The six largest cities in America, the mayors of which each signed Garcetti’s letter, are home to about 21 million people just by themselves. Add to that the 40 million people in the state of California—where governor Jerry Brown has promised (paywall) to “take significant action” if the US abandons the Paris agreement—and Trump’s potential action at the federal level is significantly undermined.

America’s urban areas are home to more than 80% of the country’s population. As social scientist and visionary political theorist Benjamin Barber, author of If Mayors Ruled the World, told Quartz in a Q&A last year, “About 80% of greenhouse gas emissions come from cities and cities also control about 80% of GDP. They can do a lot to combat climate change, whether or not Trump undermines the COP21 agreement.”

Barber, who died of cancer in April, days after his last book Cool Cities: Urban Sovereignty and the Fix for Global Warming was published, also argued that cities’ power extends far beyond complying with the Paris agreement.

That’s shown by a host of initiatives US mayors are taking to cut carbon emissions locally. Atlanta mayor Kasim Reed, for example, claims he can cut emissions by 25 percentage points in his city by picking off “low-hanging fruit” like retrofits on wasteful buildings and incentives to businesses to cut energy use. “To take action in the city of Atlanta, the center of the ninth-largest metro in the country…it takes my decision and eight votes from [the city] council,” he said in an interview last year. “In my opinion, that’s very efficient.”

It isn’t just altruism that compels the nation’s cities to take action. With solar panels becoming cheaper than fossil fuels in 2016, it’s making less and less economic sense to shy away from alternative energy sources. “Even if you didn’t believe in climate change, [green energy] is a pure driver of economic activity,” Chicago mayor Rahm Emanuel said in December. “The city is never going back from this.”

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