Climate change poses a significant danger that requires intervention today; climate denial poses a key challenge to meaningful timely intervention. In this paper, we argue that current free speech jurisprudence in the US inadequately addresses the risk of climate change because it is overly permissive of ‘professional’ climate denial and underappreciates the need to address the future harm of climate change today. We begin by clarifying the risk posed by the Supreme Court's current approach to speech with respect to climate (...) change and, relatedly, reviewing the philosophical foundations of the marketplace of ideas found in the work of John Stuart Mill. Following this, we examine three potential ways in which Supreme Court jurisprudence could be used to limit what we term ‘professional climate change denial’ while permitting a degree of ‘private’ scepticism. Largely setting aside the return to earlier free speech jurisprudence and the extension of libel law, we offer a novel solution to the problem that suggests that ‘professional’ climate denial could be treated as a categorical exception under free speech jurisprudence and thus afforded a lower level of constitutional protection than other expression. (shrink)