82 major cities are meeting this week to talk about how to limit the worst effects of climate change

Bangkok and Miami could be underwater in a matter of decades if sea levels keep rising. Cape Town could see severe food shortages because of drought.

In Mexico City, which gets so smoggy in the spring that parents keep their children indoors, hotter temperatures could make the pollution even worse.

Global warming poses an existential threat to many of the world’s cities. But cities are responsible for more than 70% of the carbon dioxide and other greenhouse gases that trap heat in the atmosphere, so they have mostly themselves to blame.

This week, officials from Los Angeles, New York, Tokyo, Beijing and 82 other major cities — including more than 40 mayors — are meeting in Mexico City to share ideas about how to reverse course.

The cities belong to C40, a network launched in 2005 to combat climate change. In an analysis released Wednesday, the group said that actions taken over the next four years will determine whether cities do their part in meeting the goals of the Paris agreement on climate change signed last year by nearly 200 countries.

The pact seeks to keep global average temperature rise to well below 2 degrees Celsius above pre-industrial times in a bid to prevent the worst effects of climate change. With the aim of meeting that goal, the C40 analysis set emissions targets for 84 cities.

It calls for a wide variety of carbon-cutting measures, including more recycling, more charging stations for electric cars, the creation of car-free zones and other green spaces. The effort would require an investment of $375 billion over the next four years and $1 trillion between now and 2050.

On Wednesday, a posse of mayors toured the Mexico City subway system and took a ride on a new electric bus.

“Cities are rising to the climate challenge,” said Paris Mayor Anne Hidalgo, the chair of C40.

The initiative comes as the Paris agreement appears to be in peril.

U.S. President-elect Donald Trump, who has called climate change a “hoax,” has vowed to “cancel” the climate pact. The United States is the world’s second-largest emitter.

Los Angeles Mayor Eric Garcetti, who attended the conference, said now more than ever it is more important that cities step up to tackle climate change.

“The actions at the local level are more important than those at the national level,” he said. “We are going to continue to make sure the power we have as mayors cannot be taken away.”

Garcetti was one of 39 U.S. mayors who recently penned a letter to Trump, calling on him to continue efforts to combat climate change.

“The cost of prevention pales in comparison to cost of inaction, in terms of dollars, property and human life,” it said.

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