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1. Healthy—but not unlimited—agency discretion is necessary for adaptability. One of the most difficult questions policymakers face in addressing climate change is how to appropriately balance administrative discretion and constraints on that discretion. Experience with the CAA suggests that broad agency discretion is imperative to adaptability, but that it should be constrained by backstops like deadlines for compliance, citizen suits and federalism arrangements to avoid inaction.
2. Durable policy is related to its adaptability. Regularized adaptability, with built-in processes for incorporating new information, will make energy policy more durable by providing clear signals to regulated parties and safeguarding against the elimination of a regulation under new political leadership.
3. Flexibility is important to a policy’s durability and adaptability. Flexibility can make long-term energy policy more durable by ensuring the cost of regulation is not prohibitive and more adaptable by producing information about cost-effective technologies that can lead to additional greenhouse gas reductions over time. 
4. Political challenges inform the balance between durability and adaptability. Evaluation of CAA programs suggests that politics matter in particular ways that are highly relevant to initial design choices about the appropriate balance between durability and adaptability. Given the gridlock besetting Congress and the highly politicized nature of the climate change issue, the authors recommend that policy should be crafted to maximize the discretion and flexibility granted to the implementing expert agency.
5. The �䴡��’s provisions are intertwined in ways that affect adaptability, durability and flexibility. The CAA programs analyzed by the authors do not act in isolation; for example, greenhouse gas regulation arose in part because of statutory provisions covering automobiles and stationary sources, but it is also the result of a broad definition of air pollutants that led the US Supreme Court to hold that greenhouse gases fit within the definition.
6. Process and cooperative federalism are key ingredients of the �䴡��’s success. The processes of the CAA, including citizen suits and notice and comment, as well as the role of cooperative federalism in sharing responsibility among levels of government, provide engines of progress and stability in achieving environmental goals. They should be important components in the future design of energy and climate policy.

The transportation and electricity sectors, as well as buildings and industrial facilities, are infrastructure intensive and involve large capital investments that often last many decades. Some power plants in the United States were built over seventy years ago.1 The average age of the nuclear fleet is approaching forty years.2 Cars built today can last for 150,000 miles or more with an average lifetime of fifteen years,3 heavy-duty trucks may stay on the road for thirty years, and US rail cars, ships and airplanes remain in operation for decades.4 The United States will need stable, durable policies across multiple decades to induce the innovations required to decarbonize these capital-intensive industries by mid-century. Stability and durability in policy will provide the signal necessary to investors and innovators to develop technologies and systems that can help accomplish the United States’ long-term emissions goals. Stability and durability will also reduce the attendant risk that accompanies investments in the research and development of these technologies.

By stability and durability, however, the authors do not mean that a policy must be fixed for the next forty-five years. Instead, they advocate for a policy framework that continues to accomplish the objectives for which it was adopted, long after it was adopted. In political terms, they argue that a durable policy outlasts its initial supporters. And the policy must accomplish its goals even in the face of changes in scientific knowledge, technological innovation and economic change.5

To maintain its effectiveness for multiple decades, a durable policy must also, then, include mechanisms to adapt to new information about science, technology and economics. The world will not remain static over the next several decades: already, the electricity and transportation sectors are transforming with a rapidity that is outpacing predictions of just a few years ago. At the same time, the methods to adapt policy to new information must be predictable, the authors believe, in order to provide clear signals to regulated industries that policies will change. Put a different way, the country needs durable yet evolving policy that—through its adaptive mechanisms—is predictable.

Flexibility adds a third element to many successful policies. Regulators want to get the biggest bang for the buck out of regulations that impose costs on the economy, but they often lack complete knowledge about where to find the biggest bang. A flexible approach will provide incentives to a group of regulated entities to reduce emissions, rewarding those individual entities that can reduce the most and operate most cleanly, while calibrating the overall effort to meet environmental goals.

