Previous research in environmental education and learning has mainly concerned students’ understanding of natural scientific knowledge, whereas research on the influence of other knowledge in learning environmental issues is marginal. Also, the interest in most studies investigating students’ natural scientific knowledge has been to capture constraints in students’ understanding, hence investigations of students’ meaning making are rare. The main objective of this thesis was to explore individual students’ reasoning regarding climate change, and the influence of knowledge on their reasoning. In Study I, students’ conceptions of the enhanced greenhouse effect (EGHE) were investigated. The results showed that students incorporated different pieces of information from different problem areas into the conceptualization of the EGHE. Setting up causal links between diversely different pieces of information seems to be a way to make meaning, and thus a necessary step in the learning process. Study II is an investigation of students’ solutions to climate change. The results indicated that students contextualized problems and solutions by addressing the individual(s), where the individual(s) was either “myself” or “someone else”. The different notions of the individual(s) became crucial as the students’ views of the environment, as well as society, changed according to the different contexts.  To further study students’ conceptions of “me” and “others”, Study III examined students’ conceptualized solutions to the dilemma between economic development and mitigating climate change. The findings suggested that students’ conceptualized nature as a “box” of resources, and that economic development would sustain and improve nature. Therefore, the dilemma between economic development and mitigating climate change or dealing with environmental problems did not exist. Results from all three studies were discussed with respect to theoretical implications. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Manuscript.