Economywide legislation to address climate change will be ineffective unless it addresses greenhouse gas emissions from agriculture and land use. Yet incorporating these sectors into the most popular policy proposal—a carbon tax—carries legal risk that policymakers and legal commentators have ignored. This Article explores whether a carbon tax, as applied to agriculture and land use, is a direct tax within the meaning of the Constitution; it concludes that text, history, and Supreme Court precedent up through National Federation of Independent Business v. Sebelius (2012) leaves such a tax open to challenge. Consequently, policymakers should avoid eliminating EPA’s regulatory authority over greenhouse gas emissions in exchange for a carbon tax.