Michael Mann is a climate scientist and researcher whose work has been at the center of the global warming debate for decades. After emails came to light concerning Mann’s statistical methods, two of his critics wrote scathing pieces arguing that Mann had “molested and tortured data in the service of politicized science,” and calling for “a fresh, truly independent investigation.” Despite such harsh criticism being par for the course in online commentary, Mann sued both writers and their publishers for libel. A three-judge panel of the D.C. Court of Appeals ruled that Mann’s libel claim could succeed in front of a jury, and allowed the case to go forward. The defendants have asked the court to reconsider the implications of its decision, and Cato has filed a brief supporting that request. Harsh words are common to the discourse of pundits and politicians alike. Op-eds and stump speeches frequently feature terms like “fraud,” “scam,” “misconduct,” and even “treason.” Whether such characterizations are apt or not is for readers and listeners to judge, but until now few imagined that using them could lead to years of litigation and a costly libel verdict. Similarly, calls for investigation and accusations of whitewashing have a long history dating back to Emile Zola’s J’accuse...! and continuing today with debates over the trials of O.J. Simpson, George Zimmerman, and many others. If Mann’s critics committed actionable libel, then so might everyone who has voiced disagreement with such verdicts, as well as everyone who has called for politicians to be investigated for corruption, fraud, or war crimes. Finally, the court wrongly held that merely comparing a public figure to a “notorious person” could be libelous. As we know from Godwin’s law, such comparisons are a time-honored tradition of American debate. Opinion writers in recent years have invoked colorful analogies to Timothy McVeigh, Charles Manson, and Jack the Ripper to express their displeasure with the conduct of public figures. Writers and historians concerned with the conduct of politicians have drawn parallels with Stalin, Mussolini, and, of course, the ubiquitous Hitler. Right or wrong, such language is unquestionably speech on subjects of public concern. The D.C. Court of Appeals should give this case a second look and reverse its earlier decision. It’s not an exaggeration to say that the court’s reasoning could put thousands of articles, blogposts, and even tweets under a cloud of potential liability, thereby chilling the speech that is the lifeblood of Washington politics. Cultural and political debates should be litigated in the court of public opinion, not law.