It's hard to upstage Bill Clinton and Al Gore when it comes to the topic of climate change. But Michael Bloomberg did just that this afternoon in Seattle. The U.S. Conference of Mayors, which has been meeting here for the past two days, heard from both the former president and the former vice president yesterday. But the real news came today when the New York mayor took to the podium. Bloomberg took the occasion to make one of the biggest policy announcements since becoming mayor: a call for a "carbon tax" to help control harmful emissions. "As long as greenhouse gas pollution is free, it will be abundant," said Bloomberg. "We have to stop ignoring the laws of economics." He took aim at the current president's alternative. "The voluntary targets suggested by President Bush would be like voluntary speed limits - doomed to fail." A carbon tax has been in the air for a while now, though few major political figures have dared endorse it. Al Gore has, but that was hardly news. That Bloomberg - a Republican politician (at least until last June!), a Wall Street billionaire, and a potential president - is now on board, greatly elevates the idea. Indeed, Michael Bloomberg taking up the carbon tax is akin to Ross Perot taking up deficit reduction (remember the charts?) - you can't ignore them. A carbon tax would, quite simply, charge companies for the greenhouse gases they are responsible for. Putting a cost on carbon, Bloomberg argues, is the only way to significantly limit it. For those who hate taxes, Bloomberg had a bone. What the federal government collects in a new carbon tax could be returned in the form of a reduction in the payroll tax. In other words, a wash. "A charge on pollution," he argued, "would be less regressive than the payroll tax, because the more energy you consume, the more you would pay." Seattle is certainly becoming a hotbed when it comes to combating climate change. Mayor Greg Nickels got the ball rolling on Kyoto from here. And, just today, Bloomberg used this City to move the national debate yet again. Who says the Emerald City is just a nickname?