This article analyzes the application of environmental impact assessment as a tool for climate change mitigation from a global comparative perspective. It firstly confirms that, despite persistent resistance in a few jurisdictions, climate effect assessment is now widely applied on a global scale. Yet the article also shows that this practice has faced recurrent practical and conceptual issues, in particular, concerning the determination of the significance of a project’s climate effect and the assessment of indirect effects. Lastly, this article assesses how states have addressed these issues and identifies good practices. In doing so, the article illustrates the potential of functionalist comparative analysis in advancing our understanding of climate law and suggesting policy-relevant conclusions.