Climate change is one of the most significant science issues facing humanity; yet, teaching students about climate change is challenging: not only is it multidisciplinary, but also it is contentious and debated in political, social and media forums. Students need to be equipped with an understanding of climate change science to be able to participate in this discourse. The purpose of this study was to examine Western Australian high school students' understanding of climate change and the greenhouse effect, in order to identify their alternative conceptions about climate change science and provide a baseline for more effective teaching. A questionnaire designed to elicit students' understanding and alternative conceptions was completed by 438 Year 10 students (14-15 years old). A further 20 students were interviewed. Results showed that students know different features of both climate change and the greenhouse effect, however not necessarily all of them and the relationships between. Five categories of alternative conceptions were identified. The categories were (1) the greenhouse effect and the ozone layer; (2) types of greenhouse gases; (3) types of radiation; (4) weather and climate and (5) air pollution. These findings provide science educators a basis upon which to develop strategies and curriculum resources to improve their students' understanding and decision-making skills about the socioscientific issue, climate change.