The news last Friday was dominated by the administration’s National Climate Assessment, a congressionally-mandated document and the fourth of its kind, issued to comply with the Global Change Research Act of 1990. Its release that day rekindled the global warming debate that had subsided since President Trump took office, and media coverage focused heavily on its predictions for agriculture and climate-induced problems in the Corn Belt. While the report focused on crop failures resulting from the rapid increase in carbon emissions and their potential effect on weather, there wasn’t a great deal of context about the role of carbon dioxide (CO2) for agriculture. Of course, as all middle school science textbooks point out, CO2 is...