Endnotes

• 1See Steven Mufson, “Vintage US Coal-Fired Power Plants Now an ‘Aging Fleet of Clunkers,’” The Washington Post, June 13, 2014, [].
• 2See US Energy Information Administration, “Frequently Asked Questions: How Old Are US Nuclear Power Plants, and When Was the Newest One Built?” June 21, 2017, [].
• 3National Highway Transportation and Safety Administration, Final Regulatory Impact Analysis, Corporate Average Fuel Economy for MY 2017–MY 2025 Passenger Cars and Light Trucks (Washington, D.C.: US Department of Transportation, 2012).
• 4California Environmental Protection Agency Air Resources Board, Draft Supporting Information For Technology Assessments: Truck And Bus Sector Description (Sacramento: California Environmental Protection Agency, 2016).
• 5Eric M. Patashnik, Reforms at Risk: What Happens after Major Policy Changes are Enacted (Princeton, N.J.: Princeton University Press, 2008).

The Clean Air Act is a remarkable statute in its accomplishments. Over the course of its fifty-year history, it has resulted in large reductions in harmful air pollutants. These reductions have occurred across all areas of the country, a wide range of pollutants and a huge number of sources, including in the electricity and transportation sectors that must be at the heart of long-term climate and energy policy. All six of the pollutants covered by the National Ambient Air Quality Standards (NAAQS) have declined significantly: Carbon monoxide is down 86 percent on average nationwide. Lead has declined 99 percent. Nitrogen dioxide has declined 60 percent and sulfur dioxide 84 percent. Ozone, which remains one of the toughest pollutants to control, is down 32 percent. And in the fifteen years since the Environmental Protection Agency (EPA) began regulating fine particulate matter (PM2.5, or particles smaller than 2.5 micrometers in diameter), concentrations have fallen 37 percent.6 All of these declines have occurred while the US economy has grown dramatically (16-fold since 1971)7 and the population has increased by 150 percent.8 The challenge facing the United States is to achieve comparable declines in greenhouse gas emissions over a roughly similar period of time, by mid-century; this makes the CAA an important statute to study.

The longevity of the CAA demonstrates another of its rather remarkable accomplishments: it has endured through significant ideological and political change, as the country has elected both Republican and Democratic presidents throughout the statute’s history. The CAA was first enacted with broad bipartisan support and signed into law by a Republican president (Nixon), saw major amendments in 1977 under a Democratic president (Carter), and another round of major amendments in 1990 under George H. W. Bush. Since its passage in 1970, very different administrators with very different ideological predilections have led the implementing agency. Yet even in the face of the current administrative retrenchment, the CAA is protected by features like federalism arrangements, statutory deadlines and citizen suits and will likely continue to move forward.

In addition to durability, the statute has also exhibited adaptability by providing a regularized administrative process that allows agencies to respond to new information. Regulated parties are on notice that the CAA will adapt, but only with significant lead time, an opportunity for broad public participation and the input of sophisticated scientific and technical experts.

Flexibility through market-based mechanisms has been another key ingredient of some of the most successful regulations under the CAA. The Acid Rain Trading Program and NO_x Budget Trading Program, for example, both led to significant pollution reductions while allowing regulated entities to reach compliance at their lowest possible cost through the trading and banking of emissions permits. However, in some cases, provisions of the CAA have undermined environmental outcomes and threatened the durability of some initiatives.

At the heart of Lessons from the Clean Air Act are five case studies in which the authors discuss how a variety of mechanisms in the CAA promote—and in some cases undermine—flexible, adaptable yet durable policy:

