Tyler Cowen makes one excellent point and one debatable one in the comments:
Cities are mostly low-carbon because they discourage people from having children, or limit the number of children people decide to have. (Taxing education, or other correlates of children, is in fact one substitute for a carbon tax.) But cities also export their complementary and indeed financially required pollution to more distant geographic areas; that means that cities are not low-carbon per se.
Dense cities are a tax on having children, but that's not an inherent quality of cities. If we had taller buildings next to spots of green space, you could have more residential space and a convenient play space, and built-in play spaces for your children. If you had school choice, you could solve many of the fears about the urban school system that lead affluent families to flee the city. If the tax system weren't set up so that localities bear the responsibility for caring for the indigent, you wouldn't have affluent families moving out to get away from the tax burden. A dense city is in many places a better place to raise a child than a suburb: you spend a lot fewer years shuttling the kids around, and there are many more options and activities for them than for suburban children. But current political culture makes them child-unfriendly.
To be sure, one cannot just wish away current political culture, but if I am imagining something as unlikely as a change in transportation subsidies, cannot I then imagine a more sanely conducted city government?
On the carbon export angle, I completely agree. City dwellers are far too self-satisfied with their allegedly low-carbon lifestyle, too willing to impose carbon taxes in the belief it won't affect them much. It is especially irritating to hear people who take multiple annual long-haul flights complain about SUV drivers, but the general phenomenon is broader than that. I expect that in the event a carbon tax is enacted, I will see a lot of my costs go up--as they should, to the extent that I am exporting my carbon emissions elsewhere. But nonetheless, I don't think they'll go up as much as those of people in suburban homes, because heating, cooling, and driving to those homes really is simply massively less efficient than doing the same thing in an urban area.