Expressions of likelihood and confidence in the IPCC uncertainty assessment process

Communication of uncertainty information in Intergovernmental Panel on Climate Change (IPCC) assessments has evolved through successive reports to provide increasingly formal classifications for subjective and objective information. The first IPCC assessments provided uncertainty information in largely subjective form via linguistic categorizations depicting different levels of confidence. Recent assessments have codified linguistic terms to avoid ambiguity and introduced probabilistic ranges to express likelihoods of events occurring. The adoption of formal schemes to express likelihood and confidence provides more powerful means for analysts to express uncertainty. However, the combination of these two metrics to assess information may engender confusion when low confidence levels are matched with very high/low likelihoods that have implicit high confidence. Part of the difficulty is that the degree to which different quantities in the assessments are known varies tremendously. One solution is to provide likelihood information in a scheme with a range of different precision levels that can be matched to the level of understanding. A version of this scheme is also part of the IPCC uncertainty guidance and is described here.