This morning the Supreme Court unanimously ruled that the federal government’s Environment Protection Agency (EPA), and only the EPA, can legally limit greenhouse gas (GHG) emissions.

The court struck down a US Appeals ruling that upheld the lawsuit brought by six states against power companies which, the states said, were contributing to global climate change by polluting the environment.

“We hold that the Clean Air Act and the EPA actions it authorizes displace any federal common law right to seek abatement of carbon-dioxide emissions from fossil-fuel fired power plants,” wrote Justice Ruth Bader Ginsburg in the court ruling.

The ruling was a victory, not only for the power companies involved-American Electric Power, Southern Company, Xcel Energy, Duke Energy and Tennessee Valley Authority (TVA)-but also for the Obama administration, which appointed an attorney for the case to represent the government-owned TVA.

Although the Supreme Court already ruled, in December 2009, that the Clean Air Act authorizes the EPA to place limits on carbon dioxide and GHG emissions, this ruling today said that the previous the ruling did not allow for independent states to set limits, unless the EPA fails to do so.

“If EPA does not set emissions limits for a particular pollutant or source of pollution, States and private parties may petition for a rulemaking on the matter, and EPA’s response will be reviewable in federal court,” the judges wrote in this ruling, on American Electric Power Company v. Connecticut.

Several states, including New York and Connecticut, and several land trusts had sued the utilities under federal law, but the Supreme court threw out the case because it ruled that any matter that was under the jurisdiction of the EPA could not be tried in federal court by anyone except the EPA.

“It is altogether fitting that Congress designated an expert agency, here, EPA, as best suited to serve as primary regulator of greenhouse gas emissions. The expert agency is surely better equipped to do the job than individual district judges issuing ad hoc, case-by-case injunctions. Federal judges lack the scientific, economic, and technological resources an agency can utilize in coping with issues of this order,” said the Supreme Court ruling.

Environmental activists used the ruling to urge the EPA to action.

“Today’s ruling reaffirms the EPA’s duty under the nation’s 40-year-old Clean Air Act to safeguard public health and welfare from dangerous carbon pollution,” said Policy Director of the Natural Resources Defense Council‘s Climate Center, David Doniger, in a statement.

Indeed, the Supreme Court ruling did urge the EPA to act on GHG emissions when necessary.

“EPA may not decline to regulate carbon-dioxide emissions from power plants if refusal to act would be “arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law,” the court decision said.

Justice Sonia Sotomayor did not take part in today’s ruling.

Picture: US Supreme Court in Washington DC