In the course of the IPCC assessment procedure, chapter teams review the published research literature, document the findings (including uncertainties), assess the scientific merit of this information, identify the key findings, and attempt to express an appropriate measure of the uncertainty that accompanies these findings using a shared guidance procedure. This process has changed over time. The early Assessment Reports (FAR and SAR) were largely qualitative. As the field has grown and matured, uncertainty is being treated more explicitly, with a greater emphasis on the expression, where possible and appropriate, of quantified measures of uncertainty.

Although IPCC’s treatment of uncertainty has become more sophisticated since the early reports, the rapid growth and considerable diversity of climate research literature presents ongoing challenges. In the wake of the TAR the IPCC formed a Cross-Working Group team charged with identifying the issues and compiling a set of Uncertainty Guidance Notes that could provide a structure for consistent treatment of uncertainty across the IPCC’s remit (Manning et al., 2004[1]). These expanded on the procedural elements of Moss and Schneider (2000)[2] and introduced calibrated language scales designed to enable chapter teams to use the appropriate level of precision to describe findings. These notes were revised between the TAR and AR4 and again between AR4 and AR5 (Mastrandrea et al., 2010)[3].

Recently, increased engagement of social scientists (e.g., Patt and Schrag, 2003[4]; Kandlikar et al., 2005[5]; Risbey and Kandlikar, 2007[6]; Broomell and Budescu, 2009[7]; Budescu et al., 2009[8]; CCSP, 2009[9]) and expert advisory panels (CCSP, 2009; InterAcademy Council, 2010[10]) in the area of uncertainty and climate change has helped clarify issues and procedures to improve presentation of uncertainty. Many of the recommendations of these groups are addressed in the revised Guidance Notes. One key revision relates to clarification of the relationship between the ‘confidence’ and ‘likelihood’ language, and pertains to demarcation between qualitative descriptions of ‘confidence’ and the numerical representations of uncertainty that are expressed by the likelihood scale. In addition, a finding that includes a probabilistic measure of uncertainty does not require explicit mention of the level of confidence associated with that finding if the level of confidence is high or very high. This is a concession to stylistic clarity and readability: if something is described as having a high likelihood, then in the absence of additional qualifiers it should be inferred that it also has high or very high confidence.


  1. Manning, M., et al., 2004: IPCC workshop Report: Describing scientific uncertainties in climate change to support analysis of risk and of options [IPCC IPCC Working Group I Technical Support Unit (ed.)]. Available at (accessed 07-10-2013), 138.
  2. Moss, R. H., and S. H. Schneider, 2000: Uncertainties in the IPCC TAR: Recommendations to lead authors for more consistent assessment and reporting. In: Guidance Papers on the Cross Cutting Issues of the Third Assessment Report of the IPCC. World Meteorological Organization, Geneva, pp. 33–51.
  3. Mastrandrea, M. D., et al., 2010: Guidance notes for lead authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties. Available at http:// (accessed 07-10-2013).
  4. Patt, A. G., and D. P. Schrag, 2003: Using specific language to describe risk and probability. Clim. Change, 61, 17–30.
  5. Kandlikar, M., J. Risbey, and S. Dessai, 2005: Representing and communicating deep uncertainty in climate-change assessments. C. R. Geosci., 337, 443–455.
  6. Risbey, J. S., and M. Kandlikar, 2007: Expressions of likelihood and confidence in the IPCC uncertainty assessment process. Clim. Change, 85, 19–31.
  7. Broomell, S., and D. Budescu, 2009: Why are experts correlated? Decomposing correlations between judges. Psychometrika, 74, 531–553.
  8. Budescu, D., S. Broomell, and H.-H. Por, 2009: Improving communication of uncertainty in the reports of the Intergovernmental Panel on Climate Change. Psychol. Sci., 20, 299–308.
  9. CCSP, 2009: Best Practice Approaches for Characterizing, Communicating, and Incorporating Scientific Uncertainty in Climate Decision Making. U.S. Climate Change Science Program, Washington, DC, USA, 96 pp.
  10. InterAcademy Council, 2010: Climate change assessments. In: Review of the Processes and Procedures of the IPCC, Amsterdam, The Netherlands.
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