• Law professor William Boyd focuses on the National Ambient Air Quality Standards to show how this centerpiece of the CAA has produced significant reductions of major pollutants across multiple decades. His chapter also sets the stage for those that follow, given the centrality of the NAAQS in the way the CAA operates.
• Law professor Hannah Wiseman examines the regulation of stationary sources of air pollution—that is, factories and industrial plants—under the CAA. Her comprehensive account of the many regulatory programs under the CAA show both real successes in establishing durable yet adaptable policy as well as some significant failures.
• Political scientist Barry Rabe addresses the regulation of mobile sources of air pollution. He focuses on the �䴡��’s unique federalism arrangement—which gives California special authority to regulate emissions from cars, trucks and other mobile sources while preempting all other state authority to regulate separately—and shows how it has been among the most durable yet adaptable provisions.
• Economist Joseph Aldy looks at the CAA’s regulation of fuels, in which the story of durability and flexibility is perhaps most mixed. The CAA has produced some great successes in its regulation of fuel—for instance, the elimination of lead from gasoline—as well as one of its greatest failures to date: the attempt to produce low-carbon renewable fuels.
• Political scientist Eric Patashnik offers an account of a regulatory approach—the use of market-based tools—that cuts across multiple parts of the CAA. Among his findings are that several market-based programs have proven to be less durable than more traditional regulatory programs, and that the CAA has not always provided EPA with the adaptability necessary to design market-based programs effectively. Other market-based programs have proven to be more successful in promoting adaptability, durability and flexibility, however, and his chapter provides important insights about how and why.

Endnotes

• 6These figures come from the US epa’s Air-Trends reporting. See US Environmental Protection Agency, “National Air Quality: Status and Trends of Key Air Pollutants,” [].
• 7According to the National Bureau of Economic Analysis, gross domestic product was, when adjusted for inflation, $1,137.8 billion in the first quarter of 1971, when the CAA took effect, and $18,164.8 billion at the end of 2015 (EPA data about pollutants are through 2015). See US Department of Commerce, Bureau of Economic Analy­sis, “National Data: GDP and Personal Income,” [].
• 8See US Census Bureau, “Historical National Population Estimates: July 1, 1900 to July 1, 1999,” [] [showing 1971 US population as 207.66 million people]; and US Census Bureau, “Annual Estimates of the Resident Population: April 1, 2010 to July 1, 2015,” [] [showing 2015 US population as 321.42 million people].

From these case studies, the book draws six overarching conclusions to help guide future climate efforts. The first three directly address the core concepts of durability, adaptability and flexibility. The fourth conclusion considers the reality that the United States has experienced, and continues to experience, sudden swings in political outcomes and the ways in which long-term policy goals have and can continue to endure in this context. The fifth conclusion reflects on the ways that the core concepts of the CAA are intertwined and mutually dependent. Finally, in search of guidance for the development of climate policy, the authors observe that two features of the CAA have come up time and again in addressing the core concepts of their study: process and cooperative federalism. These unusual features have played an impor­tant role in the success of the CAA over nearly half a century.

1. Healthy—But Not Unlimited—Discretion Is Necessary for Adaptability

One of the most difficult questions policymakers face in addressing a pollution problem on the scale of that driving climate change is how to achieve the appropriate balance between administrative discretion and constraints on that discretion. In theory, the more discretion an agency is given to address a pollution problem, the more adaptable the policy is likely to be over time. However, analysis of the discretion provided in the CAA leads to a somewhat counterintuitive conclusion: in seeking to promote adaptable governance, legislation can provide too much discretion to an agency, just as it can be too prescriptive. Wide-open discretion, unbounded by deadlines and without at least some constraining guidelines, may lead to agency inaction.

If an agency is given broad discretion to develop regulations, various mechanisms can enhance the likelihood that the discretion will be exercised. One mechanism is to set a statutory deadline for compliance that is subject to a citizen suit provision. The combination of broad delegated authority to EPA to establish the NAAQS, statutory deadlines requiring review of the NAAQS every five years and citizen suits that require EPA to meet those deadlines has led to what the authors find to be among the most powerful adaptive mechanisms in the CAA.

Another strategy is to have states as backstop or concurrent regulators. California’s special authority to regulate mobile sources, for example, has led to its long history of tightening automobile emissions standards. Once California demonstrates that a particular standard is feasible, the federal government often reacts by following suit and revising the federal standard to assimilate California’s. Hence, California’s use of its authority provides a means to push EPA to use the broad discretion it is given under the CAA to regulate mobile sources.

