The IPCC expresses its consensus conclusions using terminology that indicates the varying levels of certainty the authors have in them. (Reference: Working Group I Technical Summary, 18.64MB, pages 22-23.)

Likelihood of an outcome or result

• "Virtually certain" means greater than a 99 percent probability of occurrence.
• "Extremely likely" means greater than 95 percent.
• "Very likely" means greater than 90 percent.
• "Likely" means greater than 66 percent.
• "More likely than not" means greater than 50 percent.
• "About as likely as not" means 33 to 66 percent.
• "Unlikely" means less than 33 percent.
• "Very unlikely" means less than 10 percent.
• "Extremely unlikely" means less than 5 percent.
• "Exceptionally unlikely" means less than 1 percent.

Relative degrees of confidence in a statement

• "Very high confidence" means at least a 9 out of 10 chance of being correct.
• "High confidence" means about an 8 out of 10 chance.
• "Medium confidence" means about a 5 out of 10 chance.
• "Low confidence" means about a 2 out of 10 chance.
• "Very low confidence" means less than a 1 out of 10 chance.

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