IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability

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