Environment Counts | Communicating uncertainty in the IPCC Fifth Asessment Report
Author: Geoff Zeiss – Published At: 2013-11-02 19:11 – (829 Reads)
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Two metrics are used for communicating the degree of certainty in the findings reported in the Fifth Assessment Report (AR5) of the IPCC.
Confidence is a qualitative assessment based on the quality and consistency of the evidence and the degree of agreement between evidence from different sources. Likelihood is a quantified measure of certainty expressed as a probability. Descriptive terms such as “very likely” or “unlikely” correspond to specific ranges of probabilities. IPCC FIFTH ASSESSMENT REPORT (AR5) CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS |
- Uncertainty in measured quantities can arise from a range of sources, such as statistical variation, subjective judgement, and linguistic imprecision.
- In the modelling studies that are used to project future climate change, uncertainty is handled differently. The uncertainty may be due to approximations in the climate model itself, to uncertainty about external factors, for example, aerosol forcing, or to uncertainty about starting conditions.
Confidence
The confidence level is based on the evidence (robust, medium, and limited) and the agreement (high, medium, and low). A combination of different methods, e.g., observations and modeling, is important for evaluating the confidence level. The following summary terms are used to describe
- available evidence: limited, medium, or robust;
and
- degree of agreement: low, medium, or high.
A level of confidence is expressed using five qualifiers:
- level of confidence: very low, low, medium, high, and very high,
and typeset in italics, e.g., medium confidence. The figure shows how the combined evidence and agreement results in five levels for the confidence level used in this assessment.
For a given evidence and agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of agreement are correlated with increasing confidence.
Likelihood
The qualifier “likelihood†provides calibrated language for describing quantified uncertainty. It can be used to express a probabilistic estimate of the occurrence of a single event or of an outcome, e.g., a climate parameter, observed trend, or projected change lying in a given range. Statements made using the likelihood scale may be based on statistical or modelling analyses, elicitation of expert views, or other quantitative analyses.
| Term | Likelihood of the Outcome |
| Virtually certain | 99−100% probability |
| Very likely | 90−100% probability |
| Likely | 66−100% probability |
| About as likely as not | 33−66% probability |
| Unlikely | 0−33% probability |
| Very unlikely | 0−10% probability |
| Exceptionally unlikely | 0−1% probability |
Recently, increased engagement of social scientists and expert advisory panels in the area of uncertainty and climate change has helped clarify issues and procedures to improve presentation of uncertainty. 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.
Additionally, 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 highâ€. 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 reasonably high confidence.
IPCC FIFTH ASSESSMENT REPORT (AR5) CLIMATE CHANGE 2013: THE PHYSICAL SCIENCE BASIS
