A few hours ago, the Supreme Court ruled in Massachusetts vs. EPA that EPA has the authority to regulate carbon dioxide from auto emissions. (For background on the case, see this post.)

David Stout of the New York Times summarizes:

In a 5-to-4 decision, the court found that the Clean Air Act expressly authorizes the E.P.A. to regulate carbon dioxide emissions, contrary to the E.P.A.’s contention, and that if the agency still insists that it does not want to regulate those emissions, it must give better reasons than the “laundry list” of invalid considerations it has offered so far.

Today’s decision is surely not the last word in the continuing debate over the effects of global warming and what can, or should, be done about it. But it was still highly significant in at least two respects.

First, the majority brushed aside the Bush administration’s assertion that the Clean Air Act does not treat carbon dioxide and other heat-trapping gases as “pollutants,” and thus does not give the E.P.A. the authority to regulate them.

Secondly, the five justices declared that contrary to the administration, Massachusetts and the 11 other states and various other plaintiffs that sued the E.P.A. do indeed have legal standing to pursue their suit. In order to establish standing, a federal court plaintiff must show that there is an injury that can be traced to the defendant’s behavior, and that the injury will be relieved by the action the lawsuit seeks.

David Roberts at Gristmill notes that this decision means “California’s pioneering efforts against climate change are safe from federal interference”; Lyle Denniston at SCOTUSblog explains that now EPA must analyze the scientific data to determine whether greenhouse gases are a “danger” to the global environment.

The majority opinion (PDF here), written by Justice Stevens, refutes a number of typical Bush administration arguments against acting to slow or reverse climate change: that incremental steps are not worth taking, that regulating auto emissions alone won’t solve the problem, that China and India are major greenhouse-gas contributors … and, of course, that the science is still too uncertain.

As for the specific arguments EPA was making about the Clean Air Act (emphasis added):

Rather than relying on statutory text, EPA invokes post enactment congressional actions and deliberations it views as tantamount to a congressional command to refrain from regulating greenhouse gas emissions. Even if such post enactment legislative history could shed light on the meaning of an otherwise-unambiguous statute, EPA never identifies any action remotely suggesting that Congress meant to curtail its power to treat greenhouse gases as air pollutants. That subsequent Congresses have eschewed enacting binding emissions limitations to combat global warming tells us nothing about what Congress meant when it amended §202(a)(1) in 1970 and 1977. And unlike EPA, we have no difficulty reconciling Congress’ various efforts to promote interagency collaboration and research to better understand climate change with the agency’s pre-existing mandate to regulate “any air pollutant” that may endanger the public welfare. See 42 U. S. C. §7601(a)(1). Collaboration and research do not conflict with any thoughtful regulatory effort; they complement it.

[…]

EPA has refused to comply with this clear statutory command. Instead, it has offered a laundry list of reasons not to regulate. For example, EPA said that a number of voluntary executive branch programs already provide an effective response to the threat of global warming, 68 Fed. Reg. 52932, that regulating greenhouse gases might impair the President’s ability to negotiate with “key developing nations” to reduce emissions, id., at 52931, and that curtailing motor-vehicle emissions would reflect “an inefficient, piecemeal approach to address the climate change issue,” ibid.

Although we have neither the expertise nor the authority to evaluate these policy judgments, it is evident they have nothing to do with whether greenhouse gas emissions contribute to climate change. Still less do they amount to a reasoned justification for declining to form a scientific judgment. In particular, while the President has broad authority in foreign affairs, that authority does not extend to the refusal to execute domestic laws. In the Global Climate Protection Act of 1987, Congress authorized the State Department—not EPA—to formulate United States foreign policy with reference to environmental matters relating to climate. See §1103(c), 101 Stat. 1409. EPA has made no showing that it issued the ruling in question here after consultation with the State Department. Congress did direct EPA to consult with other agencies in the formulation of its policies and rules, but the State Department is absent from that list. §1103(b).

Nor can EPA avoid its statutory obligation by noting the uncertainty surrounding various features of climate change and concluding that it would therefore be better not to regulate at this time. See 68 Fed. Reg. 52930–52931. If the scientific uncertainty is so profound that it precludes EPA from making a reasoned judgment as to whether greenhouse gases contribute to global warming, EPA must say so. That EPA would prefer not to regulate greenhouse gases because of some residual uncertainty—which, contrary to JUSTICE SCALIA’s apparent belief, post, at 5–8, is in fact all that it said, see 68 Fed. Reg. 52929 (“We do not believe . . . that it would be either effective or appropriatefor EPA to establish [greenhouse gas] standards for motor vehicles at this time” (emphasis added))—is irrelevant. The statutory question is whether sufficient information exists to make an endangerment finding.

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

White House spokeswoman Dana Perino said she could not comment at length on the ruling.

Liz Borkowski works for the Project on Scientific Knowledge and Public Policy (SKAPP) at George Washington University’s School of Public Health and Health Services.