By Richard Black
BBC environment correspondent
Twelve US states, three cities and several prominent environmental groups told a court on Friday that the United States government had a legal duty to restrict greenhouse gas emissions.
They said the Clean Air Act mandated the government's Environmental Protection Agency (EPA) to regulate all emissions which damaged human welfare.
The hearing was the latest step in a battle dating back to 1999, before George W Bush came to power.
A number of other states, together with bodies representing industry, oppose the case, which would if successful force a policy U-turn from the Bush administration.
In 1999, a coalition of 19 environmental groups petitioned the EPA, asking it to regulate emissions of carbon dioxide and other greenhouse gases from motor vehicles, describing them as "pollutants".
Four years later, under threat of legal action, the EPA responded by denying the petition.
"Congress has not granted EPA authority under the Clean Air Act to regulate CO2 and other greenhouse gases for climate change purposes", it said at the time.
That interpretation is disputed by the parties behind Friday's court hearing.
"This case is about the EPA ignoring its responsibilities," said Massachusetts Assistant Attorney-General James Milkey, who is leading the plaintiffs' legal team.
"Since September 2003, it has hidden behind this claim that it doesn't have to regulate; in fact the Clean Air Act defines 'pollution' in a way that does include greenhouse gases and includes effects on climate."
Following that EPA decision, 13 environmental groups filed a case in the US Court of Appeals asking for a review; its wording only covers vehicle emissions, but the plaintiffs say that if successful it would automatically be extended to other sources of greenhouse gases.
Those 13 initial groups have since been joined by the authorities of states, cities and US overseas territories; many have submitted written evidence to the Court of Appeals, the second-highest tier in the American judicial system, and on Friday they presented oral arguments and answered questions from the three judges hearing the case.
Among the groups which have submitted evidence supporting the EPA's position are the Alliance of Automobile Manufacturers, the American Petroleum Institute, the National Association of Convenience Stores and the US Chamber of Commerce, as well as 11 states including George W Bush's homeland of Texas, and the EPA itself.
The key clause in the Clean Air Act is 202(a)(1), which states "The Administrator 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 his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare."
The plaintiffs say the evidence that climate change endangers the health and welfare of American citizens is clear.
"Just as one example," says James Milkey, "the State of Massachusetts owns 200 miles of coastline which is at risk from rising sea levels; and that's directly due to the EPA's reluctance to regulate greenhouse gases."
In court on Friday, Jeffrey Clark, deputy assistant attorney general for the Justice Department's environment division, said the EPA would have no easy way to regulate carbon dioxide from motor vehicles.
"For CO2, there's no catalytic converter," he argued. "There's no catch mechanism. The only way to reduce them is through fuel economy. ...The point is it would usurp National Highway Traffic Safety Administration's authority," he was reported as saying by the Associated Press.
The argument being made was that the EPA did not have the authority to act.
After their day in court, the plaintiffs were expecting to wait several months for a judgement.