As chair of the C40, Bloomberg wields an outsized amount of power in the climate-change battle. Cities account for 70 percent of the world’s greenhouse gases and contain half the world’s population.

“The federal government has largely failed to act on any of these issues,” Bloom­berg says. “Whether tackling the environment or crime or education or transportation, mayors are problem-solvers. We’re not interested in national party ­politics. Residents hold us accountable for improving conditions in their communities. That’s why cities have become laboratories of change.”

Since becoming C40’s chairman, Bloomberg has shaped an underfunded, informal organization into one doing more than almost any other group to improve the environment. He has provided significant funding through Bloomberg Philanthropies but has developed a secession plan that includes setting up C40 as a 501c3 so it can solicit donations from individuals and philanthropic organizations and engage corporate partners where appropriate.

“We always knew he wouldn’t fund it forever,” Aggarwala says. “The next chair will find the C40 with a diversified set of funders.”
 
In addition, Bloomberg worked with former President Bill Clinton to fully merge the Clinton Climate Initiative Cities Program into the C40. The C40 now has a staff of more than 30 around the world and 63 member cities­ representing 8 percent of the population and 21 percent of gross domestic product.

“The ultimate decision to integrate the two organizations was no doubt driven by the existing relationship between Mayor Bloomberg and [former] President Clinton,” says Bruce Lindsey, former CEO of the William J. Clinton Foundation. “They shared a conviction and a vision that cities can lead the way to protect our environment.”