There is widespread agreement among economists – and a diverse set of other policy analysts – that at least in the long run, an economy-wide carbon pricing system will be an essential element of any national policy that can achieve meaningful reductions of CO2 emissions cost-effectively in the United States. There is less agreement, however, among economists and others in the policy community regarding the choice of specific carbon-pricing policy instrument, with some supporting carbon taxes and others favoring cap-and-trade mechanisms. This prompts two important questions. Which – if either – of the two major approaches to carbon pricing is superior in terms of relevant criteria, including but not limited to efficiency, cost-effectiveness, and distributional equity? And which of the two approaches is more likely to be adopted in the future in the United States? This paper addresses these questions by drawing on both normative and positive theories of policy instrument choice as they apply to U.S. climate change policy, and draws extensively on relevant empirical evidence. The paper concludes with a look at the path ahead, including an assessment of how the two carbon-pricing instruments can be made more politically acceptable.

The Paris Agreement to the United Nations Framework Convention on Climate Change has achieved one of two key necessary conditions for ultimate success—a broad base of participation among the countries of the world. But another key necessary condition has yet to be achieved—adequate collective ambition of the individual nationally determined contributions. How can the climate negotiators provide a structure that will include incentives to increase ambition over time? An important part of the answer can be international linkage of regional, national, and sub-national policies, that is, formal recognition of emission reductions undertaken in another jurisdiction for the purpose of meeting a Party’s own mitigation objectives. A central challenge is how to facilitate such linkage in the context of the very great heterogeneity that characterizes climate policies along five dimensions: type of policy instrument, level of government jurisdiction, status of that jurisdiction under the Paris Agreement, nature of the policy instrument’s target, and the nature along several dimensions of each Party’s Nationally Determined Contribution. We consider such heterogeneity among policies, and identify which linkages of various combinations of characteristics are feasible; of these, which are most promising; and what accounting mechanisms would make the operation of respective linkages consistent with the Paris Agreement.