Linking Heterogeneous Climate Policies (and Activities at COP-23 in Bonn)

It’s well known that the Paris Agreement has achieved broad participation by countries accounting for some 97% of global GHG emissions (in contrast to the 14% of global emissions associated with countries taking on responsibilities under the current commitment period of the Kyoto Protocol).  That is a very important accomplishment, but as negotiations begin to elaborate key details of the Agreement (as they will in Bonn in November), a critical question is how to create incentives for countries to increase ambition over time. The ability to link different climate policies, such that emission reductions undertaken in one jurisdiction can be counted toward the mitigation commitments of another jurisdiction, may help Parties increase ambition over time.  A new paper from the Harvard Project on Climate Agreements by Michael Mehling of MIT, Gilbert Metcalf of Tufts University, and myself explores options and challenges for facilitating such linkages in light of the considerable heterogeneity that is likely to characterize regional, national, and sub-national efforts to address climate change.  The full paper is available for downloading, as is a two-page summary.

We will be presenting our results on November 13th and 14th in Bonn at the Twenty-Third Conference of the Parties (COP-23) of the United Nations Framework Convention on Climate Change.  At the end of this blog essay, I offer some details about these and other forthcoming activities of the Harvard Project on Climate Agreements at COP-23 in Bonn.


Linkage is important, in part, because it can reduce the costs of achieving a given emissions-reduction objective. Lower costs, in turn, may make it politically feasible to embrace more ambitious objectives. In a world where the marginal cost of abatement – that is, the cost to reduce an additional ton of emissions – varies widely, linkage improves overall cost-effectiveness by allowing jurisdictions with relatively higher abatement costs to finance reductions from jurisdictions with relatively lower costs. In effect, linkage drives participating jurisdictions toward a common cost of carbon, equalizing the marginal cost of abatement and producing a more efficient distribution of abatement activities. These benefits are potentially significant: The World Bank has estimated that international linkage could reduce the cost of achieving the emissions reductions specified in the initial set of NDCs submitted under the Paris Agreement 32% by 2030 and 54% by 2050.

Article 6 of the Paris Agreement provides a foundation for linkage by recognizing that Parties to the Agreement may “choose to pursue voluntary cooperation in the implementation of their” NDCs through “the use of internationally transferred mitigation outcomes” (ITMOs). In contrast to the Kyoto Protocol (which likewise included provisions for international cooperation), the voluntary and flexible architecture of the Paris Agreement allows for wide variation, not only in the types of climate policies countries choose to implement, but in the form and stringency of the abatement targets they adopt.

Heterogeneous Linkage

Linkage is relatively straightforward when the policies involved are similar. However, linkage is possible even when this is not the case: for example, when one jurisdiction is using a cap-and-trade system to reduce emissions while another jurisdiction is relying on carbon taxes. There are several potential sources of heterogeneity: type of policy instrument used (for example, taxes vs. cap-and-trade vs. performance or technology standard); level of government jurisdiction involved (for example, regional, national, or sub-national); status under the Paris Agreement (that is, whether or not the jurisdiction is a Party to the Agreement – or within a Party); nature of the policy target (for examle, absolute mass-based emissions vs. emissions intensity vs. change relative to business-as-usual); and operational details of the country’s NDC, including type of mitigation target, choice of target and reference years, and sectors and greenhouse gases covered.

Analyzing Potential Linkages (Consistent with the Paris Agreement)

The full paper examines five specific cases of linkage, with various combinations of features, to identify which types of linkage are feasible, which are most promising, and what accounting mechanisms are needed to make their operation consistent with the Paris Agreement.  Each of the cases maps to a real-world example.

Most forms of heterogeneity – including with respect to policy instruments, jurisdictions, and targets – do not present insurmountable obstacles to linkage. However, some of these characteristics present challenges and call for specific accounting guidance if linkage is to include the use of ITMOs under the Paris Agreement. In particular, robust accounting methods will be needed to prevent double-counting of GHG reductions, to ensure that the timing (vintage) of claimed reductions and of respective ITMO transfers is correctly accounted for, and to ensure that participating countries make appropriate adjustments for emissions or reductions covered by their NDCs when using ITMOs. Additional issues under Article 6 include how to quantify ITMOs and how to account for heterogeneous base years, as well as different vintages of targets and outcomes.

