Today, the U.S. Supreme Court will hear arguments on whether carbon dioxide is an air pollutant, and whether the U.S. Environmental Protection Agency has any role in regulating pollution that causes global warming.

It would seem obvious that the answers to both are a resounding "Yes."

Yet the obvious becomes contentious when the stakes are high. The case before the court — Massachusetts v. EPA — tackles these issues and is pivotal in the fight against global warming.

Massachusetts and 13 other states, as well as environmen-tal groups, contend that carbon dioxide from motor vehicles is a major contributor of the nation's greenhouse gas emissions. Furthermore, they believe regulating those emissions is the job of the EPA.

But mandating that the EPA do the job will require the Supreme Court's interpretation of the Clean Air Act.

The EPA under the Bush administration has argued the agency has no authority to regulate greenhouse gases under the law. The Clean Air Act doesn't mention carbon dioxide specifically, but it does define "air pollutant" as "any chemical substance or matter emitted into the ambient air." That would appear to qualify carbon dioxide.

If that's the case, then the law authorizes the EPA to regulate any pollutant that endangers public health or welfare, including threats to climate.

The court's decision is expected next spring. If the court rules against Massachusetts and the environmental groups, it would be disastrous. Among other things, it could even limit individual states from regulating motor vehicle emissions if the EPA chooses not to.

Considering the stakes — our future — the court must clarify the EPA's authority under the Clean Air Act, thus enabling it to play a more effective, vital role in the fight against global warming.