The Supreme Court ruled in 2007 the Clean Air Act gives the Environmental Protection Agency had the power to regulate "greenhouse gases" emitted from vehicles. Justice Anthony Kennedy, a key swing vote, joined the four liberals to form the 5-4 majority.
The court majority said climate change was real, and the EPA had the authority to do something about it when it came to vehicle emissions.
"A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere," Justice John Paul Stevens said in the majority opinion. "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.'"
The court majority concluded that "greenhouse gases fit well within the [Clean Air] Act's capacious definition of 'air pollutant,' EPA has statutory authority to regulate emission of such gases from new motor vehicles."
Riding the strength of that ruling, EPA identified greenhouse gases -- carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride.
The issue in the current case, heard Monday by the justices, is much broader: Does the EPA have the authority under the Clean Air Act to regulate greenhouse gas emissions from "stationary sources" -- such as power plants and factories -- stemming from its power to regulate emissions from new vehicles?
Most media observers declared the court, with its four conservatives and four liberals, appeared fairly evenly divided during argument, with Kennedy once again probably providing the deciding vote.
Though Kennedy kept his cards close to his vest during argument, he was occasionally critical of U.S. Solicitor General Donald Verrilli's argument for the EPA.
"I have to say in reading the brief for the states [supporting the EPA] and reading your brief, I couldn't find a single precedent that strongly supports your position" changing the threshold requirements of the Clean Air Act, Kennedy said.
From the bench, conservative Justice Antonin Scalia and liberal Justice Elena Kagan fenced with each other over whether the administration's interpretation of the Clean Air Act was reasonable.
But Chief Justice John Roberts, one of the conservatives, rejected an industry lawyer's suggestion that the 2007 ruling be put aside.
"Counsel [attorney Peter Keisler], you began that discussion by saying putting Massachusetts vs. EPA to one side," Roberts said. "I was in the dissent in that case, but we still can't do that."
That was a clear signal Roberts may not be willing to revisit the issue decided in the 2007 case -- that the EPA has the authority to regulate some greenhouse gas emissions.
Media interest in the case has been high.
The Washington Post describes the dispute as a classic standoff: The court's first big battle over climate change came in 2007, started by states and environmentalists who said the EPA under President George W. Bush had chosen not to regulate greenhouse gases.
The current case has different states and industries saying President Obama's EPA "running amok" with the power it had been granted in 2007, the Post said.
The Christian Science Monitor sees the case as an end-run around the Republican majority in the U.S. House.
One point of interest in the case: The act defines a major source of pollutant as one that emits 100 to 250 tons of conventional air pollutants, such as lead, each year.
Carbon dioxide, the most common greenhouse gas, is emitted in much larger amounts from individual sources, and those low parameters would require the regulation of millions of sources, such as hospitals, unless the threshold was raised, the New York Times explained.
To make the act enforceable, the EPA set the threshold at 100,000 tons each year, arguing that was more in line with what Congress intended in the Clean Air Act.
Though the justices are considering the case on very narrow grounds, one of the EPA's challengers, the state of Texas, said the numbers are an integral part of the equation.
In its brief, Texas asked the justices to decide in part whether the numbers change "violates the [Clean Air] Act by replacing Congress's unambiguous numerical permitting thresholds with criteria of EPA's own choosing," and whether "Congress authorized EPA to regulate greenhouse-gas emissions from stationary sources, given that the act imposes permitting thresholds that are absurdly low if applied to carbon dioxide."
In other words, Texas argued, the low threshold meant Congress could not have been talking about greenhouse gases from industry in the Clean Air Act.
In the case heard Monday, six separate challenges, brought by Texas and such entities as the U.S. Chamber of Commerce and the American Chemical Council, were consolidated for 90 minutes of argument.
Legal briefs in the consolidated case were so prolific, the justices limited their wording: 45,000 words for the challengers "in aggregate"; friend-of-the-court briefs supporting the challengers "shall not exceed 6,000 words each." The Obama administration brief "shall not exceed 15,000 words," while other briefs "shall not exceed 10,000 words each."
One of those briefs in support of the EPA came from the District of Columbia, a clutch of states -- New York, California, Connecticut, Delaware, Illinois, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New Mexico, Oregon, Rhode Island, Vermont and Washington -- and New York City.
"Citing a 'very large and comprehensive base of scientific information,' EPA found that the increase in solar energy trapped inside the Earth's atmosphere by these gases causes serious harms that are felt on a local level," the states' brief said, "including more intense, frequent and long-lasting heat waves; exacerbated smog in cities; longer and more severe droughts; more intense storms; the spread of disease, and a dramatic rise in sea levels."
The brief argued: "By its plain terms, the Clean Air Act's provisions for preventing the significant deterioration of air quality apply to major stationary emitters of any air pollutant. This [Supreme] Court has twice held, in Massachusetts and Connecticut, that the phrase 'air pollutant' in the act encompasses greenhouse gases, thereby authorizing EPA to regulate emissions of those air pollutants from both motor
vehicles and stationary sources. The identical use of this broad language in the PSD ['prevent serious deterioration'] program thus compels its application to major emitters of greenhouse gases.
"The PSD program's substantive requirements reinforce its broad scope. In particular, stationary sources subject to PSD permitting must apply the best available control technology for 'each pollutant subject to regulation' under the act -- language that unambiguously encompasses greenhouse gases."