To gain insight into complex sustainability problems and acknowledge complexity is essential and can be achieved by creating an overview of the entire system, including interaction between variables. Therefore, systems thinking is recognized as a vital cognitive skill required to grasp complex global problems. Nevertheless, the implementation of systems thinking into general education, and geography in particular, is sadly limited. Encouraging students to use appropriate tools will probably help them understand systems complexity. A lesson series to foster systems thinking in a Belgian high school geography course was developed and implemented. In this design, produced by researchers in consultation with teachers, students were required to elaborate causal diagrams based on original texts and graphs of complex geographical issues. Causal diagrams are expected to support the development of students' systems thinking ability. The interpretation of the information sources from a geographical, and thus multidimensional perspective, is the core of inquiry-based instruction aimed at fostering systems thinking. By describing the gradual use of causal diagrams as a tool to visually represent given data, this article contributes an example of this scaffolding technique in geography education. In addition to a description of the lesson series itself, we also discuss how decisions in the design process were influenced by theoretical as well as practical aspects of geography education in Flanders (Belgium). Furthermore, first reflections on the implementation are illustrated.