Officials in Cape Town, South Africa, recently announced that the city will run out of water, perhaps as soon as April. On “Day Zero,” Cape Town will turn off the taps, leaving some 4 million people without basic access to water. Residents are bracing for the worst, with many fearing a breakdown in public order amid rising social tensions.

But Cape Town isn’t the first or only major city to face the risk of running dry. In 2015, Sao Paulo, Brazil, faced a similar drought-driven disaster, with officials warning residents they might need to leave the city limits to find enough water to bathe. In the end, drastic water restrictions and short-term technical fixes averted catastrophe for Brazil’s largest city.

These aren’t isolated incidents. Here are three things to know about climate change and urban drought:

1) Technology is no longer enough to fix the problem.

Over the thousands of years that humans have been living in cities, we’ve settled pretty much everywhere, including places with little fresh water. For the most part, we’ve devised ingenious technical solutions to quench our cities’ endless thirst. Clever use of gravity and the advent of mechanized pumping made it possible to pipe water from hundreds of miles away or from aquifers deep underground.

In more recent times, railroads and refrigeration removed the need to live close to rain-fed farmland. This meant cities could bloom in previously inhospitable places, such as the Arabian Peninsula. But in some places, we may be testing the planet’s hydrological limits.

For cities like Cape Town — or Los Angeles, Beijing or other major cities facing water shortages — climate change is expected to produce longer, more frequent and more unpredictable droughts. Droughts, in turn, can push rural or semi-rural families to nearby cities in search of work. As urban populations surge in line with these and other population trends, many urban water-supply systems can’t keep up with demand, compounding the problem.

Many cities are running out of technical solutions to deal with water scarcity. In places like Sao Paulo and Cape Town, engineers have tapped most of the locally available water resources, and the economic and environmental cost of bringing water from even farther away is likely to be increasingly prohibitive.

2) Politics puts some other options out of reach.

Complex schemes to transport water from distant regions generate plenty of political and social conflict. My own research shows that, even in authoritarian countries like China, such plans often stoke tensions between urban and rural areas and generate protracted disputes between cities, states and provinces.

Besides long-distance water transfer, cities can turn to technologies such as desalination and wastewater recycling to boost the supply of available fresh water. But these are expensive technical fixes, and many cities can’t afford them. Good old-fashioned political economy also plays a role: Mayors and other elected officials are reluctant to raise prices on something as indispensable as water, which leaves many water utilities lacking the money for major capital investments.

Cities can also encourage residents to use less water, but such campaigns must be carefully designed to be effective. Moreover, there’s a limit to how much people are willing to reduce water use on their own. Cape Town’s mayor conceded with startling frankness in recent weeks that despite a major water-saving campaign, “We can no longer ask people to stop wasting water. We must force them.”

Given these drastic choices, some governments are considering encouraging people to move closer to supplies of fresh water, rather than the other way around. In 2007, a Chinese official wrote an editorial urging that the country’s capital be shifted from “water-poor” Beijing to the wetter south, an idea that has since been widely debated.

For the poorest, driest and most war-torn countries, there may be no alternative. In 2007, Yemen’s water minister advised residents of the country’s high-and-dry capital, Sanaa, to move to the coast. “Many of the city’s people,” he warned, “will simply have to move away.”

3) The cost of inaction is high.

Yemen is an extreme example, but it’s one that raises many difficult questions for scholars and policymakers. While presumably no one wants to see people forced to move for lack of water, it’s not at all clear how cities such as Sanaa can cope with a toxic cocktail of water scarcity, population growth and low technical capacity.

Richer cities have shown an impressive ability to adapt to water scarcity and shortages: Las Vegas, for example, has become highly water-efficient, in part because of aggressive water-use restrictions. But even these cities are likely to have to answer a tough political question: Just how high a price are residents willing to pay to live in a city where growth may require ever more elaborate, expensive water-supply systems?

In many places, it’s hard to answer the question directly because governments often subsidize water-supply infrastructure, as well as unsustainable practices such as over-pumping groundwater. But the costs of failing to tackle these perverse incentives is likely to be much higher. Recent research has shown that drought can be devastating for urban economies, with an impact up to four times higher than that of flooding.

In Cape Town, for instance, the critical tourism and wine industries reportedly have already suffered downturns because of the drought. But while these industries may soon recover, the city’s poor may well suffer considerably, with some already making a difficult and demeaning trek to wealthier neighborhoods blessed with natural springs.

At a minimum, the world’s urban water crisis is likely to stoke much-needed dialogue among social scientists, city planners and hydrologists about how to adapt to an era of shifting and often shrinking urban water supplies.

Scott Moore is a senior fellow at the University of Pennsylvania and the author of the forthcoming book “Subnational Hydropolitics (Oxford University Press), which examines the spread of water conflict and how to prevent it.