Thinking about remaking a whole city in these ways can be daunting, Farr said. But breaking it down into such neighborhoods--early examples of which are under construction around the country--offers a means to assembling them into far larger regions, he noted. "I believe in promoting neighborhoods that are organized in corridors. Then a region becomes a collection of corridors."

Bob Yaro, president of the Regional Plan Association, a civic group serving the New York metropolitan area, takes that idea a step farther: the association is hoping to galvanize support for emerging "megaregions" as much as 600 miles across in which several cities and their suburban regions are linked by affordable and frequent fast trains. "Much as cities were connected by interstate highways, megaregions will be organized by high-speed intercity rail," Yaro said at the conference.

Meanwhile, his group, together with New York City's government, is promoting green developments and transit enhancements within the city, part of a public-private effort that aims for a 30 percent reduction in local greenhouse-gas emissions. It is one of several such urban efforts under way around the country.

Of course, getting such visions realized won't be easy. But cities, with their economic and voting mass, can get the job done, Yaro said. "Cities have got to lead in this effort. We need the impetus of global treaties and national policy. But in the end it will be the great cities, the Washingtons and New Yorks, that will achieve these climate goals."