The Finnish upper secondary curriculum emphasizes climate change mitigation and integrates transversal competences. However, teachers trained in disciplinary-based programs often struggle with the multidisciplinary nature of climate education. This case study explores the professional learning of physics teachers taking part in the collaborative development of a project-based upper secondary school climate education teaching module in a research-practice-partnership (RPP). The RPP was a collaboration between physics teachers, climate education researchers, and climate change researchers. The collaborative development process involved iterative design and reflection meetings, with data collection on student engagement and understanding. Based on the interviews of teachers, three main challenges were recognized: integrating multidisciplinary elements in a disciplinary context, addressing values and emotions related to climate change, and adopting project-based learning. Teachers appreciated the collaborative nature of the RPP, which provided support and confidence in implementing new pedagogical approaches. The study highlights the potential of RPPs in fostering professional learning, and suggests incorporating interdisciplinary collaboration in teacher education to address global challenges like climate change.