California and other states have taken action to reduce carbon dioxide emissions from cars and trucks while the Environmental Protection Agency has looked the other way. A U.S. Supreme Court ruling that the agency no longer can "shirk its environmental responsibilities" is a major move toward a badly needed federal policy to control global warming.
The Bush administration argued that EPA had no authority to control tailpipe emissions under the Clean Air Act, which does not specifically mention carbon dioxide and other greenhouse gases. The court said the agency can "avoid taking further action" only "if it determines that greenhouse gases do not contribute to climate change." That would be a preposterous determination.

The plaintiffs included 12 states, including the three West Coast states, American Samoa, several cities and 13 environmental groups, but not Hawaii. Justice John Paul Stevens, writing for the 5-4 majority, noted that the plaintiffs submitted "uncontested affidavits" that "the rise in sea levels associated with global warming has already harmed and will continue to harm" those areas. "The risk of catastrophic harm, though remote, is nevertheless real."

Nowhere is that potential harm greater than in the Pacific. The world's sea level is projected to rise by as much as 23 inches by 2100, compared with 6 to 9 inches in the past century, the International Panel on Climate Change of the United Nations reported in February. Other estimates are more pessimistic.

Much of Waikiki could be underwater in the not-too-distance future, and the president of Kiribati has notified the United Nations that many of his 100,000 citizens will have to find other quarters when the atolls become unlivable in 50 years.

California has taken the lead with a new law to cut nearly 30 percent of carbon dioxide emissions on cars sold in the state beginning in 2016. A dozen other states have enacted similar laws, and Hawaii's Legislature is nearing approval of a bill aimed at lowering the state's greenhouse gas emissions to 1990 levels by 2020.

The effort to control emissions must be national. The high court ruling means that EPA is allowed to regulate emissions, giving momentum to Congress to eliminate any wiggle room and require that it do so.

That is understood by Rep. John Dingell, a Michigan Democrat who supports the auto industry and is chairman of the House Energy and Commerce Committee. In a prepared statement, Dingell said the ruling "provides another compelling reason why Congress must act, and the president must sign, comprehensive climate change legislation."