the face of this year’s extreme drought. At some point, water-stressed cities will have to turn to the more expensive supply-side solutions, says Maier. That means building new water infrastructure like desalinization plants and water recycling systems – if they can afford them. A desalinization plant in Israel, hailed by MIT Technology Review as the “world’s largest and cheapest,” cost $500 million to construct. They can also take a while to come online — the Israeli plant started pumping four years after construction began.

It’s hard to find the political motivation and financial capital to act against a threat that’s still in the future, says Maier. But if cities don’t act now, they could be left out to dry. “I think there’ll be more and more cases like Cape Town where there are crises,” Maier says. “People will start to think, ‘How can we prevent this?’”