Bright Lights? Big Cities.

As I type these words, representatives of more than 170 countries have assembled in Paris, where their commitments to reducing carbon emissions are being formalized and codified into what’s being billed—accurately, one hopes—as the most significant international climate agreement ever reached. News about the conference, known informally as COP21, is dominating world headlines. What will the United States really be able to deliver, in the end? Can China make good on its promise? What's Canada up to? Whither India?

Since the conference is taking place under the auspices of the United Nations, it makes perfect sense that national contributions to global emissions are the primary focus of a good deal of the media attention. But while the carbon commitments of member nations are the subject of countless news stories around the world, reporters and editors covering COP21 should be wary, as always, of burying the lede. Because the real story here may not be about what the world’s countries are doing, individually, to tackle climate change. The real story may well be about what the world’s cities are doing, collectively.

In 2014, a report from the U.N. confirmed what many demographers had been saying for decades: The majority of the earth’s human beings are now living in its cities. Between 1950 and 2014, the planet’s population of urban dwellers shot up from 30 percent to 54 percent. What’s more, by the middle of this century, fully two-thirds of us will be living in cities. Even as humanity’s total numbers continue to climb, our rural population by 2050 is expected to drop from 3.4 billion to 3.1 billion.

To get a sense of why cities need to be a bigger part of the fight against climate change, consider just a few remarkable statistics.

• There are six million more people living in the Indonesian capital of Jakarta than in all of Australia.
• The population of metropolitan Tokyo is larger than the combined populations of Greece, Norway, Ireland, and the Netherlands.
• If everyone living in Beijing and Shanghai suddenly decided to do a house-swap or apartment-swap with all of Canada, there’d still be plenty of empty houses and apartments in China’s two biggest cities—but many of Canada’s Chinese visitors would have to double up.

In Paris, nations will be the ones to sign on the dotted line. But after that momentous event has taken place, we should look to the world’s cities for signs of how we’re actually—as in physically, politically, and realistically—going to get things done.

Many of these cities began taking dramatic measures to shrink their carbon footprints long before COP21’s signatories had even booked their Paris travel. As Michael C. Bloomberg, the former mayor of New York City and the current U.N. secretary-general’s special envoy for cities and climate change, has frequently observed, cities account for 70 percent of our total greenhouse gas emissions—giving them a special responsibility for figuring out how to curb them. But, he’s quick to add, “as centers of innovation and progress…they also present the greatest opportunities for action.”

During his mayoralty, Bloomberg took major steps to lower New York City’s carbon output, including devising a plan to cut municipal energy consumption and reduce greenhouse gas emissions by 30 percent by the year 2017—a plan that will soon keep at least 34,000 tons of CO₂ from entering the earth’s atmosphere annually. Meanwhile, his counterparts in other megacities have been bringing every last ounce of their considerable executive power to bear on the issue, slashing emissions with such vigor that the entire nation in which they reside reaps the benefit.

In London, for example, mayor Boris Johnson has already undertaken a plan to cut his city’s carbon pollution by 60 percent (relative to 1990s levels) over the next 10 years. In one year alone, 2013, London cut its emissions by fully 11 percent, one of many accomplishments that allow it to now boast the lowest carbon profile of any city in England. And between 2008 and 2012, Mexico City—once considered to be among the world’s most polluted megacities—reduced its emissions by 7.7 million tons, exceeding the goal it had set for itself by more than 10 percent.

Today, when mayors of a dozen major world capitals meet at Paris’s City Hall to discuss the special place cities have in the fight against climate change, expect to hear more success stories along these lines. And when officials from member nations formally pledge their carbon reduction targets, don’t think for a second that they won’t be contemplating—as they sign—the oversize roles that their cities will have to play in keeping these promises.

As go the world’s cities, so goes the world.