Most importantly, though, Bloomberg has been able to think more grandly and reshape the role of a big-city mayor. He has placed a keen focus on public health and the impact of climate change, two issues often bogged down in bureaucracy and political yammering. In just one decade, he has made great strides in improving the quality of life for New Yorkers and has ensured that, after he leaves office, these policies will continue and grow.

“Mayor Bloomberg has what it takes to be a leader on a global scale, and his vision is one that truly showcases his leadership skills,” says Eduardo Paes, the mayor of Rio de Janeiro. “That kind of vision, both ambitious and realistic, is certainly the result of his experience as a businessman and as mayor of New York.”

With his time in office waning but his ambition growing, Bloomberg has done what any passionate, philanthropic billionaire would do: He moved from trying to save his city to trying to save the world.

Since 2010, he has served as chairman of the C40 Cities Climate Leadership Group, a collection of mayors of megacities around the world working to reduce greenhouse-gas emissions and address climate risks. Using his philanthropy, leadership and vision, he has turned the C40 into an environmental powerhouse, an on-the-ground think tank that works with cities to improve quality of life and shares best practices to speed international development of tools that reduce environmental damage. As he looks to a ­future away from public office, it is likely his role in the C40 will solidify his legacy as a pragmatic environmentalist who developed real solutions while the rest of the world bickered and watched the planet get warmer.

“We’re not elected to debate; we’re elected to get things done,” Bloomberg says of the mayors in the C40. “As chair of the C40, I’ve worked to bring cities together to share resources and to collaborate where we can. We all have the same goal: acting locally on climate-change efforts that will have a significant impact globally.”