Quantitative reasoning is a key intellectual skill, applicable across disciplines and best taught in the context of authentic, relevant problems. Here, I describe and assess a laboratory exercise that has students calculate their "carbon footprint" and evaluate the impacts of various behavior choices on that footprint. Students gather data from their own lives, use an online calculator to estimate their footprints, graph and compare data from different scenarios, and analyze and interpret the results. In the process, they recognize and modify naïve conceptions about activities that contribute greenhouse gases and gain an appreciation for how quantitative analysis can lead to unexpected insights and greater confidence in their knowledge of an issue.