Issues for the Climate Negotiators

Broader questions that bear on the opportunities for linkage under Article 6.2 include the nature of NDC targets and whether these are to be treated as strict numerical targets that need to be precisely achieved; the nature and scope of ITMOs, which have yet to be defined, let alone fully described, under the Paris Agreement; and finally, whether transfers to or from non-Parties to the Agreement (or sub-national jurisdictions within non-Parties) are possible, and if so, how they should be accounted for. Parties have differing views, however, on whether the guidance on Article 6.2 should extend to such issues.

Clear and consistent guidance for accounting of emissions transfers under Article 6 can contribute to greater certainty and predictability for Parties engaged in voluntary cooperation, thereby facilitating expanded use of linkage. At the same time, too much guidance, particularly if it includes restrictive quality or ambition requirements, might impede linkage and dampen incentives for cooperation. Given their limited mandate, Parties should exercise caution when developing guidance under Article 6.2 that goes beyond key accounting issues. This does not mean that concerns about ambition and environmental integrity should be neglected. However, if the combination of a set of common accounting rules and an absence of restrictive criteria and conditions can accelerate linkage and allow for broader and deeper policy cooperation, it can also increase the potential for Parties to scale up the ambition of their NDCs. And that may ultimately foster stronger engagement between Parties (and non-Parties), as well as with regional and sub-national jurisdictions.


The Harvard Project on Climate Agreements at COP-23 in Bonn

We will conduct three panel events at the Twenty-Third Conference of the Parties (COP-23) of the United Nations Framework Convention on Climate Change (UNFCCC) in Bonn, Germany, during the week of November 13, 2017.  If you have credentials to access the secure area of the COP, you are most welcome to attend any or all of these.  Also, COP-23 attendees who wish to meet with the Harvard Project during the conference should email: Jason Chapman (

Events in Brief:

Heterogeneous Linkage and the Evolution of Article 6
Monday, November 13
12:00 – 1:30 pm
Pavilion of the International Emissions Trading Association (IETA)

Implementing and Linking Carbon Pricing Instruments: Theory and Practice
Tuesday, November 14, 2017
11:30 am – 1:00 pm
Side Event Meeting Room 12

Carbon Pricing Policy Design
Tuesday, November 14, 2017
2:00 – 3:30 pm
Pavilion of the International Emissions Trading Association (IETA)

Events in Detail:

Heterogeneous Linkage and the Evolution of Article 6, Monday, November 13, 12:00 – 1:30 pm, Pavilion of the International Emissions Trading Association (IETA)


Jos Delbeke, Director General for Climate Action, European Commission

Kelley Kizzier, Co-Chair, Article 6, Subsidiary Body for Scientific and Technological Advice

Michael Mehling, Deputy Director, Center for Energy and Environmental Policy Research
Massachusetts Institute of Technology

Gilbert Metcalf, Professor of Economics, Tufts University

Robert Stavins, A. J. Meyer Professor of Energy and Economic Development, Harvard Kennedy School


The Paris Agreement 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 (NDCs). How can climate negotiators provide a structure that provides incentives to increase ambition over time? One part of the answer can be facilitating international linkage of regional, national, and sub-national policies. A central challenge is how to accomplish this in the context of the great heterogeneity that characterizes climate policies, along several dimensions, in the context of Paris-Agreement NDCs. Panelists will review the status of linkage in the world, the evolution of Article 6, and the relationship between the two.

Implementing and Linking Carbon Pricing Instruments: Theory and Practice, Tuesday, November 14, 2017, 11:30 am – 1:00 pm, Side Event Meeting Room 12, Co-Hosts: Harvard Project on Climate Agreements and Enel Foundation


Andrei Marcu, Senior Fellow, International Centre for Trade and Sustainable Development

Michael Mehling, Deputy Director, Center for Energy and Environmental Policy Research, Massachusetts Institute of Technology

Gilbert Metcalf, Professor of Economics, Tufts University

Simone Mori, Head of European Affairs, Enel

Robert Stavins, A. J. Meyer Professor of Energy and Economic Development, Harvard Kennedy School

Other participant(s) to be determined


The Paris Agreement 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. This panel will consider how this issue might be addressed by 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 Nationally Determined Contributions along several dimensions. 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. The panel will draw in part on a paper by Michael Mehling, Gilbert Metcalf, and Robert Stavins, “Linking Heterogeneous Climate Policies (Consistent with the Paris Agreement),” available here

Carbon Pricing Policy Design, Tuesday, November 14, 2017, 2:00 – 3:30 pm, Pavilion of the International Emissions Trading Association (IETA), Co-Hosts:  Harvard Project on Climate Agreements and Enel Foundation


