The Next Generation Science Standards (Next Generation Science Standards Lead States, 2013) and the Essential Principles for Climate Literacy (National Oceanic and Atmospheric Administration, 2009) have helped advance teaching and learning about Earth's climate and global climate change (GCC) in the formal K-12 classrooms. However, teachers report feeling challenged in understanding of the complexity of Earth's climate system, feel underprepared to teach it, and describe instruction in this area as a low priority (Hestness et al, 2011; Plutzer et al., 2016). To address this need, we are engaged in a 4-year, NSF-funded project to support secondary science teachers to engage students in model-based learning about Earth's climate and GCC through implementation of a new, 4-week curriculum unit designed around an online, computer-based NASA global climate modeling tool. After developing the initial curriculum module in Year 1, we engaged in empirical research to answer two research questions: 1) In what ways do teachers implement the project curriculum? and 2) Why do they implement it in the ways that they do? Here, we report on qualitative analysis of data from classrooms of two secondary Earth science teachers who taught the pilot curriculum in Year 1. Findings illustrate three primary themes in the two teachers' implementation of the curriculum: student autonomy, lesson adaptation, and maintaining congruence among classes. Teacher 1 tended to promote student autonomy, adapt lessons, and change the structure of lessons class-by-class. In contrast, Teacher 2 limited student autonomy, adhered more closely to the written curriculum, and maintained congruence within all class periods. These observed differences were aligned with the teachers' general priorities for instruction, as well as self-efficacy with the curriculum and climate-related concepts. Study findings have important implications for curriculum design and secondary teachers' instruction to support students' learning about Earth's climate and GCC.