Driving through Sao Paulo yesterday, I was struck by the power of cities. While cities are part of the climate change problem, they need to be part of the solution too. They are bigger and more energized than any individual or organization. Cities push and cajole; and cities act. Cities are where it all comes together.

Even more so when former President Bill Clinton, World Bank President Robert Zoellick, and New York City Mayor Michael Bloomberg joined forces in Sao Paulo. The accomplished gentlemen born less than a dozen years and 1,500 miles apart spoke and fielded questions with a worldly and gracious informality. The pleasant exchanges sat in contrast to the underlying gravity of their mission. Together they have determined to access their considerable resources to tackle one of the biggest challenges they’ve ever faced: climate change.

The location for the partnership launch is telling. With Mayor Kassab of Sao Paulo hosting this week’s C40 Large Cities Summit everyone reinforced the need for cities to be in this fight. C40 is a group of mayors of major cities of the world responsible for 12% of global emissions. It is not hard to imagine that the battle for sustainable development will be won or lost in our cities.

On behalf of the World Bank, President Zoellick and Mayor Bloomberg, representing the world’s most influential cities as Chair of C40, signed a Partnership MOU outlining how the two organizations will work more closely together and provide focused support to cities. The MOU outlines common tools and metrics, city- tailored finance, and enhanced city-to-city learning.