When designing climate policy, providing the implementing agency with little to no discretion to adapt can mean that necessary pollution reductions will not be addressed. The CAA provides some important cautionary examples. Exempting specific sources from regulation (like existing sources under the CAA) or setting emissions reduction levels by statute with no mechanism for adjustment is likely to lead to significant delay in addressing further reductions. This may be particularly true in addressing greenhouse gas emissions because of the super wicked nature of the problem. As a number of observers have described, many attributes of the problem of climate change make political action to solve it especially difficult. Thus, in crafting long-term energy policy, it is imperative that policy be made adaptable by giving agencies sufficient discretion to respond to ongoing problems.

2. Durable Policy Is Related to Its Adaptability

The authors’ analyses of policies that are durable yet adaptable lead to a second conclusion: policies that include regularized adaptability, with built-in process, can make policy more durable. Regularized process has provided both predictability and legitimacy to regulated parties, to courts reviewing EPA rulemakings and to the public. Well-designed procedures can provide clear signaling about a regulation’s future, helping stakeholders to form expectations that guide their own decisions.

Moreover, regularized process also safeguards against the potential arbitrary removal of regulations that might otherwise result from a sudden change in political leadership. Regulations under the CAA are undergirded by scientific data. While political preferences can change and democratically elected officials may even want to eliminate regulations, it takes time to unwind regulations that have been carefully and transparently built around the state of the science and technology. The process that enables policy evolution therefore also safeguards durability in the face of fickle short-run political outcomes.

A number of CAA programs demonstrate the relationship between process, durability and adaptability. The most comprehensive is the NAAQS program. Not only does the NAAQS review process contain a statutory deadline, but it also has been accompanied by a rigorous scientific review process. The process, though lengthy, contributes to public legitimacy through exhaustive analysis of the underlying science. It has also resulted in courts largely deferring to EPA judgments about where to set the NAAQS provided that those decisions are based on the best available science. And the inclusion of a regularized decision-making process allows regulated parties to anticipate and plan for strengthening of pollution standards.

By contrast, the Renewable Fuel Standard annual rulemaking process to establish yearly biofuel targets has faced a series of difficulties, occasionally resulting in the announcement of annual targets for years that have already ended. This process has led to widespread criticism and legal challenges and, unless reformed, could eventually lead to its demise.

Climate change is a long-run challenge, and mitigating greenhouse gas emissions requires long-term commitments to research, investments and infrastructure. In designing climate policy, then, it is important to put in place a process that requires the administering agency to gather and incorporate new scientific and economic information into a regularized process that requires policy to evolve in response to new information.

3. Flexibility Is Important to a Policy’s Durability and Adaptability

Incorporating flexibility into the implementation and enforcement of CAA policies is important to produce cost-effective measures to reduce pollution that draw upon the expertise and incentive structure of private regulated entities. But flexibility is also important to the durability of various provisions of the CAA. Indeed, the cost of regulation in a complex and dynamic setting could be prohibitive without flexible approaches to pollution reduction, and inflexible policies could ultimately undermine the political viability of the CAA. Flexibility can make policies adaptable by producing information about cost-effective technologies that can lead to additional pollution reductions over time.

The most well-known flexible mechanism, cap-and-trade, served as the centerpiece of the Acid Rain Trading Program. In addition to geographic flexibility in deciding where emissions reductions could be achieved at least cost, the program conveyed temporal flexibility through the ability to bank emissions allowances in anticipation of a tighter cap in the second phase of the program. This option provided firms flexibility to rationalize their investment decisions and led to earlier-than-required environmental improvement. The success of the Acid Rain Trading Program led, in turn, to other applications of emissions trading, including a regional cap-and-trade program to reduce NO_x emissions, which EPA extended to additional states. Successful implementation of the flexible implementation mechanism through the Acid Rain Program, in other words, led to adaptive regulation in other programs to address other pollutants that cross state borders.

