In January, Seattle’s Greg Nickels, used his state of the city address to ask, “If the federal government is not going to sign on to the Kyoto Protocol, why can’t we just do it at the local level?” In the five months since, mayors representing 32 million Americans, pledged that their cities would meet Kyoto Protocol guidelines of a 7 per cent cut in greenhouse gas emissions. Denver, Los Angeles, New York and more than 140 other American cities have signed up to the Kyoto targets.

Toronto is leading the way, coming up with imaginative measures to reduce emissions. It is using cold water from deep in Lake Ontario to cool its buildings in summer, saving up to 90 per cent on electricity compared with conventional air-conditioning systems. It has also established an Atmosphere Fund to support local investment in energy efficiency and renewables. Phil Jessup, who heads the fund, was in London last month helping the city’s mayor, Ken Livingstone, draw up a similar initiative. Livingstone has said he wants every large new building in London to be equipped with solar panels.

“Toronto is undoubtedly the greenest city in North America,” says veteran climate campaigner Adam Markham. “But big US cities like Seattle, Austin in Texas, Portland in Oregon and Salt Lake City have all launched big climate initiatives, and they are starting to make the White House very nervous. Not every mayor that signs the Seattle Kyoto pledge is going to be able to deliver, but a huge amount is happening on the ground at municipal level now.”

Tomorrow, World Environment Day, city mayors from five continents will meet in San Francisco to push their environmental agenda. They plan to sign up to a set of 21 “urban environmental accords” covering energy, waste reduction, urban design and other goals, complete with a scorecard to measure their progress.

“Municipal governments have the power to shape the future of the world’s environment,” says Jared Blumenfeld, environmental director for San Francisco, which has signed up to the accords.

I believe that these city governments are demonstrating that there is an enormous, latent, green sentiment that is building, and could one day be a decisive political force. Signs of green sentiments are now popping up in the most unexpected places–from evangelical Christians even to some corporate board rooms.

Turning a green movement into a Green movement is the challange facing our party. We must be sucking up every creative green idea that comes along and molding it into a coherent Green worldview. Much is riding on the outcome.