Cities are at the heart of the world’s social and environmental challenges, so it’s nice to see the World Bank and mayors of the world’s biggest cities getting together to figure out a way to put financial innovation to work to solve those challenges. That’s the gist of an agreement signed yesterday at the C40 large cities climate meeting in Sao Paulo, Brazil, aimed at smoothing the path for financing city-wide climate change reduction projects. Reducing carbon emissions, pollution and waste is necessarily a holistic process, and cities tie together multiple entities — public transportation and traffic planning, electric and gas and water utility services, waste collection and recycling, and a host of departments with their own physical plant and workforce to manage more efficiently — that can coordinate efficiency improvements in a way individual private sector actors can’t. But when it comes to paying for it, cities need a common set of measures and standards to assess just what bang for the buck they’re getting out of these complicated, cross-departmental projects. That’s one of the key goals of the new World Bank-C40 agreement, and it could apply to “smart city” projects that use IT and networking to link city departments and city residents in energy and resource-saving projects as well. After all, the technology is at hand to help cities engage in “smart city” projects, but business models for delivering the technology aren’t as easy to figure out.