In two experiments replicating an online shopping context, we studied the effectiveness of two traffic-light carbon labeling systems that differ according to the set of reference with which product comparisons are made. While the inter-category system allows consumers to compare all food products to each other, the intra-category system only allows consumers to compare products within the same food category. We also examined whether providing a descriptive social norm cue could increase theirs effects. The impact of the labels was studied on: (1) the frequency of low/medium/high carbon impact products purchased, (2) the carbon footprint of consumers' baskets. Study 1, conducted on university students (n = 228), showed that both carbon labeling systems tested had beneficial effects on consumers' behaviors and carbon footprint. Study 2 (n = 260) on a general population sample, which was less responsive to carbon labels, showed that the addition of a descriptive social norm cue enhanced the labels' effectiveness. Overall, the intra-category traffic-light labeling system shifted purchasing the most.