The ultimate planetary impact of worldwide greenhouse gas emissions remains very much a scientific uncertainty. Some climatologists insist that unchecked carbon dioxide emissions will warm the earth enough to melt completely the polar ice caps and bury entire coastal regions under water. Others project more moderate warming, the benefits of which might outweigh potential environmental inconveniences.

On April 2, the Supreme Court traded black robes for white coats, feigning scientific expertise to put a legal end to the debate and declare greenhouse gases a harmful pollutant. With a 5-4 decision, the high court ruled that provisions within the federal Clean Air Act provide authority for the Environmental Protection Agency to set nationwide CO2 emissions standards for new vehicles.

In his majority opinion, Justice John Paul Stevens wrote that "the harms associated with climate change are serious and well-recognized." He went on to purport "a strong consensus among qualified experts" for impacts such as "a precipitate rise in sea level, severe and irreversible changes in natural ecosystems, a significant reduction in winter snowpack with direct and important economic consequences, and increases in the spread of disease and the ferocity of weather events."

Treating those projected dangers as matters of fact, the court scolded the EPA for not regulating emissions. Stevens, with the support of Justices Stephen Breyer, Ruth Bader Ginsburg, Anthony Kennedy, and David Hackett Souter, concluded that in order to continue its inaction, the EPA would need to determine that greenhouse gases do not contribute to climate change.

That undue burden exposes the court's poor understanding of the global warming debate. Almost universally, scientists agree that greenhouse gases contribute to climate change. Disagreement centers on how much and whether a natural planetary climate cycle might soon reverse the current warming trend and mitigate projected harms.

Citing such uncertainties, the EPA has consistently declined to include carbon dioxide among the pollutants it regulates in accordance with the Clean Air Act. In this case, Massachusetts, 11 other states, three cities, and 13 environmental organizations challenged that EPA position, arguing that the agency's failure to act endangers the "public health or welfare."

Questions remain as to whether the plaintiffs even possessed legal standing to levy such a charge. Courts may only hear cases where there exists a clear causal connection between personal injury and a defendant's alleged illegal conduct and where a ruling to cease that conduct would prevent further injury.

How could Massachusetts possibly meet that high standard when its claims of personal injury stem from disputed scientific projections of future damages? Furthermore, how could the Commonwealth possibly prove that EPA action would prevent those future damages given the widespread consensus that U.S. efforts to curb CO2 emissions are meaningless without massive changes from China and India?

Justice Stevens removed such hurdles, writing that Massachusetts' quasi-sovereign interests entitle it to "special solicitude in our standing analysis." In other words, states get special treatment over other organizations or individuals.

In his dissent, Chief Justice John Roberts attacked that conclusion as having "no basis in our jurisprudence, and support for any such 'special solicitude' is conspicuously absent from the Court's opinion." Roberts argued that by softening the standing requirement, his colleagues had transgressed the constitutional role of the courts, namely "to decide concrete cases-not to serve as a convenient forum for policy debates."

The extent to which the ruling will affect federal policy remains in question. The decision does not require EPA regulation of greenhouse gas emissions, but it does open the agency to future lawsuits should it fail to impose such standards. California would almost certainly bring litigation in an effort to procure federal authority for its court-challenged statute, which mandates CO2 reductions of 30 percent in new vehicles by 2016.

More than a dozen other states are in the process of adopting or have already passed plans similar or identical to California's phased-in auto-emissions reductions, slated to begin in 2009 (see sidebar). Such momentum combined with Supreme Court affirmation is almost certain to push the EPA toward implementing federal restrictions, something President George W. Bush has consistently opposed due to economic concerns and the scientific uncertainties of climate change.

Early indications suggest the Bush administration will not fight that development. The president responded to the Supreme Court ruling with assurances that he also believes global warming to be an important issue and is only wary about economic damage.

Kenneth Green, a scholar of environmental policy with the American Enterprise Institute, believes the ruling's limitation to the automobile sector may preclude it from reaching the level of what Bush finds worthwhile to fight as his term winds to a close. "This is not going to hit the energy sector; it's not going to hit the overall economy; and it's not going to require the institution of a nationwide cap-and-trade," he told WORLD.

But economic costs will likely still prove significant and with very little environmental gain. Automakers forced to pursue more efficient technologies for greater fuel economy will pass those costs to consumers. Meanwhile, higher-gas-mileage vehicles will encourage more driving, pushing greenhouse gas emissions back up near original levels.

Still, the president will not likely choose to resist, as his sole avenue of recourse would require pushing Congress to pass legislation amending the Clean Air Act, a political battle with a high degree of difficulty and limited chances for success. But conservatives are not without hope: Some believe that top-down EPA bureaucracy will operate more slowly than the state-by-state movement to restrict emissions, a scenario that would buy time for the creation of new technologies and fuels and could ease some economic pain.

"People that are worried about global warming don't seem to consider that if they're in a hurry, EPA is probably the worst way to get there," Green said. "The EPA process is so unbelievably tedious, so lengthy and open to challenge and comment and public disputation."

Much of that EPA process will revolve around determining appropriate levels of greenhouse gas emissions. Unlike other pollutants, which produce measurable environmental harm at particular levels, CO2 is far more difficult to nail down. Scientists differ dramatically in their estimates of how tight restrictions need to be to stop man-made climate change. And some researchers believe a little man-made climate change could prove beneficial.

In light of such uncertainties, federal standards will be arbitrary and highly vulnerable to repeated court challenges. Justice Stevens should keep his white coat handy.

Air power

In 2002, California passed legislation requiring the state's Air Resources Board (ARB) to regulate the greenhouse gas emissions of cars, light trucks, and SUVs. On Jan. 1 of last year, the ARB made official its mandatory plan that automakers begin reducing the amount of CO2 emitted per mile in 2009 and continue those reductions up to 30 percent by 2016.

The automobile industry has challenged the law in court on the grounds that it amounts to fuel economy regulation, which only the U.S. Department of Transportation can impose. California lawmakers insist the statute is justifiable under the Clean Air Act and have petitioned the EPA for enforcement. That petition had stalled but received new life with the recent Supreme Court decision.

Many other states have followed California's lead, some adopting identical standards, others adjusting them slightly to accommodate particular needs or political climates.

States that have adopted California's standards:

Connecticut Maine Maryland Massachusetts Vermont New Jersey New York Oregon Pennsylvania Rhode Island Vermont Washington

States considering adoption of California's standards:

Arizona Illinois Minnesota Nevada New Mexico North Carolina Texas

Other related state legislation:

Arkansas: A House bill would set clean car standards and promote the availability of cars that use bio-fuel.

Hawaii: A House resolution would urge the state's congressional delegation to introduce and support federal legislation enabling Hawaii to adopt California's emissions standards.

Tennessee: A Senate bill would require Tennessee to adopt California's emissions standards if other states totaling 40 percent of the U.S. population did so.