A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere. Respected scientists believe the two trends are related. For when carbon dioxide is released into the atmosphere, it acts like the ceiling of a greenhouse, trapping solar energy and retarding the escape of reflected heat. It is therefore a species—the most important species—of a ‘‘greenhouse gas.’’ 

Calling global warming ‘‘the most pressing environmental challenge of our time,’’ a group of States, local governments, and private organizations, alleged * * * that the Environmental Protection Agency (EPA) has abdicated its responsibility under the Clean Air Act to regulate the emissions of four greenhouse gases, including carbon dioxide. Specifically, petitioners asked us to answer two questions * * * : whether EPA has the statutory authority to regulate greenhouse gas emissions from new motor vehicles; and if so, whether its stated reasons for refusing to do so are consistent with the statute.

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Section 202(a)(1) of the Clean Air Act, [citation], provides:

The [EPA] Administrator shall by regulation prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.

The Act defines ‘‘air pollutant’’ to include ‘‘any air pollution agent or combination of such agents, including any physical, chemical, biological, radioactive … substance or matter which is emitted into or otherwise enters the ambient air.’’ [Citation.] ‘‘Welfare’’ is also defined broadly: among other things, it includes ‘‘effects on … weather … and climate.’’ [Citation.]

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In the late 1970’s, the Federal Government began devoting serious attention to the possibility that carbon dioxide emissions associated with human activity could provoke climate change. In 1978, Congress enacted the National Climate Program Act, [citation], which required the President to establish a program to ‘‘assist the Nation and the world to understand and respond to natural and man-induced climate processes and their implications,’’ [citation]. President Carter, in turn, asked the National Research Council, the working arm of the National Academy of Sciences, to investigate the subject. The Council’s response was unequivocal: ‘‘If carbon dioxide continues to increase, the study group finds no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible.… A wait-and-see policy may mean waiting until it is too late.’’

Congress next addressed the issue in 1987, when it enacted the Global Climate Protection Act, [citation]. Finding that ‘‘man-made pollution—the release of carbon dioxide, chlorofluorocarbons, methane, and other trace gases into the atmosphere—may be producing a long-term and substantial increase in the average temperature on Earth,’’ [citation], Congress directed EPA to propose to Congress a ‘‘coordinated national policy on global climate change,’’ [citation], and ordered the Secretary of State to work ‘‘through the channels of multilateral diplomacy’’ and coordinate diplomatic efforts to combat global warming, [citation]. * * *

Meanwhile, the scientific understanding of climate change progressed. In 1990, the Intergovernmental Panel on Climate Change (IPCC), a multinational scientific body organized under the auspices of the United Nations, published its first comprehensive report on the topic. Drawing on expert opinions from across the globe, the IPCC concluded that ‘‘emissions resulting from human activities are substantially increasing the atmospheric concentrations of … greenhouse gases [which] will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface.’’

Responding to the IPCC report, the United Nations convened the ‘‘Earth Summit’’ in 1992 in Rio de Janeiro. The first President Bush attended and signed the United Nations Framework Convention on Climate Change (UNFCCC), a nonbinding agreement among 154 nations to reduce atmospheric concentrations of carbon dioxide and other greenhouse gases for the purpose of ‘‘prevent[ing] dangerous anthropogenic [i.e., human-induced] interference with the [Earth’s] climate system.’’ [Citation.] The Senate unanimously ratified the treaty.

Some five years later—after the IPCC issued a second comprehensive report in 1995 concluding that ‘‘[t]he balance of evidence suggests there is a discernible human influence on global climate’’—the UNFCCC signatories met in Kyoto, Japan, and adopted a protocol that assigned mandatory targets for industrialized nations to reduce greenhouse gas emissions. Because those targets did not apply to developing and heavily polluting nations such as China and India, the Senate unanimously passed a resolution expressing its sense that the United States should not enter into the Kyoto Protocol. [Citation.] President Clinton did not submit the protocol to the Senate for ratification.

On October 20, 1999, a group of 19 private organizations filed a rulemaking petition asking EPA to regulate ‘‘greenhouse gas emissions from new motor vehicles under * * * the Clean Air Act.’’ * * *

Fifteen months after the petition’s submission, EPA requested public comment on ‘‘all the issues raised in [the] petition,’’ adding a ‘‘particular’’ request for comments on ‘‘any scientific, technical, legal, economic or other aspect of these issues that may be relevant to EPA’s consideration of this petition.’’ [Citation.] EPA received more than 50,000 comments over the next five months. [Citation.]

