One of the most encouraging developments in the battle against climate change is that people around the world are moving to cities.

By the time this year’s college graduates reach retirement age, 3 out of every 4 people on the planet will live in metropolitan areas. Compared to rural residents, people who live in urban areas are more likely to take mass transit—or bike or walk—to get to work, and they are more likely live in smaller homes that require less energy to heat and cool.

It’s personal

The fact is, the carbon emissions and other pollutants that come with burning fossil fuels and that drive climate change are bad for our health. People do not want to live in cities where the simple act of breathing makes them sick and cuts their lives short. As more people demand that political leaders improve their air quality, carbon emissions will decline.

"Today, New York City’s air is cleaner than it has been in a half-century."

In New York City, we reduced our carbon footprint by 19 percent in just six years, and we did it while creating a record number of jobs. Today, New York City’s air is cleaner than it has been in a half-century.

New York is far from alone. Mayors around the country are taking action to improve their air quality and reduce their carbon emissions because they recognize that promoting economic growth and fighting climate change go hand-in-hand. They are investing in parks and public transportation, energy efficiency and renewable power. And they are experimenting with innovative, new ideas—some of which you’ll read about in this special section—and sharing their experiences with each other.

Uniting urban areas

A growing number of U.S. cities have also joined the Compact of Mayors, which commits cities to publicly disclosing their carbon reduction goals and reporting the progress they are making in meeting them, using a common measurement so that the public can hold them accountable.

City leaders are helping to show the international community that global cooperation on climate change is not only possible; it’s economically beneficial.