Within a 24-hour period starting Thursday, Nov. 1, three national figures will speak in Seattle: Bill Clinton, Al Gore, and Michael Bloomberg. The audience will be the same: the U.S. Conference of Mayors. And so will the topic: global warming.

Cities have taken a big lead when it comes to the issue. It's in city government, not in the federal government, where most of the public sector ambition and innovation on tacking climate change is located.

Take Seattle. Frustrated that the U.S. was not a signatory to the Kyoto Protocol a few years back, Mayor Greg Nickels decided to do a little "treaty-making" of his own. His aim: get as many U.S. mayors to sign onto the carbon emission limits called for in that international pact. It was substance and symbolism. He wanted to show Washington, D.C., not to mention the rest of the world, that Americans were serious about climate change.

As he often likes to say: "I want other countries to know that there are signs of intelligent life in the U.S."

To date, those signs consist of nearly 700 mayors who have agreed to the Kyoto limits within their communities - areas that include 75 million Americans. Over 120 of those leaders will be here in Seattle this week.

The Mayors Conference is part congrats (keynote speeches) and part confab (working sessions). Nickels, who helped organize the event, knows that cities need more than just targets, they need tools.

"We want to accelerate the transfer of best practices and best ideas," says Steve Nicholas, the mayor's point person for sustainability efforts.

And just what do those best practices need to accomplish? The Kyoto goal is a 7 percent reduction in greenhouse gas emissions from 1990 levels by 2012.

It sounds doable enough. It's all single-digit stuff, after all. But even Seattle, the greenest of the greens when it comes to this, may have a hard time meeting the Kyoto target. We just learned this week that while much progress has been made locally (more than almost any big city), the trend lines aren't good. Because of the huge migration expected into the area - and the resulting car trips that will inevitably follow - our "carbon footprint" is going to be hard to hold back. Not impossible, but a much heavier lift than some of the earlier successes we've logged, including taking City Light down to zero net emissions. You can only do that once, and then you have to find other triumphs. Like a massive move of people from cars to transit.

Moreover, the scientific consensus is that it will take much more than Kyoto to really make a difference globally. Are cities, which are just getting going on this effort, ready for Kyoto II?

The conference's schedule includes a former president, a former future president, and a future potential president. Bill Clinton speaks on Thursday afternoon. Al Gore speaks via satellite that same day. (Is that a smaller carbon footprint than flying out in person?) Michael Bloomberg, who has pushed for aggressive targets in New York, will speak on Friday.

"It'll be the first meeting of the signatories of the Mayor's Climate Protection Agreement," notes Nicholas, referring to the actual document that has been making its way across the country.

Greg Nickels has received the kind of accolades that come with high-profile leadership of this kind. The U.S. Conference of Mayors recently voted him as their new president, starting in 2009. And he has enjoyed much national and even international media coverage for his work. His picture has adorned the pages of Time, Newsweek, Vanity Fair, and even Rolling Stone.

But not all his press has included a glossy. For instance, he has been criticized by some environmentalists for pushing a big tunneled roadway along the waterfront to replace the Alaskan Way Viaduct. (He dropped his support after voters rejected it earlier this year.) And, on a more personal level, he took some flak for waiting so long to get a hybrid car when he travels on city business.

Even locally, his leadership can sometimes be upstaged. In what seems to be a continuation of the often awkward "green race" between Nickels and Ron Sims, the King County executive who recently helped launch a similar carbon-reduction agreement for county governments across the U.S. Sims' targets are stronger: an 80 percent reduction in emissions by 2050.

But Nickels will surely be basking in a glow this week when mayors from across America make the trek to Seattle to learn more.

And no doubt the keynoters will offer some kudos, as well. Al Gore already has. Last year he visited and told a crowd of locals: "I want a future in which those children learn that the real action ... began in a city between the mountains and the ocean. I want them to read and understand that the real change began right here in Seattle."

That history, of course, has yet to be written. But for now, Seattle is a pretty good candidate to fulfill Gore's vision.