MEXICO CITY — On bad days, you can smell the stench from a mile away, drifting over a nowhere sprawl of highways and office parks.

When the Grand Canal was completed, at the end of the 1800s, it was Mexico City’s Brooklyn Bridge, a major feat of engineering and a symbol of civic pride: 29 miles long, with the ability to move tens of thousands of gallons of wastewater per second. It promised to solve the flooding and sewage problems that had plagued the city for centuries.

Only it didn’t, pretty much from the start. The canal was based on gravity. And Mexico City, a mile and a half above sea level, was sinking, collapsing in on itself.

In the immense neighborhood of Iztapalapa — where nearly two million people live, many of them unable to count on water from their taps — a teenager was swallowed up where a crack in the brittle ground split open a street. Sidewalks resemble broken china, and 15 elementary schools have crumbled or caved in.

Much is being written about climate change and the impact of rising seas on waterfront populations. But coasts are not the only places affected. Mexico City — high in the mountains, in the center of the country — is a glaring example. The world has a lot invested in crowded capitals like this one, with vast numbers of people, huge economies and the stability of a hemisphere at risk.

One study predicts that 10 percent of Mexicans ages 15 to 65 could eventually try to emigrate north as a result of rising temperatures, drought and floods, potentially scattering millions of people and heightening already extreme political tensions over immigration.

The effects of climate change are varied and opportunistic, but one thing is consistent: They are like sparks in the tinder. They expose cities’ biggest vulnerabilities, inflaming troubles that politicians and city planners often ignore or try to paper over. And they spread outward, defying borders.

That’s what this series is about — how global cities tackle climate threats, or fail to. Around the world, extreme weather and water scarcity are accelerating repression, regional conflicts and violence. A Columbia University report found that where rainfall declines, “the risk of a low-level conflict escalating to a full-scale civil war approximately doubles the following year.” The Pentagon’s term for climate change is “threat multiplier.”

And nowhere does this apply more obviously than in cities. This is the first urban century in human history, the first time more people live in cities than don’t, with predictions that three-quarters of the global population will be urban by 2050. By that time, according to another study, there may be more than 700 million climate refugees on the move.

For many cities around the world, adapting to climate change is a route to long-term prosperity. That’s the good news, where societies are willing to listen. But adaptation can also be costly and slow. It can run counter to the rhythms of political campaigns and headlong into powerful, entrenched interests, confounding business as usual. This is, in effect, what happened in New Orleans, which ignored countless warning signs, destroyed natural protections, gave developers a free pass and failed to reinforce levees before Hurricane Katrina left much of the city in ruins.

Unlike traffic jams or crime, climate change isn’t something most people easily feel or see. It is certainly not what residents in Mexico City talk about every day. But it is like an approaching storm, straining an already precarious social fabric and threatening to push a great city toward a breaking point.

As Arnoldo Kramer, Mexico City’s chief resilience officer, put it: “Climate change has become the biggest long-term threat to this city’s future. And that’s because it is linked to water, health, air pollution, traffic disruption from floods, housing vulnerability to landslides — which means we can’t begin to address any of the city’s real problems without facing the climate issue.”

There’s much more at stake than this city’s well being. At the extreme, if climate change wreaks havoc on the social and economic fabric of global linchpins like Mexico City, warns the writer Christian Parenti, “no amount of walls, guns, barbed wire, armed aerial drones or permanently deployed mercenaries will be able to save one half of the planet from the other.”

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https://www.nytimes.com/interactive/2017/02/17/world/americas/mexico-city-sinking.html