So, just what is the carbon footprint of a climate summit? I refer, of course, to the U.S. Conference of Mayors gathering in Seattle this past week. Over 120 city leaders from around the country came here (mostly by plane) to discuss global warming. As far as I know, no one calculated how many greenhouse gases the event produced as they gassed about how many greenhouse gases they want reduced. There was a commendable effort, though, to keep the gases to a minimum. Perhaps the most obvious was using bio-diesel buses to transport delegates between locations. (Can you imagine a climate conference not using such things?) Less obvious, and not really touted much, were the carbon "offsets" purchased by the organization. They were meant to counter the emissions attributable to the air travel required of the mayors and their staffs to get to and from the conference. It was never announced just how many offsets were bought, but it was probably significant, though it can't have been an easy number to measure. Was someone really checking who flew non-stop and who had layovers? Surely the latter take more fuel, don't they? Does first class count the same as economy? And you can't tell me there wasn't a mayor or two who came by private plane. Carbon measurement, in other words, gets pretty complicated pretty quickly. Interestingly, the issue of measurement is one of the bigger themes coming out of the conference. Meeting or beating the Kyoto Protocol limits (which is precisely what the mayors were here to reaffirm) isn't possible unless you know what your city's emissions have been, and you know what they will be. Otherwise, it's like saying you'll eat fewer calories this year than you did last year without having the foggiest idea of what your intake was then and now. But that's sort of the impression you get when talking to the mayors. Over the course of the conference I interviewed nearly two dozen mayors, learning that few even knew their community's carbon footprint, much less what it was in 1990. (Kyoto calls for a 7 percent reduction in emissions from 1990 levels by 2012.) One clear result of the conference is new urgency that some kind of standard emissions measurement is necessary, and that communities, especially small ones, will need help doing the calculations. It's certainly not required that mayors have perfect information before agreeing to the Kyoto limits; most didn't when they signed. Sometimes you just have to bite the bullet to get things going. But if carbon targets are really going to be achieved, then these mayors (and the U.S. Conference) will have to get pretty serious, pretty quickly about committing resources and energy to the measurement effort.