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On September 8, 2003, EPA entered an order denying the rulemaking petition. [Citation.] The agency gave two reasons for its decision: (1) that contrary to the opinions of its former general counsels, the Clean Air Act does not authorize EPA to issue mandatory regulations to address global climate change, [citation]; and (2) that even if the agency had the authority to set greenhouse gas emission standards, it would be unwise to do so at this time, [citation].

In concluding that it lacked statutory authority over greenhouse gases, EPA observed that Congress ‘‘was well aware of the global climate change issue when it last comprehensively amended the [Clean Air Act] in 1990,’’ yet it declined to adopt a proposed amendment establishing binding emissions limitations. * * * 

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Having reached that conclusion, EPA believed it followed that greenhouse gases cannot be ‘‘air pollutants’’ within the meaning of the Act. * * *

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The agency furthermore characterized any EPA regulation of motor-vehicle emissions as a ‘‘piecemeal approach’’ to climate change and stated that such regulation would conflict with the President’s ‘‘comprehensive approach’’ to the problem, [citation]. That approach involves additional support for technological innovation, the creation of nonregulatory programs to encourage voluntary private-sector reductions in greenhouse gas emissions, and further research on climate change—not actual regulation. [Citation.] According to EPA, unilateral EPA regulation of motor-vehicle greenhouse gas emissions might also hamper the President’s ability to persuade key developing countries to reduce greenhouse gas emissions. [Citation.]

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The harms associated with climate change are serious and well recognized. Indeed, the NRC Report itself— which EPA regards as an ‘‘objective and independent assessment of the relevant science,’’ [citation]—identifies a number of environmental changes that have already inflicted significant harms, including ‘‘the global retreat of mountain glaciers, reduction in snow-cover extent, the earlier spring melting of rivers and lakes, [and] the accelerated rate of rise of sea levels during the 20th century relative to the past few thousand years.’’ [Citation.]

Petitioners allege that this only hints at the environmental damage yet to come. * * *

That these climate-change risks are ‘‘widely shared’’ does not minimize Massachusetts’ interest in the outcome of this litigation. * * * The severity of that injury will only increase over the course of the next century: If sea levels continue to rise as predicted, one Massachusetts official believes that a significant fraction of coastal property will be ‘‘either permanently lost through inundation or temporarily lost through periodic storm surge and flooding events.’’ [Citation.] Remediation costs alone, petitioners allege, could run well into the hundreds of millions of dollars. [Citations.]

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EPA does not dispute the existence of a causal connection between man-made greenhouse gas emissions and global warming. At a minimum, therefore, EPA’s refusal to regulate such emissions ‘‘contributes’’ to Massachusetts’ injuries.

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* * * Even leaving aside the other greenhouse gases, the United States transportation sector emits an enormous quantity of carbon dioxide into the atmosphere—* * * more than 1.7 billion metric tons in 1999 alone. [Citation.] That accounts for more than 6% of worldwide carbon dioxide emissions. [Citations.] To put this in perspective: Considering just emissions from the transportation sector, which represent less than one-third of this country’s total carbon dioxide emissions, the United States would still rank as the third-largest emitter of carbon dioxide in the world, outpaced only by the European Union and China. Judged by any standard, U.S. motor-vehicle emissions make a meaningful contribution to greenhouse gas concentrations and hence, according to petitioners, to global warming.

While it may be true that regulating motor-vehicle emissions will not by itself reverse global warming, it by no means follows that we lack jurisdiction to decide whether EPA has a duty to take steps to slow or reduce it. [Citation.] Because of the enormity of the potential consequences associated with man-made climate change, the fact that the effectiveness of a remedy might be delayed during the (relatively short) time it takes for a new motor-vehicle fleet to replace an older one is essentially irrelevant. * * *

We moreover attach considerable significance to EPA’s ‘‘agree[ment] with the President that ‘we must address the issue of global climate change,’’’ [citation], and to EPA’s ardent support for various voluntary emission-reduction programs. * * *

In sum—at least according to petitioners’ uncontested affidavits—the rise in sea levels associated with global warming has already harmed and will continue to harm Massachusetts. The risk of catastrophic harm, though remote, is nevertheless real. That risk would be reduced to some extent if petitioners received the relief they seek. We therefore hold that petitioners have standing to challenge the EPA’s denial of their rulemaking petition.

