CHICAGO – The cities must plant the seed.

That was a key message to mayors and other officials from around the world who gathered Thursday in Chicago to talk about sustainable growth and the battle against global warming, pollution and other threats to the environment.

“While nations talk, but too often drag their heels, I think it is fair to say it is the cities that are acting,” New York Mayor Michael Bloomberg said during a news briefing at an Organization for Economic Cooperation and Development conference.

During the conference, Chicago Mayor Rahm Emanuel touted the need for sustainable growth, saying efforts such as Chicago’s planned massive retrofitting project will not only create scores of jobs but will also drive down the city’s energy costs.

Emanuel talked about the city’s recent creation of a trust in which private financial organizations will invest in the retrofitting project and subsequent major infrastructure projects. And, he said, he hoped to hear what other cities are doing.

“Other mayors are facing similar, though unique challenges, and the question is do they have a strategy or interesting way of having done something that can be imported, adopted, and then done for your own economic growth uniquely your own way,” he said.

In that vein, Bloomberg announced the creation of an initiative by the so-called C40 Climate Leadership group that is made up of mayors from the world’s largest cities.

“Cities will learn from one another and work together in creating, evaluating and replicating new financing structures for improved mass transit, alternative power generation and other green projects,” he said.