The CAA’s experience with regulation fuels, however, provides a cautionary tale of flexibility mechanisms in the reformulated gasoline program, which gives refiners the ability to decide which volatile organic compounds (VOCs) to reduce. The result has been that many refiners have reduced the less-reactive VOCs, which has led to lower total ozone reduction benefits than had refiners reduced more reactive VOCs. This case illustrates that effective flexibility mechanisms must allow regulated entities to reduce their compliance costs without giving them the flexibility to deviate from the desired environmental outcomes.

Mitigating climate change will be the most expensive environmental policy in history, and achieving long-term goals depends on doing so in a cost-effective way. It will be important, for example, to allow emitters both temporal and geographic compliance flexibility in making large greenhouse gas reductions. If cap-and-trade is the chosen regulatory mechanism to reduce emissions, allowance banking and a broad geographic reach will be critical to providing the flexibility necessary to solidify long-term policy. If, instead, a carbon tax is the chosen means to regulate, designers should ensure sufficient geographic and temporal flexibility to achieve greenhouse gas reductions while maximizing cost-effectiveness.

4. Political Challenges Inform the Balance between Durability and Adaptability

It may seem axiomatic that politics matter in crafting long-term energy policy. But the fourth broad conclusion suggests that politics matters in particular ways that are highly relevant to initial design choices about the appropriate balance between durability and adaptability.

The climate policy process will take decades and ideally would include relatively frequent Congressional evaluation to ensure that the goals are being met in the most equitable, cost-effective and efficacious ways possible. The Clean Air Act amendment process worked that way for two decades after its passage, with two major amendments in 1977 and 1990 to address areas that needed reform or updating. Since 1990, however, Congress has largely stayed on the sidelines and let EPA do the regulating with its extant authority.

This inaction raises the question of how much discretion to afford an agency tasked with decarbonizing the United States’ energy sectors. If the current gridlock besetting Congress persists, in designing climate policy it might be safe to assume that should Congress eventually act to reduce greenhouse gases, it will get only one crack at the effort. Conversely, if regulatory issues remain highly politicized, swings in policy goals could make durable policy challenging and expensive, with progress only in fits and starts. Given the current political environment, policy should be crafted to maximize the discretion and flexibility granted to the implementing expert agency, consistent with other conclusions about including regularized process and backstops like deadlines, citizen suits and federalism arrangements.

5. The �䴡��’s Provisions Are Intertwined in Ways that Affect Adaptability, Durability and Flexibility

A fifth conclusion is that the CAA programs analyzed by the authors do not act in isolation. Here are several examples:

• California uses its unique mobile source authority in large part because it is required to meet National Ambient Air Quality Standards.
• The success story of eliminating lead from gasoline is in part the result of EPA authority to regulate both tailpipe emissions and fuels, in part because the agency chose effective and flexible tools for doing so and in part because EPA listed lead as a NAAQS and eliminating lead from gasoline was one of the most effective tools for meeting the NAAQS.
• The regulation of greenhouse gases from automobiles, existing power plants and new sources is in part the result of statutory provisions covering automobiles and stationary sources. But it is also the result of a broad definition of air pollutants that led the US Supreme Court to hold that greenhouse gases fit within the definition.
• The regulation of mercury and other hazardous air pollutants from coal-fired power plants leads to large reductions in other pollutants, including NAAQS pollutants like fine particulates. Thus, even though the existing/new source distinction the CAA makes in regulating stationary sources has kept many coal-fired power plants in operation long after initial predictions, the regulation of hazardous air pollutants is now bringing about NAAQS reductions indirectly.
• The market-based programs that arose out of the good neighbor provision of the CAA have helped achieve cost-effective NAAQS reductions from existing stationary sources, such as coal-fired power plants that might otherwise be exempt from stationary source provisions, like New Source Review. This is because the flexible compliance mechanisms of the cap-and-trade programs allow the regulated party, rather than the regulator, to decide where to make the reductions.

