What’s worse than a public policy debate that turns bitter and impolite? Well, for one, having the courts step into the marketplace of ideas to judge which side of a debate has the best “facts.” Yet that’s what Michael Mann has invited the D.C. court system to do. In response to some scathing criticism of his methodologies and an allegation of scientific misconduct, the author of the infamous “hockey stick” models of global warming—because they resemble the shape of a hockey stick, with temperatures rising drastically beginning in the 1900s—has taken the global climate change debate to a record low by suing the Competitive Enterprise Institute, National Review, and two individual commentators. The good Dr. Mann claims that some blogposts alleging his work to be “fraudulent” and “intellectually bogus” were libelous. The D.C. trial court rejected the defendants’ motion to dismiss this lawsuit, holding that their criticism could be taken as a provably false assertion of fact because the EPA, among other bodies, have approved of Mann’s methodologies. In essence, the court seems to cite a consensus as a means of censoring a minority view. The defendants have appealed to the D.C. Court of Appeals (the highest court in the District of Columbia). Cato has filed a brief, joined by three other think tanks, in which we urge the court to stay out of the business of refereeing scientific debates. We argue that the First Amendment demands that failing to leave room for the marketplace of ideas to operate stifles academic and scientific progress, and that judges are ill‐suited to officiate policy disputes—as history has shown time and again. The lower court clearly got it wrong here—and there are numerous cases where courts have more judiciously treated similarly harsh assertions for what they really are: expressions of disagreement on public policy that, even if hyperbolic, are among the forms of speech most deserving of constitutional protection. The point in this appeal is that courts should not be coming up with new terms like “scientific fraud” to squeeze debate over issues impacting government policy into ordinary tort law. Dr. Mann is not like a corner butcher falsely accused of putting his thumb on the scale or mixing horsemeat into the ground beef. He is a vocal leader in a school of scientific thought that has had major impact on government policies. Public figures must not be allowed use the courts to muzzle their critics. Instead, as the U.S. Supreme Court has repeatedly taught, open public debate resolves these sorts of disputes. The court here should let that debate continue outside the judicial system.