An established research result is that people's environmental attitudes only loosely translate into actions effectively reducing their environmental impact, something known as the attitude-behaviour gap. On the other hand, correct information and environmental education are often considered a key to promote sustainability, which raises the question of when attitudes can actually work as a lever to promote environmental objectives and, conversely, when other factors have a better chance to succeed. To answer these questions, we tested the effect of environmental attitudes in an online experiment with real money at stake and real-world climate mitigation consequences. We found that environmental attitudes mainly affected behaviour in a low cost situation, while their effect was reduced when the stakes were higher. This finding is consistent with the low cost hypothesis of environmental behaviour and has important consequences for the shaping of more effective climate policies in a democratic context.