Semantics “Very Likely” a Problem in IPCC Communications

August 5, 2014

The United Nations Intergovernmental Panel on Climate Change (IPCC), the leading international organization for assembling and disseminating information about climate change, apparently has a language problem.  Authors of a recent study found that lay people around the world frequently misinterpret IPCC terminology that describes the probability of future climate change.

The IPCC uses probability statements with associated likelihood values ranging from 1% to 99% to describe the strength of their data and conclusions.  Relying on terms like “exceptionally unlikely,” “about as likely as not,” “very likely,” and “virtually certain” to describe probability, the language has often been criticized as confusing. Budescu, et al. (2014) tested lay people from 24 countries in 17 languages on their ability to accurately interpret these probability statements and found that the charge has merit.  Across all languages and nationalities people consistently underestimated the strength of statements like “very likely” and “virtually certain” and overestimated the strength of statements like “exceptionally unlikely.”

The authors then presented numerical ranges to laypeople instead of probability statements and found that the accuracy of people’s understanding of IPCC reports increased. These results suggest that scientists need to focus on how to effectively craft public statements about climate change and that the IPCC specifically should update the language used in their reports to enhance the public’s grasp of their message.       


Budescu D, H Por, SB Broomell, M Smithson (2014) The interpretation of IPCC probabilistic statements around the world. Nature Climate Change:DOI: 10.1038/NCLIMATE2194