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Food production is a major cause of energy use and GHG emissions, and therefore diet change is an important behavioural strategy for reducing associated environmental impacts. However, a severe obstacle to diet change may be consumers' underestimation of the environmental impacts of different types of food. Here we show that energy consumption and GHG emission estimates are significantly underestimated for foods, suggesting a possible blind spot suitable for intervention. In a second study, we find that providing consumers with information regarding the GHG emissions associated with the life cycle of food, presented in terms of a familiar reference unit (light-bulb minutes), shifts their actual purchase choices away from higher-emission options. Thus, although consumers' poor understanding of the food system is a barrier to reducing energy use and GHG emissions, it also represents a promising area for simple interventions such as a well-designed carbon label.