Ensuring the Climate and Environmental Integrity of Alternative Fuels.

Public and private investments in alternative fuels─molecular energy carriers with potentially lower net greenhouse gas emissions than conventional fossil fuels─are rapidly expanding due to their perceived decarbonization potential. However, alternative fuels like hydrogen, ammonia, and methanol are not inherently "good" simply because they can be produced with low-carbon methods. Their climate and environmental impacts vary widely depending on how they are produced, handled, and used─factors that most assessments fail to fully account for. This can result in poorly informed decision-making, with overestimated benefits of deployment choices. In this Perspective, we highlight potential environmental impacts from alternative fuel value chains and provide an actionable guide for their responsible deployment. Specifically, we provide a brief overview of alternative fuel attributes, assess climate and air quality implications across production, handling, and use, and discuss technological, political-economic, social, and other environmental considerations for responsible deployment. We recommend improvements to alternative fuel assessments and suggest safeguards to maximize their climate and environmental integrity, including best-in-class practices and enabling policies. Given the urgency of achieving global climate goals, it is critical to ensure that scaling up of alternative fuels adheres to sustainable practices from the outset.