Climate projections and other predictions are often described as outcomes that can happen, indicating possibilities that are imaginable, but uncertain. Whereas the meanings of other uncertainty terms have been extensively studied, the uses of modal verbs like can and will have rarely been examined. Participants in five experiments were shown graphs and verbal statements showing projections of future global warming, sea level rise, and other climate-related issues. All studies gave support for the extremity hypothesis, which states that people use can-statements to describe the topmost values in a distribution of outcomes, regardless of their actual probabilities. Despite their extremity, outcomes that can happen are believed to have a substantial likelihood of occurrence. The extremity effect was replicated in two languages (Norwegian and English), and with several related terms (can, possible, could, may). The combination of extremity and exaggerated likelihood conveyed by such statements could lead to serious miscommunications.
It can become 5 °C warmer: The extremity effect in climate forecasts