This study reports on an effort to illustrate the coupling of educational research with ongoing curriculum development to promote effective and evidence-based online learning. The research findings have been used to inform the "EarthLabs" curriculum development team as they revise existing modules and create new modules, in order to represent the ways in which such research findings can be used to improve similar online curriculum materials and enhance student learning outcomes. "EarthLabs" curriculum is a suite of online inquiry-based activities that promote understanding of Earth system science. Assessments were employed to understand student learning about complex climate systems as a result of their engagement with the online "EarthLabs" curriculum. Collection of pre- and postcourse student assessment data (n = 205), classroom observations during implementation (n = 6), teacher interviews (n = 7), and eye-tracking data (n = 49) were included in the study. Qualitative and quantitative findings show that "EarthLabs" classroom implementation significantly improves students' conceptual and systems understanding and that students and external users are appropriately engaged with the online materials. These findings have been applied to evaluating the efficacy of the "EarthLabs" program in reaching target programmatic and learning goals, as well as to developing a broader understanding of the cognitive challenges students have in navigating complex Earth systems phenomena, where continued "EarthLabs" program revision has occurred through design-based research.