So far, there has been mixed evidence in the literature regarding the relationship between environmental attitudes and actual `green' actions, something known as the attitude-behavior gap. This raises the question of when attitudes can actually work as a lever to promote environmental objectives, such as climate change mitigation, and, conversely, when other factors would be more effective. This paper presents an online experiment with real money at stake and real-world consequences designed to test the effect of environmental attitudes on behavior under various conditions. We found that environmental attitudes affected behavior only in low-cost situations. This finding is consistent with the low-cost hypothesis of environmental behavior postulating that concerned individuals will undertake low-cost actions in order to reduce the cognitive dissonance  between their attitudes and rational realization of the environmental impact of their behavior but avoid higher-cost actions despite their greater potential as far as environmental protection. This finding has important consequences for the design of more effective climate policies in a democratic context as it puts limits on what can be achieved by raising environmental concern alone.