Daniele Agostini, Head of Low Carbon Policies and Carbon Regulation, Enel

Joseph Aldy [via videoconference], Associate Professor of Public Policy, Harvard Kennedy School

Gilbert Metcalf, Professor of Economics, Tufts University

Robert Stavins, A. J. Meyer Professor of Energy and Economic Development, Harvard Kennedy School

Other participant(s) to be determined


This panel will review experiences with cap-and-trade and carbon-tax policies, and draw lessons from those experiences. Panelists will also examine the choice between — and design of — such policies, through a political-economy lens, in order to highlight important public policy principles and policy options in carbon-pricing-policy design. The panel will draw in part on a paper by Joseph Aldy, “The Political Economy of Carbon Pricing Policy Design,” available here.


The UN Climate Summit and a Key Issue for the 2015 Paris Agreement

World leaders converged at the United Nations in New York City this past week for Secretary-General Ban Ki-moon’s much anticipated Climate Summit, a lead-up to global negotiations that will take place in Lima, Peru, in December of this year, and culminate a year later in Paris.  The challenge before negotiators is great, because there are significant obstacles to reaching a meaningful agreement, as I describe in an Op-Ed that appeared in The New York Times on Sunday, September 21st, “Climate Realities.”

However, partly because of the new path that is being taken under the Durban Platform for Enhanced Action, in which all countries will be included under a common legal framework in a politically realistic hybrid policy architecture, the prognosis for a meaningful international agreement is better now than it has been in decades.  I discuss this briefly at the end of the Times article, and emphasize it in a follow-up Op-Ed that appeared in The Boston Globe on September 23rd, “UN summit can accelerate momentum to a new approach to climate change.”  (Also, for my overall assessment of the UN Climate Summit, see this interview carried out by the Harvard Kennedy School’s Doug Gavel.)

A New Development at the UN Climate Summit

The most significant development at the UN Climate Summit this past week was the degree to which carbon pricing became central to so many discussions, including with leaders from the business community.  As carbon pricing – in particular, cap-and-trade systems – have emerged as the policy instrument of choice in many parts of the world, interest in linking these systems together has grown.  Linkage (unilateral or bilateral recognition of allowances) among carbon markets — and, for that matter links with non-market-based systems — can reduce the aggregate cost of achieving climate targets.  And lower compliance costs can in turn encourage countries to increase the ambition of their contributions under the 2015 Paris agreement.

New Research from Harvard

Because of this, the Harvard Project on Climate Agreements has been collaborating with the International Emissions Trading Association (IETA) to explore the role of linkage in the new international climate change agreement to be completed in Paris.  In this new research, my co-authors (Daniel Bodansky of Arizona State University, Seth Hoedl of Harvard Law School, and Gilbert Metcalf of Tufts University) and I examine linkage — not only among cap-and-trade systems, but among cap-and-trade, carbon tax, and non-market regulatory systems — and the role that linkage should play in the 2015 agreement.  We look both at what would inhibit or even prevent linkage and should therefore be avoided in the 2015 agreement, and what – in a positive sense – should be included in the agreement to facilitate effective linkage of regional, national, and sub-national climate policies.

We released an Executive Summary of our research paper (“Facilitating Linkage of Heterogeneous Regional, National, and Sub-National Policies Through a Future International Agreement”) in New York City on September 22nd at an event co-sponsored by IETA and the Harvard Project, on the sidelines of UN Climate Summit, “Carbon Pricing and the 2015 Agreement” (the agenda of the event is available here).

In the executive summary (which can be downloaded in full here), we conclude that among the design elements the 2015 agreement should avoid because they would inhibit linkage are so-called “supplementarity requirements” that require parties to accomplish all (or a large, specified share) of their emissions-reduction commitments within their national borders. The 2015 agreement also should avoid including detailed linkage rules in the core agreement; an agreement with more flexibility would allow rules to evolve on the basis of experience.

Importantly, we also find that, to advance linkage, the 2015 agreement should:  define key terms, in particular the units that are used for compliance purposes; establish registries and tracking mechanisms; and include default or model rules, from which nations are free to deviate at their discretion.  Overall, the most valuable outcome of the Paris Agreement regarding linkage may simply be including an explicit statement that parties may transfer portions of their emissions-reduction contributions to other parties — and that these transferred units may be used by the transferees to implement their own commitments.

