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The goal of this study was to evaluate the impact of a two-quarter freshman course series entitled Food: A Lens for Environment and Sustainability (Food cluster) on the carbon footprint of food choices by college freshmen attending a large public university in California. Students enrolled in the course completed a baseline questionnaire about their diets in early fall quarter and then again at follow-up, about 6months later at the end of the winter quarter. The control group consisted of freshmen enrolled in a different course series entitled Evolution of the Cosmos and Life (Cosmos cluster). The instruction in the Food cluster included lecture material on general environmental science and life cycle analyses of food, an analysis of a reading comparing the environmental footprint of various types of meats, and classroom exercises to calculate the environmental footprint of typical foods. The Cosmos cluster instruction included climate change, but no information about food. While the two groups were statistically indistinguishable at baseline, throughout the period of the study, Food cluster students decreased (a) their overall dietary carbon footprint for a 2000-kcal normalized diet by 7% (p=0.062), (b) the beef component of their dietary carbon footprint by 19% (p=0.024), and (c) their reported ruminant consumption by 28% (p<0.001). At follow-up, the overall dietary footprints for Food cluster students were 4153 and 5726g CO2-eq/day for female and male students, respectively, compared to 4943 and 6958g CO2-eq/day for female and male Cosmos students. In the Food cluster, both genders decreased their reported ruminant meat consumption by about a serving per week, while reported ruminant meat consumption increased for males in the control group. Modest, voluntary dietary changes such as those observed in this study could play an important role in mitigating climate change. Extrapolated across the entire US population, the difference in dietary carbon footprint observed between the Food cluster and control group would amount to 33% of the reduction required for the 2013 President's Climate Action Plan (2013).