This study explored the use of collaborative action research (CAR) in a year-long professional development (PD) for global climate change (GCC) education. The purpose was to understand how high school science teachers' engagement in CAR affected their classroom practice. The teachers exchanged stories of practice, shared and tried out new ideas in their classrooms, and engaged in systematic inquiry. The study was guided by these questions: What are the effects of extended professional development on the teachers' classroom practice? What role did action research play in reaching our goals for professional development? What affordances and obstacles affect their change in pedagogy? Data included recordings of CAR meetings, classroom observations, and post-interviews with teachers. Data were analyzed through coding processes and the development of learning progressions to understand how the teachers' knowledge developed throughout the PD. Results indicated teachers changes their practice and demonstrated increased confidence in teaching GCC science. They incorporated discussion and argumentation into their teaching practice, utilized a more place-based approach to GCC education, and presented GCC as real and anthropogenic. It was also found that the complexities of teachers' lives inhibited the full participation of all the teachers who volunteered to engage in CAR.