F. Communication of uncertainty in the Working Group II Fourth Assessment
A set of terms to describe uncertainties in current knowledge is common to all parts of the IPCC Fourth Assessment, based on the Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties1, produced by the IPCC in July 2005.
Description of confidence
On the basis of a comprehensive reading of the literature and their expert judgement, authors have assigned a confidence level to the major statements in the Report on the basis of their assessment of current knowledge, as follows:
| Terminology | Degree of confidence in being correct |
| Very high confidence | At least 9 out of 10 chance of being correct |
| High confidence | About 8 out of 10 chance |
| Medium confidence | About 5 out of 10 chance |
| Low confidence | About 2 out of 10 chance |
| Very low confidence | Less than a 1 out of 10 chance |
Description of likelihood
Likelihood refers to a probabilistic assessment of some well-defined outcome having occurred or occurring in the future, and may be based on quantitative analysis or an elicitation of expert views. In the Report, when authors evaluate the likelihood of certain outcomes, the associated meanings are:
| Terminology | Likelihood of the occurrence/ outcome |
| Virtually certain | >99% probability of occurrence |
| Very likely | 90 to 99% probability |
| Likely | 66 to 90% probability |
| About as likely as not | 33 to 66% probability |
| Unlikely | 10 to 33% probability |
| Very unlikely | 1 to 10% probability |
| Exceptionally unlikely | <1% probability |