To date, C40 cities have taken on more than 5,000 initiatives that could reduce greenhouse-gas emissions by more than 1 billion tons by 2030 (the equivalent of ­making both Canada and Mexico carbon neutral). Los Angeles has replaced its outdoor lighting with LEDs, reducing energy use by more than 60 percent. The Empire State Building’s environmental retrofit will save it $4.4 million a year in energy costs. And Chicago is on track to reduce its greenhouse-gas emissions by 80 percent below 1990 levels by 2050.

These successes will be passed on to the next cities.

“C40 provides a good forum to exchange ideas and work with other municipalities to gain wider audience for our efforts,” says Chicago Mayor Rahm Emanuel.

One stark example of how this information sharing can improve environmental conditions rapidly is in the move to hybrid and electric buses in Latin America. São Paulo and Rio de Janeiro, along with Bogotá, Colombia, and Santiago, Chile, all were considering purchasing electric buses. Time spent testing in each city would have been as long as two years before purchasing decisions would have been made. By coordinating through C40, the four cities cut their testing times by 75 percent, speeding the delivery of clean public transit.

“Cities are pioneering new solutions in large part because mayors are pragmatists who are directly responsible for delivering results,” Bloomberg explains. “Cities understand the importance of immediate action, and that’s why I got involved in C40.”