Challengers to U.S. EPA's greenhouse gas emissions rules faced stiff questioning from a panel of federal judges today on why the court should overturn the agency's determination that the gases pose a threat to human health.

At one point, Chief Judge David Sentelle, who was appointed to the U.S. Court of Appeals for the District of Columbia Circuit by President Reagan, said the logic of one of the petitioners' arguments was "engaged in some martial arts."

The first round of today's arguments focused on EPA's endangerment finding, the agency's original conclusion that greenhouse gases pose a health risk and, thus, should be regulated under the Clean Air Act. That rule came after the 2007 Massachusetts v. EPA decision, in which justices ruled for the first time that greenhouses gas are a pollutant subject to the statute.

The challengers -- which include states, energy companies as well as mining, farming and chemical interests -- had a high bar to clear because EPA was acting within its authority. The lawyers sought to establish that EPA acted in an arbitrary and capricious manner or exceeded its regulatory authority.

Patrick Day of the Coalition for Responsible Regulation argued that the finding "erroneously decided to completely divorce" the risk assessment from the agency's regulatory decisions. EPA's inadequate endangerment finding, he said, authorizes "ineffective and perhaps even fruitless regulations."

Judge David Tatel, who was particularly aggressive in questioning both sides, sharply dismissed Day's logic.

"I do not understand your argument," he said. "Why don't you try again?"

Tatel and Sentelle both seemed concerned that the petitioners were asking the court to overturn the scientific basis for EPA's endangerment finding or the Supreme Court's decision in Massachusetts. Both judges indicated that was not their job.

Jeffrey Bossert Clark, appearing on behalf of the U.S. Chamber of Commerce, argued that the Supreme Court decision centered not on science but rather on the fact that EPA must first consider the Clean Air Act and what its purpose is before making an endangerment finding.

That drew a stern rebuke from Sentelle.

"Sometimes in reading the petitioners' briefs," he said, "I get the impression that Massachusetts had not been decided."

Lawyers then turned to arguments over the agency's tailpipe rule, which used the endangerment finding to set greenhouse gas emissions standards for cars and light-duty trucks beginning with 2012 models.

Notably, Sentelle seemed to take issue with EPA's justification for the rule. In particular, he noted that of the six greenhouse gases EPA considered, two of them are not emitted by motor vehicles. That, he said, "disturbs me."

"Why are they treated as a group?" he asked.

Angeline Purdy of the Justice Department, representing EPA, responded that the agency made that determination because "that's how the science looks at them."

The arguments continue tomorrow when the court will turn to EPA's timing and tailoring rules. The timing rule required greenhouse gas emission standards from stationary sources to go into effect on Jan. 2, 2011, at the same time as the car and light-duty truck limits.

EPA's tailoring rule, which many legal observers expect to be the most vulnerable of the set, interpreted the Clean Air Act such that only the heaviest polluters were required to obtain greenhouse gas permits. EPA's intent was to exempt nonindustrial sources like schools or apartment buildings from having to obtain the permits. But to accomplish that, the agency has to rewrite part of the Clean Air Act.

The court will also hear arguments tomorrow challenging several of EPA's past Clean Air Act regulations, many of which have already held up in previous legal challenges. Petitioners are seeking to use EPA's recent application of the greenhouse gas rule to undermine previous air efforts in other areas.