6. Process and Cooperative Federalism Are Key Ingredients of the �䴡��’s Success

Finally, it is important to emphasize that two design elements have played highly important roles in the history of the CAA: process and cooperative federalism.

Process subsumes many of the features the authors mention, including formal fact finding, peer review, notice and comment, citizen lawsuits and other details of making, implementing and enforcing rules and regulations. Process can be time consuming, tedious and frustrating. Sometimes critics of the frequently slow pace of promulgating regulations complain that process gets in the way of progress. But when political tides change, the same process can protect the carefully constructed design of regulatory programs from short-run political gamesmanship.

A second design element built into the CAA that frequently shines is cooperative federalism, the division of authority between levels of government. Cooperative federalism has often led to policy and technical innovation and has played an essential role in enabling the adaptability of the CAA and ensuring the durability of environmental outcomes. The authors have observed examples of this in the implementation of the NAAQS and in the very first applications of emissions trading and other approaches to incentive-based regulation, in which states have been test beds for new ideas. In standard-setting and planning, subsidiary jurisdictions must meet federal standards, but also are sometimes permitted to set more ambitious ones. Under the CAA, this idea has enabled state-level ratcheting of stringency, which sometimes propagates into an evolution into federal standards. With respect to climate policy under the CAA, the technical findings of EPA, which provided a legal determination of adequately demonstrated technologies for greenhouse gas emissions reductions under the Clean Power Plan, were in fact built almost entirely on experience and actions taken by the states in various energy efficiency, renewable technology and carbon dioxide cap-and-trade programs.

The process of the CAA and the role of cooperative federalism in sharing responsibility among levels of government might serve well in the future design of energy and climate policy. For example, even when the federal government enacts comprehensive policy including carbon pricing, that policy might be implemented by an expert agency, such as EPA, which could pursue a formal process to evaluate and reform the policy in order to achieve statutory goals. In the likely event that policy has multiple dimensions affecting ubiquitous emissions sources, Congress might again invoke the power of cooperative federalism to achieve outcomes in innovative ways. Indeed, it is worth remembering that under the CAA, the original authority for regulation rests with the states and is only taken up to the federal level through overt action. Congress would do well to consider that balance carefully.

The hope is that these six broad conclusions, and the more in-depth analy­ses the authors have conducted in Lessons from the Clean Air Act, can provide guidance for a long-lasting, evolving and cost-effective energy and climate policy for the rest of this century.

Finding the right balance between discretion and prescription will be critical to making long-term energy policy adaptable. Perhaps counterintuitively, the CAA demonstrates that adaptability may actually produce policy durability and will be crucial in enabling long-term emissions reduction in US energy sectors. Flexibility in compliance, too, can enhance both durability and adaptability, particularly given the scope and scale of the climate change problem.

In designing long-term climate policy, legislators will also need to be sensitive to the likelihood that they will engage in necessary revisions in the future; if revision and reform are unlikely, or might come too often, the implementing agency should be given more discretion to adhere to science and adapt to new information (but with processes designed to check excess discretion).

Additionally, paying attention to the interactions between and among a statute’s provisions may well enhance the adaptability, durability and flexibility of the statute. The authors find that policies across a complex statute like the CAA can be mutually reinforcing so that broad definitions combined with regulatory discretion, for example, can produce adaptable, effective regulation.

And finally, future energy and climate policy should consider the crucial roles of regulatory process executed by an expert agency and cooperative federalism as mechanisms that maintain momentum while enabling democracy in pursuit of initial and potentially far-reaching legislative goals.

Designing long-term climate and energy policy in a very dynamic and complex world is one of today’s greatest policy challenges. The �䴡��’s successes in continuing to address pollution problems for fifty years in innovative and cost-effective ways provide a story that can guide policymakers in addressing this challenge.