* At issue: Should judges decide greenhouse gas curbs
* Ruling expected by end of June
* Most important environmental case since 2007
WASHINGTON, April 19 (Reuters) - The Supreme Court on Tuesday questioned whether a global warming lawsuit against five big power companies can proceed, with several justices saying the U.S. Environmental Protection Agency, not federal judges, should deal with the issue.
The high court justices sounded a skeptical note during arguments when they asked whether complicated environmental issues, such as how much greenhouse gas pollution is allowable and how it should be curbed, should be left to federal judges.
The big environmental case stemmed from a 2004 lawsuit claiming that five coal-burning utilities have created a public nuisance by contributing to climate change. Its consequences, such as rising seas, reduced crop yields and destruction of some hardwood trees, would harm the states' citizens.
The lawsuit, now involving six states, seeks to have a federal judge in New York order the utilities to cut their carbon dioxide emissions.
Both liberal and conservative justices questioned whether the lawsuit can go forward.
Justice Ruth Bader Ginsburg, one of the court's liberals, took aim at the states' case, saying what they want is what the Environmental Protection Agency is trying to do with its proposed regulations of carbon emissions.
"How does a district judge decide what's reasonable and effective?" conservative Justice Samuel Alito asked rhetorically.
Another conservative, Chief Justice John Roberts, also asked whether a district court judge could conduct the cost-benefit analysis to determine what was reasonable to reduce global warming: "I think that's a pretty big burden to impose on a district court judge."
Liberal Justice Elena Kagan said the facts at issue in the lawsuit usually were determined by an agency such as the EPA, rather than the courts. "There is an administrative agency. There is a Clean Air Act," she said.
Lawyers for the power companies, including an Obama administration attorney representing the government-owned Tennessee Valley Authority, said the scope of the lawsuit was unprecedented in U.S. history, involving national and international issues outside the power of the courts.
"In the 222 years that the court has been sitting, there has never been a case with so many potential perpetrators and so many potential victims," said Neal Katyal, the administration's top courtroom lawyer.
The power companies -- American Electric Power Co Inc (AEP.N), Southern Co (SO.N), Xcel Energy Inc (XEL.N) and Cinergy Corp, which Duke Energy Corp (DUK.N) acquired in 2006, along with TVA -- want the lawsuit dismissed.
The states -- California, Connecticut, Iowa, New York, Rhode Island and Vermont -- said their citizens have been harmed by global warming and urged the top court to allow their lawsuit to go forward.
It is the most sweeping climate change case to come before the high court since its landmark 2007 ruling that authorized the EPA to regulate greenhouse emissions if they endanger human health.
In their comments on Tuesday, the justices said Congress has given the federal environmental agency the authority to do just that.
But even though the EPA has found officially that greenhouse pollution poses a health hazard, it has not yet gone forward to impose regulations on emissions, and Republicans in Congress have sought to limit the agency's ability to do so.
Coal-fired power plants emit about twice as much carbon dioxide -- which warms the Earth by trapping solar heat in the atmosphere -- as natural gas-fired plants. Nuclear power plants emit virtually no greenhouse gases.
The five power utilities account for 10 percent of U.S. carbon dioxide emissions, said Barbara Underwood, who argued the case for the states.
A ruling is expected by the end of June. (Editing by Bill Trott)