Looking Forward

We will release the complete research paper in November of this year, prior to the Twentieth Conference of the Parties (COP-20) of the United Nations Framework Convention on Climate Change in Lima, Peru, in December 2014, where the Harvard Project and IETA plan to conduct a side-event that will focus on this work.

When the full paper is released in November, I will provide a more complete description at this blog of our research methods and our findings.

[Additional press coverage is here, here, here, here, here, here, here, here, here, and here.]


The Promise and Problems of Pricing Carbon

Friday, October 21st was a significant day for climate change policy worldwide and for the use of market-based approaches to environmental protection, but it went largely unnoticed across the country and around the world, outside, that is, of the State of California.  On that day, the California Air Resources Board voted unanimously to adopt formally the nation’s most comprehensive cap-and-trade system, intended to provide financial incentives to firms to reduce the state’s greenhouse gas (GHG) emissions, notably carbon dioxide (CO2) emissions, to their 1990 level by the year 2020, as part of the implementation of California’s Assembly Bill 32, the Global Warming Solutions Act of 2006.  Compliance will begin in 2013, eventually covering 85% of the state’s emissions.

This policy for the world’s eighth-largest economy is more ambitious than the much heralded (and much derided) Federal policy proposal – H.R. 2454, the Waxman-Markey bill – that was passed by the U.S. House of Representatives in June of 2009, and then died in the U.S. Senate the following year.  With a likely multi-year hiatus on significant climate policy action in Washington now in place, California’s system – which will probably link with similar cap-and-trade systems being developed in Ontario, Quebec, and possibly British Columbia – will itself become the focal point of what may evolve to be the “North American Climate Initiative.”

The Time is Ripe for Reflection

California’s formal adoption of its CO2 cap-and-trade system is an important milestone on the multinational path to carbon pricing policies, and signals that the time is ripe to reflect on the promise and problems of pricing carbon, which is the title of a new paper that Joe Aldy and I have written for a special issue of the Journal of Environment and Development edited by Thomas Sterner and Maria Damon on “Experience with Environmental Taxation” (“The Promise and Problems of Pricing Carbon:  Theory and Experience,” October 27, 2011).  [For anyone who is not familiar with my co-author, let me state for the record that Joseph Aldy is an Assistant Professor of Public Policy at the Harvard Kennedy School, having come to Cambridge, Massachusetts, from Washington, D.C., where he served, most recently, during 2009 and 2010, as Special Assistant to the President for Energy and Environment.  Before that, he was a Fellow at Resources for the Future, the Washington think tank.]

Why Price Carbon?

In a modern economy, nearly all aspects of economic activity affect greenhouse gas – in particular, CO2 – emissions.  Hence, for a climate change policy to be effective, it must affect decisions regarding these diverse activities.  This can be done in one of three ways:  mandating that businesses and individuals change their behavior; subsidizing businesses and individuals; or pricing the greenhouse gas externality.

As economists and virtually all other policy analysts now recognize, by internalizing the externalities associated with CO2 emissions, carbon pricing can promote cost-effective abatement, deliver powerful innovation incentives, and – for that matter – ameliorate rather than exacerbate government fiscal problems.  [See the concise and compelling argument made by Yale Professor William Nordhaus in his essay, “Energy:  Friend or Enemy?” in The New York Review of Books, October 27, 2011.]

By pricing CO2 emissions (or, more likely, by pricing the carbon content of the three fossil fuels – coal, petroleum, and natural gas), governments wisely defer to private firms and individuals to find and exploit the lowest cost ways to reduce emissions and invest in the development of new technologies, processes, and ideas that could further mitigate emissions.

Can Market-Based Instruments Really Work?

Market-based instruments have been used with considerable success in other environmental domains, as well as for pricing CO2 emissions.  The U.S. sulfur dioxide (SO2) cap-and-trade program cut U.S. power plant SO2 emissions more than 50 percent after 1990, and resulted in compliance costs one half of what they would have been under conventional regulatory mandates.

The success of the SO2 allowance trading program motivated the design and implementation of the European Union’s Emission Trading Scheme (EU ETS), the world’s largest cap-and-trade program, focused on cutting CO2 emissions from power plants and large manufacturing facilities throughout Europe.  The U.S. lead phase-down of gasoline in the 1980s, by reducing the lead content per gallon of fuel, served as an early, effective example of a tradable performance standard.  These and other positive experiences provide motivation for considering market-based instruments as potential approaches to mitigating GHG emissions.

