"There are various ways, including a carbon tax or cap-and-trade program, to ensure that at least some of [the] external costs {associated with greenhouse gas emissions} are paid by whoever is responsible for them. . . . The idea isn't to punish people for their greenhouse gases; it's to create an incentive for inventors to create competitive carbon-free alternatives. By progressively increasing the price of carbon to reflect its true cost, governments can nudge producers and consumers toward more efficient decisions and encourage innovation . . . ."