This case study investigated the nature of in-service science teachers' argumentation and personal epistemology about global climate change during a 3-year professional development program on climate change education. Qualitative analysis of data from interviews and written assessments revealed that while these teachers grounded their arguments on climate issues in evidence, the evidence was often insufficient to justify their causal claims. Compared with generating arguments for their own views, teachers had more difficulties in constructing evidence-based arguments for alternative perspectives. Moreover, while these teachers shared some similarities in their epistemology about climate science, they varied in their beliefs about specific aspects such as scientists' expertise and the credibility of scientific evidence. Such similarities and distinctions were shown to relate to how teachers used evidence to justify claims in their arguments. The findings also suggested a mismatch between teachers' personal epistemology about science in general and climate science, which was revealed through their argumentation. This work helps to further the ongoing discussions in environmental education about what knowledge and skills teachers need in order to teach climate issues and prepare students for future decision making. It constitutes first steps to facilitate reasoning and argumentation in climate change education and provides important implications for future design of professional development programs.