What Policy Instruments Can be Used for Carbon Pricing?

In our paper, Joe Aldy and I critically examine the five generic policy instruments that could conceivably be employed by regional, national, or even sub-national governments for carbon pricing:  carbon taxes, cap-and-trade, emission reduction credits, clean energy standards, and fossil fuel subsidy reduction.  Having written about these approaches many times in previous essays at this blog, today I will simply direct the reader to those previous posts or, better yet, to the paper we’ve written for the Journal of Environment and Development.

Although it is natural to think and talk about carbon pricing using the future tense, a few carbon pricing regimes are already in place.

Regional, National, and Sub-National Experiences with Carbon Pricing

Explicit carbon pricing policy regimes currently in place include the European Union’s Emissions Trading Scheme (EU ETS); the Regional Greenhouse Gas Initiative in the northeast United States; New Zealand’s cap-and-trade system; the Kyoto Protocol’s Clean Development Mechanism; a number of northern European carbon tax policies; British Columbia’s carbon tax; and Alberta’s tradable carbon performance standard (similar to a clean energy standard).  We describe and assess all of these in our paper.

Also, the Japanese Voluntary Emissions Trading System has operated since 2006 (Japan is considering a compulsory emissions trading system), and Norway operated its own emissions trading system for several years before joining the EU ETS in 2008.  Legislation to establish cap-and-trade systems is under debate in Australia (combined with a carbon tax for an initial three-year period) and in the Canadian provinces of Ontario and Quebec.  And, of course, California is now committed to launching its own GHG cap-and-trade system.

International Coordination Will Be Needed

Of course, climate change is truly a global commons problem:  the location of greenhouse gas emissions has no effect on the global distribution of damages.  Hence, free-riding problems plague unilateral and multilateral approaches, because mitigation costs are likely to exceed direct benefits for virtually all countries.  Cost-effective international policies – insuring that countries get the most environmental benefit out of their mitigation investments – will help promote participation in an international climate policy regime.

In principle, internationally-employed market-based instruments can achieve overall cost effectiveness.  Three basic routes stand out.  First, countries could agree to apply the same tax on carbon (harmonized domestic taxes) or adopt a uniform international tax.  Second, the international policy community could establish a system of international tradable permits, – effectively a nation-state level cap-and-trade program.  In its simplest form, this represents the Kyoto Protocol’s Annex B emission targets and the Article 17 trading mechanism.  Third and most likely, a more decentralized system of internationally-linked domestic cap-and-trade programs could ensure internationally cost-effective emission mitigation.  We examine the merits and the problems associated with each of these means of international coordination in the paper.

What Lies in the Future?

In reality, political responses in most countries to proposals for market-based approaches to climate policy have been and will continue to be largely a function of issues and factors that transcend the scope of environmental and climate policy.  Because a truly meaningful climate policy – whether market-based or conventional in design – will have significant impacts on economic activity in a wide variety of sectors and in every region of a country, proposals for these policies inevitably bring forth significant opposition, particularly during difficult economic times.

In the United States, political polarization – which began some four decades ago, and accelerated during the economic downturn – has decimated what had long been the key political constituency in the Congress for environmental action, namely, the middle, including both moderate Republicans and moderate Democrats.  Whereas Congressional debates about environmental and energy policy had long featured regional politics, they are now fully and simply partisan.  In this political maelstrom, the failure of cap-and-trade climate policy in the U.S. Senate in 2010 was essentially collateral damage in a much larger political war.

It is possible that better economic times will reduce the pace – if not the direction – of political polarization.  It is also possible that the ongoing challenge of large budgetary deficits in many countries will increase the political feasibility of new sources of revenue.  When and if this happens, consumption taxes (as opposed to traditional taxes on income and investment) could receive heightened attention, and primary among these might be energy taxes, which can be significant climate policy instruments, depending upon their design.

That said, it is probably too soon to predict what the future will hold for the use of market-based policy instruments for climate change.  Perhaps the two decades we have experienced of relatively high receptivity in the United States, Europe, and other parts of the world to cap-and-trade and offset mechanisms will turn out to be no more than a relatively brief departure from a long-term trend of reliance on conventional means of regulation.  It is also possible, however, that the recent tarnishing of cap-and-trade in U.S. political dialogue will itself turn out to be a temporary departure from a long-term trend of increasing reliance on market-based environmental policy instruments.  It is much too soon to say.