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Supreme Court Decision Paves Way for Federal Regulation of Greenhouse Gas Emissions from Vehicles

The Supreme Court’s decision in Massachusetts v. EPA today (earlier post) may prove to be a landmark in the movement toward national regulation of greenhouse gas emissions from mobile as well as stationary sources. In the majority decision, the Supreme Court held that carbon dioxide and greenhouse gases are pollutants can be regulated under the Clean Air Act, and that as such the Environmental Protection Agency has the authority to set regulatory standards for greenhouse gas emissions from motor vehicles.

The Court ordered the EPA to reconsider its earlier decision not to regulate greenhouse gases, and to ground its arguments in the Clean Air Act.

The case originated when a group of private organizations petitioned the Environmental Protection Agency (EPA) to begin regulating greenhouse gases, including carbon dioxide, under the Clean Air Act. The Clean Air Act requires that the EPA establish “standards applicable to the emission of any air pollutant from any class of new motor vehicles which...cause[s], or contribute[s] to, air pollution reasonably anticipated to endanger public heath or welfare.

EPA denied the petition, reasoning that:

  1. The Clean Air Act does not authorize it to issue mandatory regulations to address global climate change, and

  2. Even if it did have the authority to set greenhouse gas emission standards, it would have been unwise to do so at that time because a causal link between greenhouse gases and the increase in global surface air temperatures was not unequivocally established.

The EPA also characterized any theoretical regulation of greenhouse gas emissions from motor vehicles as a piecemeal approach to climate change that would conflict with the Administration’s non-regulatory approach to encouraging reduction, among other factors.

The original petitioners, joined by Massachusetts and other state and local governments, sought review of the decision in the DC Circuit, which denied review in a 2-1 decision. The case moved to the Supreme Court, and was heard 29 Nov 2006. In the 5-4 decision today, the Court held that:

  1. The Petitioners do in fact have standing to challenge the EPA’s denial of the rulemaking petition.

  2. 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, 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 ...

  3. EPA’s failure to dispute the existence of a causal connection between man-made greenhouse gas emissions and global warming, its refusal to regulate such emissions, at a minimum, “contributes” to Massachusetts’ injuries. EPA overstates its case in arguing that its decision not to regulate contributes so insignificantly to petitioners’ injuries that it cannot be haled into federal court, and that there is no realistic possibility that the relief sought would mitigate global climate change and remedy petitioners’ injuries, especially since predicted increases in emissions from China, India, and other developing nations will likely offset any marginal domestic decrease EPA regulation could bring about.

    ...EPA overstates its case. Its argument rests on the erroneous assumption that a small incremental step, because it is incremental, can never be attacked in a federal judicial forum. Yet accepting that premise would doom most challenges to regulatory action. Agencies, like legislatures, do not generally resolve massive problems in one fell regulatory swoop...They instead whittle away at them over time, refining their preferred approach as circumstances change and as they develop a more-nuanced understanding of how best to proceed.

    That a first step might be tentative does not by itself support the notion that federal courts lack jurisdiction to determine whether that step conforms to law.

    And reducing domestic automobile emissions is hardly a tentative step. Even leaving aside the other greenhouse gases, the United States transportation sector emits an enormous quantity of carbon dioxide into the atmosphere according to the MacCracken affidavit, more than 1.7 billion metric tons in 1999 alone. That accounts for more than 6% of worldwide carbon dioxide emissions.

    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, US motor-vehicle emissions make a meaningful contribution to greenhouse gas concentrations and hence, according to petitioners, to global warming.

  4. While regulating motor-vehicle emissions may not by itself reverse global warming, it does not follow that the Court lacks jurisdiction to decide whether EPA has a duty to take steps to slow or reduce it.

    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. Nor is it dispositive that developing countries such as China and India are poised to increase greenhouse gas emissions substantially over the next century: A reduction in domestic emissions would slow the pace of global emissions increases, no matter what happens elsewhere.

  5. Despite an agency’s refusal to initiate enforcement proceedings not ordinarily being subject to judicial review, the Court held that it “may reverse [it if it finds it to be] arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.

  6. Because greenhouse gases fit well within the Act’s capacious definition of “air pollutant,” EPA has statutory authority to regulate emission of such gases from new motor vehicles.

    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...§7602(g) (emphasis added). 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 hydrofluorocarbons are without a doubt “physical [and] chemical...substance[s] which [are] emitted into...the ambient air.”  The statute is unambiguous.

  7. EPA’s duty is to protect the public health and welfare, even if by setting standard in doing so, it overlaps with the responsibilities of other agencies.

