In the debate on climate change commentators often express pessimistic or optimistic views. We see this mainly in the media and popular literature, but also in various academic fields. The aim of this paper is to investigate different kinds of pessimistic and optimistic views put forward in this debate and suggest explanations of the diversity of views. The paper concludes that pessimism and optimism may concern, for example, climate change as an unmitigated or poorly mitigated process, mitigation of climate change (...) or specific measures of mitigation. These aspects are important to distinguish, because a person can be pessimist concerning climate change as an unmitigated or poorly mitigated process and optimist concerning mitigation of climate change, and be pessimist concerning one specific mitigation measure and optimist concerning another. It is suggested that the diversity of pessimistic and optimistic views is due to the uncertainty of scientific climate models and the influence of evaluative and ideological assumptions. (shrink)