To effectively contribute to the mitigation of climate change, individuals require accurate representations of the greenhouse gas emissions related to climate actions across life domains. Although this carbon competence is generally low, the related psychological barriers remain largely unexplored, and few attempts have been made to improve it. Here we investigate how heuristic and motivated reasoning processes relate to mitigation potential estimates and evaluate corrective interventions. Across three preregistered studies ( n _total = 2,924) we show that low- to medium-impact actions are more frequently adopted than higher-impact actions, and that knowledge about the differential impact of climate actions is limited. Our data suggest that inaccuracies are driven by individuals consistently relying on the availability heuristic and motivated reasoning to form their judgements. To address this, we identify perceived behavioural costs and social prevalence as reasonable proxies of the actual impact of a broad range of climate actions. Leveraging this insight in two corrective boosting interventions and informing about the most impactful climate actions only partly improved impact estimates, with considerable variations across individuals. Accounting for heterogeneity through tailored interventions appears crucial to overcome pervasive psychological barriers to carbon competence on a large scale.