    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. 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,” a statutory obligation wholly independent of DOT’s mandate to promote energy efficiency. The two obligations may overlap, but there is no reason to think the two agencies cannot both administer their obligations and yet avoid inconsistency.

  8. EPA’s alternative argument for its decision—that even if it had authority to regulate greenhouse gases, the timing was not good—is “divorced from the statutory text” and is “impermissible.”Its action was “arbitrary, capricious, or otherwise not in accordance with law.” EPA must reconsider its decision, and ground its reasons for action or inaction in the Clean Air Act statute.

Justice Stevens delivered the opinion of the Court, in which Justices Kennedy, Souter, Ginsburg, and Breyer joined. Chief Justice Roberts filed a dissenting opinion, in which Justices Scalia, Thomas, and Alito joined. Justice Scalia also filed a dissenting opinion, in which Chief Justice Roberts and Justices Thomas and Alito joined.

Reaction from the Alliance of Automobile Manufacturers was somewhat muted. In a statement concerning the decision, Alliance president and CEO Dave McCurdy said:

The Alliance of Automobile Manufacturers believes that there needs to be a national, federal, economy-wide approach to addressing greenhouse gases.  This decision says that the US Environmental Protection Agency will be part of this process. The Alliance looks forward to working constructively with both Congress and the administration, including EPA and the National Highway Traffic Safety Administration, in developing a national approach.

The Alliance is one of the groups supporting the challenge in court to California’s law regulating motor vehicle greenhouse gas emissions.




If reduced throttling losses were all of it, a smaller engine would be better than an Atkinson cycle.  The only reason a diesel won't derive as much benefit is because its end gases are cooler and have less remaining recoverable expansion.


The narrow nature of this ruling demonstrates that the Democratic congress should do everything in its power to resist any future nominee to the Supreme Court who would vote along the lines of Roberts, Alito, of Scaglia with respect to issues of this nature. It's not all about abortion.

Having said that, the ball is still in Congress' court. There will not be anything meaningful coming out of this admistration. So, while I welcome the ruling, it's just a feel good exercise given the proclivity of this adminitration to either disobey the law or make up its own laws through signing statements.


I didn't say reduced throttling losses were "all of it". Atkinson gains efficiency in two main ways: extracting more energy from the expansion stroke and reducing pumping losses. A diesel might improve efficiency via the former, but not the latter.

That's why I said Atkinson doesn't help diesel "as much". If throttling losses were all of it I would have said it doesn't help diesel "at all".


Talking of TV´s it is a fact when colour tv was introduced into England in 1967 , a colour tv set cost in the region of 750 pounds , at this time you could buy a brand new Ford Cortina family car for 750 pounds then the total of an average years
wages ,
And guess what , people bought them ! they could not make enough of them !

If we are ever allowed to buy a mass market BEV PHEV , I am sure they
will sell in large numbers , it is just a question of when .


I believe that these are the steps needed to get the alternative fuels debate on track:

1. Significantly revamp Corporate Average Fuel Economy (CAFE) standards to drive improvements in fuel efficiency
2. Legislate to require MPG in advertising
3. Legislate to require fuel consumption instrumentation in the dashboard
4. Define an allowance and taxation system based on fuel consumption at vehicle purchase and during annual registration/licensing, revenues to fund research, development and adoption.
5. Support the Bio-diesel industry, facilitating fast infrastructure permitting and making Bio-diesel and Bio-diesel blends available nation-wide
6. Align Global (US, Japanese and European) emissions standards

See more detail at



Example number 2 then:

P25 mandated for all (Public Safety) radios. You canNOT get federal grants for your communications systems in ANY state unless the equipment you will be purchasing is the P25 "interoperability" radios. Seems to have been a boon to everyone even though a Motorola XTS 5000 is about 3-4 times the cost of a Motorola HT1550 LS+ with nearly the same feature set.

This is a govt. mandated one size fits all technology only purchasable through one particular vendor (the codec that is; not the radios).

Or would you prefer no govt mandates and more OK city bombings, and 9/11s where someone is having to run from one agency to another with notes on a piece of paper and helicopters are reporting that a building is going to collapse but the firefighters can't communicate with them to get out of the building in time.


