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
2223.)
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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