When hundreds of thousands of marchers thronged New York City in September demanding action against climate change, they probably didn't imagine they were affecting the "shadow" price of carbon set by their bete noire, ExxonMobil. Pricing carbon would address what economist Sir Nicholas Stern in a 2006 report called the greatest market failure in history. Economists from right to left agree that taxing carbon is the most efficient way to cut emissions. A rising, predictable carbon price would unleash the power of the market against climate change, giving both producers and consumers an incentive to shift to lower-carbon energy choices. In a 2009 speech, ExxonMobil's CEO, Rex Tillerson, said if governments do price carbon, they should impose a revenue-neutral carbon tax. Under such a policy, all carbon tax revenue would be refunded to the public -- for example, through reduced payroll taxes -- so higher energy prices wouldn't reduce personal incomes and gross domestic product or hurt the poor.