This past weekend, over 1,400 mayors from cities across the country gathered in Miami Beach for the annual meeting of the U.S. Conference of Mayors. This year — just weeks after President Donald Trump announced he was withdrawing from the Paris Climate Agreement — the conference officially endorsed a pledge supporting cities that commit to 100 percent renewable energy by 2035.

They're not the only local leaders moving quickly on environmental policy in the wake of Trump's decision. A bill to shift California to 100 percent renewable energy by 2045 just cleared a major hurdle by passing the state's Senate.

"This is the good news if one is looking for a silver lining in a very dark cloud," said Bill McKibben, environmental activist and founder of the grassroots climate campaign

"At least some people are starting to stand up, and the way they're doing it is, increasingly, to come together around these plans to take these cities or plans to 100 percent renewable energy in the pretty near future," he continued.

McKibben says scientific and engineering advancements in the last decade have brought down the costs of renewable energy dramatically — and made these political commitments possible.

"Ten years ago, maybe even five years ago, it was hard to imagine how we were going to get off gas or oil anytime soon," McKibben explained. "In most of the world, wind and sun are now the cheapest ways to generate energy."

To hear more from environmentalist Bill McKibben, click the audio player above.