The U.S. Supreme Court handed down two decisions yesterday that could create an unprecedented opportunity for setting new standards for reducing greenhouse gases from automobiles and lead to emissions reductions by a major utility.

In Massachusetts et al v. Environmental Protection Agency et al, the court ruled 5 to 4 against the Bush administration’s contention that it didn’t have the authority to regulate carbon dioxide emissions from motor vehicles.

The EPA had resisted calls by Massachusetts and other states to impose limits, saying it didn’t have the authority to do so. Yesterday, the court rejected every argument made by the EPA to do nothing.

Environmentalists hailed the decision as a landmark. “It is hard to overstate the importance of this decision for combating climate change — the most pressing environmental challenge of our time,” said Cynthia Giles, head of the Conservation Law Foundation in Rhode Island.

Local officials said the decision should provide support for new vehicle-emission standards being proposed in Rhode Island. Those standards are being challenged in federal court by automakers.

The second case appears to have more limited impact. In Environmental Defense et al v. Duke Energy Corp. et al, the court ruled unanimously (although Justice Clarence Thomas agreed to most, but not all of the decision) to vacate a lower court ruling that allowed the power company to upgrade 29 of its generating plants without having to conform to stricter air emissions rules. The case was sent back to the lower court for further review.

In the Massachusetts climate change case, the EPA argued that a link between the increase in greenhouse gases and global surface air temperatures “cannot be unequivocally established.”

But Justice John Paul Stevens disagreed. Writing for the majority, Stevens first established that Massachusetts had standing to file its case. “When a state enters the Union, it surrenders certain sovereign prerogatives,” he wrote. “Massachusetts cannot invade Rhode Island to force reductions in greenhouse gas emissions; it cannot negotiate an emissions treaty with China or India....”

Those prerogatives, Stevens wrote, are lodged in the federal government and “Congress has ordered EPA to protect Massachusetts (among others) by prescribing standards” to limit air pollution.

As for establishing injury, Stevens noted evidence of “the global retreat of mountain glaciers, reduction in snow-cover extent, the earlier spring melting of rivers and lakes, and the accelerated rise of sea levels during the 20th century.” Also cited was the loss of coastal land in Massachusetts.

Stevens wrote that a 2004 affidavit from climate scientist Michael MacCracken asserting that loss of wetlands and worsening hurricanes threaten New Orleans was “eerily prescient” when Hurricane Katrina struck a year later.

“In short, EPA has offered no reasoned explanation for its refusal to decide whether greenhouse gases cause or contribute to climate change,” Stevens wrote. “Its action was therefore ‘arbitrary and capricious ... or otherwise not in accordance with the law.”

Chief Justice John Roberts, along with Justices Antonin Scalia, Thomas, and Samuel Alito, all dissented.

“Global warming may be a ‘crisis,’ even the most pressing environmental problem of our time,” Roberts wrote. “Indeed, it may ultimately affect nearly everyone on the planet in some potentially adverse way, and it may be that governments have done too little to address it.”

But he said climate change has not escaped the attention of federal policymakers; the suit stems from those who are dissatisfied with the “pace of progress on this issue.”

Roberts asserted the court committed a “sleight of hand” in failing to show how limiting domestic motor vehicle emissions will redress the loss of coastal land in Massachusetts due to sea-level rise caused by global warming.

“The realities make it pure conjecture to suppose that EPA regulation of new automobile emissions will likely prevent the loss of Massachusetts coastal land,” Roberts wrote.

In the Duke Energy case, the company replaced components in 29 of its 30 coal-fired power plants in North Carolina and South Carolina, and maintained it didn’t need new EPA permits because none of the projects amounted to a “major modification” that would trigger the need for a new permit.

The Supreme Court directed the lower court to revisit the case.

Both decisions were hailed by environmental groups, and by Rhode Island Attorney General Patrick Lynch, who had joined other states in pressing both lawsuits.

“They might be inconvenient truths for the Bush Administration, but the Clean Air Act is the law of the land, carbon dioxide is a pollutant, and the Environmental Protection Agency can’t continue to flout the law,” Lynch said.

“On the most important environmental-protection case in at least a generation, the Supreme Court has delivered a ringing victory for everybody who genuinely cares about the fate of our nation and our planet.”

Matt Auten, of Environment Rhode Island, said the decisions were “huge” and the climate change case bodes well for Rhode Island’s proposed auto-emission standards.

plord@projo.com