By Eugene Scott CNN

(CNN) -- The head of the Environmental Protection Agency administrator said Thursday he does not believe that carbon dioxide is a primary contributor to global warming -- a stance at odds with leading climate scientists.

Scott Pruitt made the comment to CNBC's Joe Kernen when he was asked if he believes that carbon dioxide "is the primary control knob for climate."

"I think that measuring with precision human activity on the climate is something very challenging to do and there's tremendous disagreement about the degree of impact, so no, I would not agree that it's a primary contributor to the global warming that we see," he said on CNBC's "Squawk Box."

"But we don't know that yet," he added. "We need to continue the debate and continue the review and the analysis."

Pruitt, who frequently sued the EPA while attorney general of Oklahoma, was repeatedly questioned during his confirmation hearings because of his positions on climate change and other environmental issues. At a hearing, Pruitt told senators, "Science tells us the climate is changing and human activity in some matter impacts that change."

The EPA itself has said that net emissions of greenhouse gases from human activities increased by 35% from 1990 to 2010, and emissions of carbon dioxide, which account for about three-fourths of total emissions, increased by 42% over this period.

Pruitt's predecessor at the agency, Gina McCarthy, said she "cannot imagine" any additional information the EPA administrator would need on the subject.

"I cannot imagine what additional information the administrator might want from scientists for him to understand that," McCarthy said in a statement. "Giving pink slips to scientists across the federal government, including 43% of EPA scientists, and proposing to eliminate the US Climate Global Research Program in its entirety makes one question who this administration will rely on for scientific research and facts."

Pruitt's comments Thursday also were swiftly denounced by Democrats and environmental groups.

Hawaii Sen. Brian Schatz called Pruitt a "climate change denier" unqualified to head the EPA.

"If there was ever any doubt that Scott Pruitt is a climate denier, this settles it. Anyone who denies over a century's worth of established science and basic facts is unqualified to be the administrator of the EPA," said Schatz, co-chair of the Senate Climate Action Task Force. "Now more than ever, the Senate needs to stand up to Scott Pruitt and his dangerous views."

The executive director of the Sierra Club, a leading environmental organization, tweeted that Pruitt should be fired for his position.

"Pruitt loves baseball so put it this way: An EPA head denying science is like Jeter refusing to use a bat. He'd be fired and so should Pruitt," tweeted Michael Brune.

Pruitt previously declined to answer repeated questions about whether he would "allow" climate-change research.

"We have many priorities at the agency. We must focus on those," he told CNN's Wolf Blitzer last week on "The Situation Room," rattling off other needs, such as improving air quality.

CNN's Rene Marsh contributed to this report.

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