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Effective communication of uncertainty in the IPCC reports

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Abstract

The Intergovernmental Panel on Climate Change (IPCC) publishes periodical assessment reports informing policymakers and the public on issues relevant to the understanding of human induced climate change. The IPCC uses a set of 7 verbal descriptions of uncertainty, such as unlikely and very likely to convey the underlying imprecision of its forecasts and conclusions. We report results of an experiment comparing the effectiveness of communication using these words and their numerical counterparts. We show that the public consistently misinterprets the probabilistic statements in the IPCC report in a regressive fashion, and that there are large individual differences in the interpretation of these statements, which are associated with the respondents’ ideology and their views and beliefs about climate change issues. Most importantly our results suggest that using a dual (verbal—numerical) scale would be superior to the current mode of communication as it (a) increases the level of differentiation between the various terms, (b) increases the consistency of interpretation of these terms, and (c) increases the level of consistency with the IPCC guidelines. Most importantly, these positive effects are independent of the respondents’ ideological and environmental views.

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Notes

  1. Budescu et al. (2009) also asked respondents to indicate a range of values that best describes the word. Our sample size was inversely proportional to the length of the questionnaire, so we decided to focus only on the mean values (as a proxy for the ranges), in an effort to maximize sample size.

  2. The two numbers are different because there were two items associated with each word.

  3. We re-ran the same analysis with the complete data (including all the extreme responses), and found significant differences between words, presentation formats and an interaction between words and presentation formats. Similarly, the differences between words is most (least) pronounced in the VN (Control) group. This analysis confirms the robustness of our results. The detailed results are described in Section 6 of the Online Resource.

  4. Ten respondents were not included in this analysis because they used extreme values (either 0 or 100) for all items and these were coded as missing values.

  5. Relatively fewer respondents scored 3 or more on the numeracy scale and this contributed to higher standard errors observed for these scores

  6. We replicated most of the key ANCOVA results using the complete data. We found significant differences between words, but not between formats. Most importantly, the interaction between words and formats is significant with the VN group showing the best differentiation between terms (see Section 6 in the Online Resource for the detailed results).

  7. Extracted from IPCC AR4 Working Group I Report “The Physical Science Basis”, Chapter 10.

  8. The translation table is ambiguous as it does not specify whether the various ranges are mutually exclusive or overlapping (for example, it is unclear whether likely applies to all probabilities above 66%, or only to values between 67% and 90%). The recently published guidelines for the 5th assessment (Mastrandea, et al. 2010) explicitly state that they are not exhaustive, i.e. likely can be used for any probability between 66% and 100% (not only for the 67–90% range). Although we have no data on this point we suspect that most people will find this solution counterintuitive and confusing.

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Acknowledgements

This work was supported by the National Science Foundation under Grant No. 0345925, and the data were collected by Time-sharing Experiments for the Social Sciences, NSF Grant 0818839 (Jeremy Freese and Penny Visser, Principal Investigators). The third author was supported by the National Geospatial-Intelligence Agency (under Grant No. HM1582-09-1-0020). The opinions, findings and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the funding entity.

Many thanks to Profs. Klaus Keller and Thomas Wallsten and three reviewers for useful comments and suggestions on an earlier version.

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Authors and Affiliations

  1. Department of Psychology, Fordham University, 441 East Fordham Road, Bronx, NY, 10458, USA

    David V. Budescu & Han-Hui Por

  2. Department of Social and Decision Sciences, Carnegie Mellon University, Pittsburgh, PA, 15213, USA

    Stephen B. Broomell

  3. College of Information Sciences and Technology, Pennsylvania State University, University Park, PA, 16802, USA

    Stephen B. Broomell

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  1. David V. Budescu
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Correspondence to David V. Budescu.

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Budescu, D.V., Por, HH. & Broomell, S.B. Effective communication of uncertainty in the IPCC reports. Climatic Change 113, 181–200 (2012). https://doi.org/10.1007/s10584-011-0330-3

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