ARTICLES PUBLISHED ONLINE: 20 APRIL 2014 | DOI: 10.1038/NCLIMATE2194 The interpretation of IPCC probabilistic statements around the world David V. Budescu 1 *, Han-Hui Por 1 , Stephen B. Broomell 2 and Michael Smithson 3 The Intergovernmental Panel on Climate Change (IPCC) uses verbal descriptions of uncertainty (for example, Unlikely) to convey imprecision in its forecasts and conclusions. Previous studies showed that the American public misinterprets these probabilistic statements. We report results from a multi-national study involving 25 samples in 24 countries and 17 languages. As predicted, laypeople interpret IPCC statements as conveying probabilities closer to 50% than intended by the IPCC authors. We show that an alternative presentation format supplementing the verbal terms with numerical ranges increases the correspondence between the public’s interpretations and the IPCC guidelines, and the terms are better differentiated. These qualitative patterns are remarkably stable across all samples and languages. In fact, interpretations of the terms in various languages are more similar under the new presentation format. These results suggest changing the way the IPCC communicates uncertainty. T he IPCC assembles and disseminates information about global climate change (GCC). Findings and conclusions are compiled into periodical Assessment Reports informing policymakers and the public on issues relevant to the understanding of GCC. One important issue facing the IPCC is how to communi- cate the uncertainties in its models and predictions. This challenge applies to all risk communications, but the debates surrounding climate change are much more intense and politicized. Questions about the reality, severity and sources of GCC and the best ways to address it frequently occur in public and political debates. Some sources of uncertainty are inherent to the climate science. Others reflect the public’s imperfect understanding of climate-related issues and misperceptions about scientific consensus on the topic 1 . Probabilistic judgments can be communicated as precise numerical probabilities (for example, there is a 0.4 chance that X will occur), imprecise numerical probabilities (for example, the probability that X will occur is between 0.3 and 0.6) or probability phrases (for example, it is improbable that X will occur). The challenge facing the IPCC is to convey information with the level of precision warranted by the available evidence 2 . Using precise (numerical) probabilities could be misleading, as it would imply too high a level of precision, and of the consensus among experts. In recent assessments the IPCC has used verbal descriptions of uncertainty such as Likely accompanied by a translation table reproduced in Table 1 (ref. 3). Recent empirical work 4,5 has questioned the efficiency of this method and has documented the superiority of an alternative dual-scale combining probability phrases and numerical ranges. We report results of an international study designed to document and compare the efficiency of this method of uncertainty communication in various countries and languages. We also explore the relationship between this efficiency and the perceptions of, and beliefs about, GCC. Communication of uncertainty by probability phrases Psychologists have documented large individual differences in the ways people understand, communicate and use probability phrases 6,7 . They found that representations and interpretations of probability terms are context dependent 8–10 ; recipients of verbal forecasts interpret them as less extreme and more imprecise than intended by the communicators 11,12 ; most people prefer to communicate their opinions verbally, but prefer receiving precise numerical information 13,14 ; probability terms are susceptible to self- serving interpretations 15 ; and verbal lexicons vary markedly across individuals 16–18 , as does the interpretation of most verbal terms 19–21 . These results induce an illusion of communication that stems from the (intuitive, but false) assumption that everyone interprets terms similarly across contexts and circumstances. Some researchers 22,23 have suggested using standardized lists of terms accompanied by numerical ranges to reduce errors in communicating uncertainty. However, it is difficult to ‘legislate’ language. For example, National Weather Service weather forecasters, trained to use a set of phrases in meteorological forecasts, reverted to the colloquial meaning of the same phrases when they were embedded in different contexts 9 . Nevertheless, this is the solution that the IPCC adopted. Authors are instructed to use a scale employing 7 verbal terms to convey uncertainties (Table 1). Even if all authors comply with these instructions, it is important to test whether the readers of the reports understand these terms as intended by the authors. This is a serious concern because critics use uncertainty as an excuse to dismiss the findings all together. Participants in previous studies 4,5 read sentences containing probabilistic terms extracted from IPCC reports, and judged the probabilities intended by the authors. The consistency between readers and authors, as measured by the degree to which the participants’ judgments matched the conversion table, was low. Responses were highly regressive—underestimating high probabilities, and overestimating low values—and the variability in readers’ interpretations far exceeded the uncertainty implied by the IPCC terms. A reanalysis of these data 24 found that negatively worded phrases caused responses to be more regressive and more varied than their positively worded counterparts. Furthermore, the interpretation of the phrases was correlated with the respondents’ beliefs in, and experience with, GCC, their education, numeracy and ideology. 1 Department of Psychology, Fordham University, 441 East Fordham Road, Bronx, New York 10458, USA, 2 Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania, 15213, USA, 3 Research School of Psychology, Bldg 39, The Australian National University, Canberra A.C.T. 0200, Australia. *e-mail: budescu@fordham.edu NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1 © 2014 Macmillan Publishers Limited. 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