It can become 5 °C warmer: The extremity effect in climate forecasts.

Abstract

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 2 languages (Norwegian and English), and with several related terms (can, possible, could, and may). The combination of extremity and exaggerated likelihood conveyed by such statements could lead to serious miscommunications. (PsycINFO Database Record

More than 50% or Less than 70% Chance: Pragmatic Implications of Single‐Bound Probability Estimates

“…Assimilative comparisons will have the opposite effect, as terms like nearly , almost, or up to indicate that high values are reachable. Recent studies indicate a similar usage of statements about outcomes that can happen (Teigen, Filkukova, & Hohle, ). Such statements (“it can be 5 °C warmer”) typically point to the upper extreme of a distribution, while at the same time highlighting this value as a real, obtainable possibility.…”

Entirely possible overruns: how people think and talk about probabilistic cost estimates

“…But it is hard to legislate natural language, and formal definitions of selected phrases may conflict with natural usage of the same expressions (Berry et al, 2003;Budescu et al, 2012). A recent line of research has shown that the "translation approach" (which probabilities correspond to a good chance) should be complemented with a "which outcome approach" (which outcomes are characterized as having a good chance) (Juanchich et al, 2013;Teigen et al, 2014). For instance, unlikely is commonly translated as corresponding to a 10-30% probability (Mastrandrea et al, 2010;Theil, 2002).…”

“…However, it turns out that outcomes described as unlikely are those that have never (hitherto) occurred, in other words values with a minimal (close to zero) probability of occurrence, mostly values beyond the top of the distribution. This phenomenon has been labelled an extremity effect (Jenkins et al, 2018; which still, due to their extremeness, have a relatively low probability of occurrence (Teigen, Filkuková and Hohle, 2018). As these terms are often "translated" to denote a medium probability (around 50%), a lack of understanding of the extremity effect can lead to serious miscommunications.…”

“…A recent study of climate-related predictions showed that people were willing to consider all outcomes that happened to fall inside a wide 90% prediction interval as having been equally well predicted. Outcomes outside the prediction interval indicated that the prediction had been wrong, this time in proportion to the distance between the outcome value and the interval boundaries (Teigen, Filkuková and Hohle, 2018; Teigen, Løhre and Hohle, 2018). A narrow, but compatible 60% interval, was in contrast viewed as producing wrong predictions.…”

“…This phenomenon has been labeled an extremity effect (Jenkins et al , 2018; Teigen et al , 2013). Statements about what is “possible”, or what “can”, “could” and “may” occur will typically describe top values that are a bit more realistic than the “unlikely” ones, but which still, due to their extremeness, have a relatively low probability of occurrence (Teigen, Filkuková and Hohle, 2018; Teigen, Løhre and Hohle, 2018). As these terms are often “translated” to denote a medium probability (around 50 percent), a lack of understanding of the extremity effect can lead to serious miscommunications.…”