Michael Mann, an influential climatologist who has spent years in the center of the debate over climate science, has sued two organizations that have accused him of academic fraud and of improperly manipulating data.

Mann, director of Pennsylvania State University's Earth System Science Center, on Monday sued the National Review and the Competitive Enterprise Institute, along with two of their authors, Rand Simberg and Mark Steyn.

The lawsuit, Mann's lawyer said in a statement, was based upon their "false and defamatory statements" accusing him of academic fraud and comparing him to a convicted child molester, former Penn State assistant football coach Jerry Sandusky.

Neither Mann nor his lawyer, John B. Williams of the Washington, D.C.-based law firm Cozen O'Connor, were available for comment Tuesday afternoon. But on Facebook, where news of the lawsuit was initially posted Tuesday, Mann said the lawsuit was part of "a battle" to assist climate scientists in the fight against those who attack their work.

"There is a larger context for this latest development," he wrote, "namely the onslaught of dishonest and libelous attacks that climate scientists have endured for years by dishonest front groups seeking to discredit the case for concern over climate change."

But he faces a high bar: Mann has played a key role in climate science for decades, and the law generally requires a much higher burden of proof from public figures, said CEI general counsel Sam Kazman.

"I don't think he's got a shot at reaching it," Kazman said in an interview. "Our stuff may have been debatable, but it was solidly based and we had a perfect right to say what we did."

"We plan to defend the suit vigorously but we think it is a totally unfounded lawsuit."

In 1999 Mann published a timeline of global temperatures stretching back almost 1,000 years. The graph showed a fairly stable trend until 1900, when temperatures spiked sharply upward. That so-called "hockey stick" diagram became a lightning rod in the debate on whether humans were influencing the climate.

In 2007 he shared the Nobel Peace Prize with former Vice President Al Gore and authors of the Intergovernmental Panel on Climate Change report for work connecting human activities to global warming.

$500,000 and two years

Two years later a cache of emails illegally obtained from University of East Anglia in the United Kingdom purported to show climate scientists manipulating data. Many of the emails were to or from Michael Mann.

Upwards of seven organizations, from the National Science Foundation to Penn State, conducted investigations into Mann's work. All declared baseless allegations of academic fraud.

Yet the attacks persisted: Virginia Attorney General spent $500,000 and two years unsuccessfully suing to obtain email correspondence from the University of Virginia, where Mann worked from 1999 to 2005.

A separate attempt by American Traditions Institute, a conservative think tank headed by members of other conservative groups such as the Competitive Enterprise Group and Cooler Heads Coalition, was equally unsuccessful.

One Sandusky reference came in a July National Review blog post by Mark Steyn, who repeated another blogger's comments comparing Mann to the disgraced football coach: "He has molested and tortured data in the service of politicized science that could have dire economic consequences for the nation and planet."

Kazman, CEI's counsel, said the organization removed "a sentence or two" relating to the Penn State sex abuse scandal shortly after it published a post.

Lawyers for the National Review in New York City were not immediately available for comment Tuesday afternoon.

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.