MBTE was mandated to clean auto emissions which it did.It also contaminated ground water supplies.
ULSD reduces emissions from diesel fuel.I believe it is refined with very large amounts of hydrogen to get to ULSD standards.Havent I read here that creating the hydrogen releases large amounts of co2.Is ULSD that much better then?
Beware of unintended consequences with legislation.I just urge proceeding with much forethought and care.
I recall President Clinton taxing yachts to get the rich to pay their fair share.U.S yacht building was devastated and the blue collar workers lost their jobs.the rich put off buying or bought foreign built yachts.he ended up repealing the tax.
Im a little queasy about legal precedent also.Can we open ourselves to international litigation because the supreme cout has determined we are drowning Bangladesh?legal precedent is also fraught with unintended consequences.


The only reason a diesel won't derive as much benefit is because its end gases are cooler and have less remaining recoverable expansion. -- E-P

IOW, the diesel expansion ratio is higher than what's used in current Atkinson engines, especially for turbodiesels.

Atkinson gains efficiency in two main ways: extracting more energy from the expansion stroke and reducing pumping losses. -- ddw

How does Atkinson reduce throttle losses?

ULSD reduces emissions from diesel fuel.I believe it is refined with very large amounts of hydrogen to get to ULSD standards.Havent I read here that creating the hydrogen releases large amounts of co2.Is ULSD that much better then? -- Earl

Same issue with gasoline, only the gasoline limit is 30 ppm (that will come down as well). This problem -- gasoline or diesel -- is only going to get worse as remaining petroleum stocks get sourer, and the gasoline (and to an extent diesel) refining costs will only go higher as the stocks get heavier.

This is a tradeoff -- high aerosol levels appear to have mitigated GHG induced warming. No one disputes that PM has serious health effects, though. Of course, using less fossil fuel reduces both PM and CO2, but that's probably too obvious a solution.


Plenty of good comments here, but I don't see anyone addressing the idea that The Supreme Court might have overstepped the boundries of their function in our government. Am I the only one that sees a problem in that area?



This ruling is opening the whole can of worms. The scale of consequences is hard to imagine. Interesting comments (including some legal opinions from lawyers) could be found on climateaudit.org:



Bob:  The Supreme Court did nothing except tell EPA (a part of the executive branch, charged with executing the laws of the land) what the Clean Air Act (written by the legislative branch) means.  That's exactly what the Supreme Court's job is:  resolving disputes about what the law (including the Constitution itself) means.

AutoXprize - 100MPG challenge

Actually, I don't think any government agency can manage/enforce this without wasting billions upon billions

Let the free market work freely. Just look at the March sales numbers and you'll see how Toyota and Honda are forcing GM/Ford/etc toward a fuel efficient (possibly hybrid) lineup

March US auto sales:

BMW Group –1.4% at 28980 (3/06: 28,352)
Chrysler Group –8% at 206,435 (3/06: 216,412)
Ford Motor Co –12.24% at 264,975 (3/06: 291,146)
General Motors –7.7% at 349,867 (3/06: 365,375)
Honda America +7.3% at 143,392 (3/06: 128,806)
Nissan North America +3.9% at 111,119 (3/06: 103,095)
Toyota Motor Co. +7.7% at 242,675 (3/06: 217,286)
Volkswagen –19.3% at 17,355 (3/06: 20,730)


There won't be any true free-market solutions until people have to pay for their carbon emissions.


How does Atkinson reduce throttle losses? --cidi

By reducing intake manifold vacuum at low load. The compression cylinder's intake valve stays open during the first part of the stroke. This pushes charge out of the compression cylinder and back into the intake manifold, where it feeds the intake cylinder. The intake cylinder does not have to suck fresh air past the nearly-closed throttle plate during this phase, so manifold pressure is higher and pumping losses lower.

The effect is especially noticeable at low load when the valve overlap maxes out.


This pushes charge out of the compression cylinder and back into the intake manifold, where it feeds the intake cylinder. -- ddw

Interesting, thanks. Still and all, it strikes me that if you insist on the ICE, diesels do more on less fossil fuel than gassers.


I think the ruling says that the EPA CAN do something about it, it does not say they HAVE to. It opens the door for legal action if they do not have a good reason for not taking action. It holds them accountable for not taking action when public health and safety are at stake. It also said that if they have a good reason, like it is not cost effective, they do not have to take action..stay tuned for further developments on this one.


In consideration of this and noting that we have been moving forward in this nation to consume renewable energy resources such as ethanol and biodiesel, have we considered the impact that manufacturing ethanol and biodiesel has on our immission as well. Along with increased CO2 distrubution in making ULSD given the fact that we burn coal in the manufacturing process of making ethanol again what have we gained. Coal is deemed the most highest in polluting the air in that when burned it to emits CO2.. much more than any everyday driving would ever produce.

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