It is now widely understood that societies must rapidly decarbonize to avoid the worst impacts of future climate change. To contribute to this urgent effort, academic libraries should develop an understanding of the carbon footprint resulting from their activities. This article shares the results of an effort at the Penn State University Libraries to calculate the greenhouse gas emissions generated in one year. It will define the elements of a greenhouse gas (GHG) inventory, explain the context and process for undertaking this effort at Penn State, share findings, and discuss potential implications for academic libraries.