Political and media debate on the existence and causes of climate change has become increasingly factious in several western countries, often resting on claims and counter-claims about what most citizens really think. There are several well-established phenomena in psychology about how people perceive the prevalence of opinions, including the false consensus effect1 (a tendency to overestimate how common one’s ‘own’ opinion is) and pluralistic ignorance2 (where most people privately reject an opinion, but assume incorrectly that most others accept it). We investigated these biases in people’s opinions about the existence and causes of climate change. In two surveys conducted 12 months apart in Australia (n = 5,036; n = 5,030), respondents were asked their own opinion about the nature of climate change, and then asked to estimate levels of opinion among the general population. We demonstrate that opinions about climate change are subject to strong false consensus effects, that people grossly overestimate the numbers of people who reject the existence of climate change in the broader community, and that people with high false consensus bias are less likely to change their opinions.