Yes, said five justices  on the U.S. Supreme Court in 2007, when they ruled that greenhouse gases qualified under the Clean Air Act’s definition of a “pollutant.” Under the Constitution, the Supreme Court is the final arbiter of Congress’s intent in passing legislation, so politicians on both sides of the aisle figured that issue was settled. Unless, that is, Congress were to amend the Clean Air Act to rule out certain gases from regulation.
As expected, the new political tide on climate change  has brought a raft of legislative proposals to do just that. A key pair: a finalized bill  by Senator John Barrasso (R-WY) and a "discussion draft " by the House of Representatives Energy and Commerce Committee Chair Fred Upton (R-I) and Senator Jim Inhofe (R-OK).
The lawmakers and their allies know the 2007 decision means Congress must pass legislation if it wants to force the U.S. Environmental Protection Agency (EPA) to stop its march toward regulating greenhouse gas emissions . That hasn’t stopped them from seeking to “clarify,” as Upton put it this week , that “the Clean Air Act was not written by Congress to address climate change.” It’s a common  argument  among critics  of  President Barack Obama's EPA’s moves on climate.
Congress explicitly allowed for evolving atmospheric science in the Clean Air Act when it overhauled existing air quality law in 1970 and amended that bill subsequently. And discussion of the nascent science of climate change is part of the congressional record in the original version of the law and its updates. Still, the question of whether CO2 is pollution is now at the fore of the debate about whether to amend the act again.
The Clean Air Act named six known pollutants , including lead and soot. But it also set up a process called the “endangerment finding” that EPA would use to decide whether additional pollutants should be regulated under the act or adjust its standards for allowable pollution.
“Congress said to the EPA: We want you to be watching the science. You’re supposed to be on guard. When the science shows there’s a danger, then you need to act. Don’t come to us for instructions,” says David Doniger, an attorney with the Natural Resources Defense Council in Washington, D.C.
There’s evidence that during the drafting of the act and its subsequent amendments that climate was one of the dangers Congress was thinking about. In 1970, as mentioned here , Senator Caleb Boggs (R-DE) said during debate on the law that “Air pollution alters climate and may produce global changes in temperature.” As laid out here , in 1977 a report by the House that accompanied an update to the law mentioned “possible weather and climate modifications” among the risks to particulate matter it was seeking to regulate.
But advocates who say the Supreme Court got it wrong say Congress would have been more explicit if CO2 and other greenhouse gases were meant to be regulated. Their arguments are likely to be repeated as the debate about the EPA’s efforts today heats up. In a brief  to the high court in 2007, car companies and other industries quoted from several dictionaries on the definition of “pollutant” and went on to argue that:
Greenhouse gases such as carbon dioxide and water vapor do not contaminate the air we breathe; rather, they are an integral part of a complex, dynamic climate system. Although increased or decreased levels of these naturally occurring compounds may affect global ambient temperatures, that does not mean that the air is becoming more or less polluted. The textual provisions of the Clean Air Act, which vest EPA with authority to promulgate regulations designed to “clean” the air and make it fit to breathe, cannot be blithely transformed into an immensely broad mandate to reshape the composition of the Earth’s atmosphere.
The Supreme Court flatly denied this reasoning, saying it had “little trouble” seeing the contrary and that the statute was “unambiguous” on the question. Members of Congress trying to amend the Clean Air Act to exclude climate regulation face an even steeper battle: They may have a tough time corralling votes for such a bill to pass the Senate and even if they did, Obama has said he would veto any such bill.
But there's another approach that opponents are taking. Lawsuits  are coming fast and furious. Matthew McGill of law firm Gibson, Dunn and Crutcher  in Washington, D.C., told ScienceInsider that the main issue isn't whether CO2 is a pollutant. Even though the Supreme Court decision meant that EPA could regulate CO2, McGill says that the ruling didn’t cover “the legality of any particular regulation.” So legal challenges to individual efforts by the agency to clamp down on greenhouse gas emissions may fare better than attacks on the court’s central reasoning in 2007.