The scope of our review of the merits of the statutory issues is narrow. As we have repeated time and again, an agency has broad discretion to choose how best to marshal its limited resources and personnel to carry out its delegated responsibilities. * * *

* * * We therefore ‘‘may reverse any such action found to be … arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.’’ [Citation.]

On the merits, the first question is whether * * * the Clean Air Act authorizes EPA to regulate greenhouse gas emissions from new motor vehicles in the event that it forms a ‘‘judgment’’ that such emissions contribute to climate change. We have little trouble concluding that it does. In relevant part, [the Clean Air Act] provides that EPA

shall by regulation prescribe … standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in [the Administrator’s] judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare. [Citation.]

Because EPA believes that Congress did not intend it to regulate substances that contribute to climate change, the agency maintains that carbon dioxide is not an ‘‘air pollutant’’ within the meaning of the provision.

The statutory text forecloses EPA’s reading. The Clean Air Act’s sweeping definition of ‘‘air pollutant’’ includes ‘‘any air pollution agent or combination of such agents, including any physical, chemical … substance or matter which is emitted into or otherwise enters the ambient air.’’ [Citation.] On its face, the definition embraces all airborne compounds of whatever stripe, and underscores that intent through the repeated use of the word ‘‘any.’’ Carbon dioxide, methane, nitrous oxide, and hydro fluoro-carbons are without a doubt ‘‘physical [and] chemical … substance[s] which [are] emitted into … the ambient air.’’ The statute is unambiguous.

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EPA finally argues that it cannot regulate carbon dioxide emissions from motor vehicles because doing so would require it to tighten mileage standards, a job (according to EPA) that Congress has assigned to DOT. [Citation.] But that DOT sets mileage standards in no way licenses EPA to shirk its environmental responsibilities. EPA has been charged with protecting the public’s ‘‘health’’ and ‘‘welfare,’’ [citation], a statutory obligation wholly independent of DOT’s mandate to promote energy efficiency. [Citation.] The two obligations may overlap, but there is no reason to think the two agencies cannot both administer their obligations and yet avoid inconsistency.

While the Congresses that drafted §202(a)(1) might not have appreciated the possibility that burning fossil fuels could lead to global warming, they did understand that without regulatory flexibility, changing circumstances and scientific developments would soon render the Clean Air Act obsolete. The broad language [the Clean Air Act] reflects an intentional effort to confer the flexibility necessary to forestall such obsolescence. [Citation.] * * *

The alternative basis for EPA’s decision—that even if it does have statutory authority to regulate greenhouse gases, it would be unwise to do so at this time—rests on reasoning divorced from the statutory text. While the statute does condition the exercise of EPA’s authority on its formation of a ‘‘judgment,’’ [citation], that judgment must relate to whether an air pollutant ‘‘cause[s], or contribute[s] to, air pollution which may reasonably be anticipated to endanger public health or welfare,’’ [citation]. Put another way, the use of the word ‘‘judgment’’ is not a roving license to ignore the statutory text. It is but a direction to exercise discretion within defined statutory limits.

If EPA makes a finding of endangerment, the Clean Air Act requires the agency to regulate emissions of the deleterious pollutant from new motor vehicles. [Citation.] EPA no doubt has significant latitude as to the manner, timing, content, and coordination of its regulations with those of other agencies. But once EPA has responded to a petition for rulemaking, its reasons for action or inaction must conform to the authorizing statute. Under the clear terms of the Clean Air Act, EPA can avoid taking further action only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do. * * *

EPA has refused to comply with this clear statutory command. Instead, it has offered a laundry list of reasons not to regulate. * * *

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In short, EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change. Its action was therefore ‘‘arbitrary, capricious, … or otherwise not in accordance with law.’’ [Citation.] We need not and do not reach the question whether on remand EPA must make an endangerment finding, or whether policy concerns can inform EPA’s actions in the event that it makes such a finding. [Citation.] We hold only that EPA must ground its reasons for action or inaction in the statute.

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