By Nick Miles
BBC News, Washington
The US Supreme Court is to consider whether to force the government to regulate carbon dioxide emissions from energy producers and cars.
Car manufacturers could be forced to improve fuel efficiency
A dozen states and environmental groups asked the court to take up the case after a lower court ruled against them.
They argue the onus should be on the government's Environmental Protection Agency to limit CO2 emissions.
They say CO2 is the primary greenhouse gas causing a warming of the Earth and so should be categorised a pollutant.
The US government says that CO2 is not a pollutant under federal laws and that even if it was, it would have discretion over whether or not to regulate it.
A federal appeals court recently sided with the government.
If the Supreme Court disagrees when it makes its ruling later this year, it could have a profound impact on American life.
It could pave the way, for example, for car manufacturers to be forced to improve fuel efficiency as a way of reducing CO2 emissions.
That would be opposed by US President George W Bush.
When he first ran for office, he expressed support for regulating CO2 but since then, he has favoured voluntary steps to reduce emissions.
Anything else, he argues, would harm business and cost jobs.