As climate change becomes an increasingly important topic for science educators, it is critical to learn how teachers may be able to increase students' knowledge about it. We conducted two consecutive quasi-experimental studies that investigated the role of interest in predicting middle school students' knowledge gains from a unit about how scientists use mathematical models to predict climate change's impacts on forests. The studies measured the intervention's effects on students' knowledge about climate change and examined how their interest in the topic and related factors were associated with their knowledge before and after the intervention. Participants in the two studies included 467 treatment and 177 comparison group students (Study 1) and 363 treatment and 219 comparison group students (Study 2). Multilevel modeling analyses revealed increases in students' knowledge about climate change after participating in the unit. Path modeling analyses showed that students' interest in climate change was indirectly related to their knowledge about climate change, mediated by students' existing desire to learn more, their interest in the unit, their belief that climate change was important (Studies 1 and 2), as well as their behavioral self-efficacy (Study 2). Students' interest in science was positively associated with their knowledge about climate change (Study 1) but their perception of threats posed by climate change was not (Study 2). Findings suggest that science educators can improve students' knowledge about climate change by connecting the topic to students' lives and ensuring they feel empowered to act on climate change.