The Sierra Club has failed in its attempt to label Scott Pruitt a violator of the Environmental Protection Agency's Scientific Integrity Policy.

Pruitt is off the hook for expressing an opinion about global warming, as a scientific review panel ruled that it is within EPA policy to call for rigorous debate on the issue, according to a letter obtained by the Washington Free Beacon.

"The freedom to express one's opinion about science is fundamental to EPA's Scientific Integrity Policy even (and especially) when that point of view might be controversial," wrote Dr. Thomas Sinks, the director of the EPA's Office of the Science Advisor.

The Sierra Club filed a complaint to the inspector general against Pruitt in March, alleging Pruitt violated the Scientific Integrity Policy by saying during an interview that there is disagreement among scientists about whether CO2 is the primary driver of global warming.

The inspector general referred the allegation to Dr. Francesca Grifo, the EPA's Scientific Integrity Official, who then passed along the Sierra Club's complaint to the EPA Scientific Integrity Committee.

The committee cleared Pruitt of the charge, and, in fact, defended the EPA administrator for expressing his opinion and encouraging "vigorous debate."

"In his response, the Administrator expressed his opinion regarding contributions to global warming and called for more debate, review, and analysis as a precursor to any future EPA policy decision on the matter," the Scientific Integrity Review Panel said. "This expression of opinion, which was not made in a decisional context, is fully within the protections of EPA's Scientific Integrity Policy and does not violate that Policy. We also note that, in his remarks, the Administrator did not suppress or alter Agency scientific findings."

The EPA panel went further in refuting the Sierra Club's interpretation of the Scientific Integrity Policy. The liberal environmentalist group had branded Pruitt a "climate denier" in its complaint.

"Expressing an opinion about science is not a violation of the EPA Scientific Integrity Policy," the EPA panel said. "Indeed, the Scientific Integrity Policy—in the spirit of promoting vigorous debate and inquiry—specifically encourages employees to express their opinion should the employee disagree with scientific data, scientific interpretations, or scientific conclusions."

The Sierra Club did not immediately respond to a request for comment.

The group's complaint was spurred by an interview Pruitt gave to CNBC, where he was asked, "Do you believe that it's been proven that carbon dioxide is the primary control knob for climate?"

Pruitt responded: "No. 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. But we don't know that yet… We need to continue the debate and continue the review and the analysis."

The letter was signed by Dr. Sinks, and sent to Elena Saxonhouse, a senior attorney for the Sierra Club, and Joanne Spalding, the group's chief climate counsel.