In the Times, Harvard economist Gregory Mankiw joins Al Gore and calls for the implementation of a carbon tax, and using the revenue gained to cut payroll taxes.

Yet this natural aversion to carbon taxes can be overcome if the revenue from the tax is used to reduce other taxes. By itself, a carbon tax would raise the tax burden on anyone who drives a car or uses electricity produced with fossil fuels, which means just about everybody. Some might fear this would be particularly hard on the poor and middle class.

But Gilbert Metcalf, a professor of economics at Tufts, has shown how revenue from a carbon tax could be used to reduce payroll taxes in a way that would leave the distribution of total tax burden approximately unchanged. He proposes a tax of $15 per metric ton of carbon dioxide, together with a rebate of the federal payroll tax on the first $3,660 of earnings for each worker.

This is such a smart idea—and payroll taxes are regressive, anyway—that it will almost surely never happen. (Sorry, that was cynical.)

What’s really interesting is that Mankiw is an adviser to Mitt Romney, who has been vociferous in his opposition to any new taxes….