Does it matter whether contribution decisions regarding environmental publicgoods are arrived at through intuition or reflection? Experimental research in behavioral economics has recently adopted dual-system theories of the mind from psychology in order to address this question. This research uses response time data in public good games to distinguish between the two distinct cognitive processes. We extend this literature towards environmental public goods by analyzing response time data from an online experiment inwhich over 3400 subjects from the general population faced a dichotomous choice between receiving a monetary payment or contributing to climate change mitigation efforts. Our evidence confirms a strong positive link between response times and contributions: The average response time of contributors is 40% higher than that of non-contributors. This suggests that reflection, not intuition, is at the root of pro-environmental contributions. This result is robust to a comprehensive set of robustness checks, including a within-subjects analysis that controls for potentially unobserved confounds and recovers the relationship at the individual level.