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In one of its most important environmental decisions in years, the Supreme Court on Monday ruled that the Environmental Protection Agency has the authority to regulate heat-trapping gases in automobile emissions. The court further ruled that the agency could not sidestep its authority to regulate the greenhouse gases that contribute to global climate change unless it can provide a scientific basis for its refusal.

The 5-4 decision was a strong rebuke to the Bush administration, which has maintained that it does not have the right to regulate carbon dioxide and other greenhouse gases under the Clean Air Act, and that even if it did, it would not use the authority. The ruling does not force the EPA to regulate auto emissions, but it would almost certainly face further legal action if it fails to do so.

Writing for the majority, Justice John Paul Stevens said the only way the agency could "avoid taking further action" now is "if it determines that greenhouse gases do not contribute to climate change" or provides a good explanation why it cannot or will not find out whether they do.

Beyond the specific context for this case — so-called "tailpipe emissions" from cars and trucks, which account for about one-fourth of the country's total greenhouse gas emissions — the decision is highly likely to have a broader impact on the debate over government efforts to address global warming.

Court cases around the country had been placed on hold to await the decision in this case. Among them is a challenge to the EPA's refusal to regulate carbon dioxide emissions from power plants, now pending in the federal appeals court here. Individual states, led by California, are also moving aggressively into what they have seen as a regulatory vacuum.

Stevens, joined by Justices Anthony M. Kennedy, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer, said that by providing nothing more than a "laundry list of reasons not to regulate," the Environmental Protection Agency had defied the Clean Air Act's "clear statutory command." He said a refusal to regulate could be based only on science and "reasoned justification," adding that while the statute left the central determination to the "judgment" of the agency's administrator, "the use of the word 'judgment' is not a roving license to ignore the statutory text."

The court decided a second Clean Air Act case on Monday, adopting a broad reading of the EPA's authority over factories and power plants that add capacity or make renovations that increase emissions of air pollutants. In doing so, the court reopened a federal enforcement effort against the Duke Energy Corp. under the Clean Air Act's "new source review" provision. The vote in the second case, Environmental Defense v. Duke Energy Corp., was 9-0.

The two decisions left environmental advocates exultant. Many said they still harbored doubts about the federal agency and predicted that the decision would help push the Democratic-controlled Congress to address the issue. Even in the nine months since the Supreme Court agreed to hear the case, Massachusetts v. Environmental Protection Agency, and accelerating since the elections in November, there has been a growing interest among industry groups in working with environmental organizations on proposals for emissions limits.