The First Amendment provides robust protection for political and scientific debate, but it faces a new threat from a climate activist determined to silence his critics. In a case pending before the District of Columbia Court of Appeals, Penn State professor Michael Mann is waging an aggressive campaign of lawfare, accusing of defamation those who dare to question his work. So far, the courts have given this assault on free speech a green light.

Mr. Mann is famous as the creator of the “hockey stick” graph, which portrays...