“Dr. Mann has supplied sufficient evidence for a reasonable jury to find, by a preponderance of the evidence, that statements in the articles written by Mr. Simberg and Mr. Steyn were false, defamatory, and published by appellants to third parties, and, by clear and convincing evidence, that appellants did so with actual malice,” wrote Senior Judge Vanessa Ruiz in the court’s opinion.

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The decision suggests that, even as the climate-skeptical Trump administration comes into office, a high profile lawsuit could be underway in Washington, D.C., that also partly turns on the evidence for, and against, climate change.

The origins of the lawsuit

Mann and several other colleagues first published the hockey stick graph in the late 1990s, and it has since become one of the most recognizable visual illustrations of human-caused–or anthropogenic–climate change. The graph used temperature data acquired from a variety of sources including tree rings, coral samples and ancient sediments to depicts a sharp uptick in global temperatures in the 20th century in comparison with prior centuries. Scientists attribute the rise to a spike in atmospheric carbon dioxide concentrations brought on by human industrial activities. (Here’s one depiction presented by the Intergovernmental Panel on Climate Change.)

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The graph received widespread criticism from climate doubters, whose skepticism was further fueled by a 2009 incident now known as “Climategate,” in which hackers uncovered and anonymously published thousands of emails from Mann and other climate scientists. At the time, many climate skeptics argued that the emails’ content suggested global warming was a conspiracy, the product of manipulated data and scientific fraud.

As a result, multiple agencies — including committees from both Penn State and the U.S. government — launched investigations into the affair. They all concluded that Mann had not committed any misconduct.

The current lawsuit revolves around a pair of blog posts written in 2012 after the investigations were complete. The first was allegedly authored by Rand Simberg for the Competitive Enterprise Institute and suggested that “Mann could be said to be the Jerry Sandusky of climate science, except that instead of molesting children, he has molested and tortured data in the service of politicized science that could have dire economic consequences for the nation and planet.” (This line has since been removed from the post.)

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The second was posted allegedly by Mark Steyn in National Review — it quoted from Simberg’s post and added that “Michael Mann was the man behind the fraudulent climate-change ‘hockey-stick’ graph, the very ringmaster of the tree-ring circus.” Both posts suggested that Penn State’s investigation may have resulted in a cover-up.

The current case brings a defamation lawsuit against Steyn, Simberg, National Review and the Competitive Enterprise Institute “for their utterly false and defamatory statements against Dr. Mann—accusing him of academic fraud and comparing him to a convicted child molester, Jerry Sandusky, the disgraced former football coach at Pennsylvania State University.”

When the lawsuit was first brought in a lower court, the defendants asked the judge to dismiss the case and were denied. They appealed, and now the D.C. Court of Appeals has also denied the motion to dismiss, finding that “Dr. Mann has presented evidence sufficient to defeat the special motions to dismiss as to some of his claims.”

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Moving forward

The case will now move to trial court, although how quickly it proceeds remains to be seen. For now, what may be most interesting about the appeals court’s opinion is its effort to distinguish between the larger phenomenon of climate change doubt that exists in the U.S. and the specific accusations of fraud that a jury could potentially find defamatory.

“Tarnishing the personal integrity and reputation of a scientist important to one side may be a tactic to gain advantage in a no-holds-barred debate over global warming,” wrote Senior Judge Vanessa Ruiz in the court’s opinion. “That the challenged statements were made as part of such debate provides important context and requires careful parsing in light of constitutional standards. But if the statements assert or imply false facts that defame the individual, they do not find shelter under the First Amendment simply because they are embedded in a larger policy debate.”

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This distinction has been pointed out by other climate scientists following the case as well. In a series of tweets on Thursday, NASA climatologist Gavin Schmidt called the ruling “nuanced” and “well argued.”

“Judges make clear that there is a large constitutional gap btw criticism of scientific conclusions or methodology… & allegations of misconduct and fraud,” he tweeted.

That said, the court of appeals has only ruled that the case may proceed to trial. From there, a jury will decide whether the content of the blog posts actually constitutes defamation — and because Mann is considered a “limited public figure” in this case, the decision could hinge on whether the jury finds that the defendants acted with “actual malice.”

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In a recent blog post, defendant Mark Steyn expressed his disagreement with the ruling.

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“The ‘sufficient evidence’ Dr Mann has supplied are a series of mendacious claims to have been ‘investigated‘ and ‘exonerated‘ by multiple Anglo-American bodies that did, in fact, do neither,” he wrote (his links).

And there are others — even those who accept the science of anthropogenic climate change — who take issue with the court’s decision from a constitutional perspective.

“While a direct accusation of scientific fraud may be actionable — particularly when made against a non-public figure — challenges to scientific conclusions and interpretations of scientific studies are clearly protected by the First Amendment,” wrote Jonathan Adler, a law professor at the Case Western University School of Law, in a recent blog post. “So are erroneous interpretations of scientific conclusions and — particularly relevant here — criticisms of the conclusions of investigatory bodies.”

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“In refusing to dismiss claims against Steyn and Simberg, the D.C. Court of Appeals placed tremendous weight on the fact that Penn State and other institutions investigated Mann and did not find evidence of academic misconduct,” he continued. “Yet it is the alleged inadequacy of Penn State’s investigation that was the focus of the very posts at issue.”

And over the years, the defendants have received similar expressions of support from other sources. In 2014, The Reporters Committee for Freedom of the Press and 26 other organizations — including The Washington Post and other media groups — filed an amicus brief in support of the defendants, suggesting that “defendants were expressing constitutionally protected opinions about both (a) the validity of Mann’s scientific conclusions on climate change and (b) the findings of governmental bodies that had endorsed his views.”

At the time, the brief called for the court to hear the appellants’ appeal and reverse the denial of the motion to dismiss, lest the decision “work a profound chill on expressions of opinion about important scientific and public policy issues.”

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But the most recent court opinion points out that multiple committees conducted investigations and found no evidence of misconduct, adding that “appellants do not counter any of these reports with other investigations into the CRU emails that reach a contrary conclusion about Dr. Mann’s integrity.”