Uncertainty Terminology

In assessing the state of knowledge about climate change, scientists have developed a careful terminology for expressing uncertainties around both statements of fact about a current situation (for example, "most observed warming can be attributed to human action") and statements about the likelihoods of various future outcomes (for example, "sea level could rise by several feet by 2100"). The IPCC, in particular, has devoted serious debate and discussion to appropriate ways of expressing and dealing with uncertainty around such statements (Moss and Schneider, 2000), and all recent IPCC assessments have adopted a set of terminology to describe the degree of confidence in conclusions (see, e.g., Manning et al., 2004). In estimating confidence, scientific assessment teams draw on information about "the strength and consistency of the observed evidence, the range and consistency of model projections, the reliability of particular models as tested by various methods, and, most importantly, the body of work addressed in earlier synthesis and assessment reports" (USGCRP, 2009). It is easier to employ precise uncertainty language in situations where conclusions are based on extensive quantitative data or models than in areas where data are less extensive, important research is qualitative, or models are in an earlier stage of development. Statements about the future are also generally more uncertain than statements of fact about observed changes or current trends.

Table D.1 shows the language adopted by the IPCC to describe confidence about facts or the likelihood that a statement is accurate. The U.S. Global Change Research Program's recent assessment report on Global Climate Change Impacts on the United States(USGCRP, 2009) uses similar language. In this report, Advancing the Science of Climate Change, when we draw directly on the statements of the formal national and international assessments, we adopt their terminology to describe uncertainty. However, because of the more concise nature and intent of this report, we do not attempt to quantify confidence and certainty about every statement of the science.

TABLE D.1 Language Adopted by the IPCC to Describe Confidence About Facts or the Likelihood of an Outcome

Terminology for Describing Confidence About Facts

Very high confidence

At least 9 out of 1C chance of being correct

High confidence

About 8 out of 1C chance

Medium confidence

About 5 out of 1C chance

Low confidence

About 2 out of 1C chance

Very low confidence

Less than 1 out of 1C chance

Terminology for Describing Likelihood of an Outcome

Virtually certain

More than 99 chances out of 1


Extremely likely

More than 95 chances out of 1


Very likely

More than 9C chances out of 1



More than 65 chances out of 1


More likely than not

More than 5C chances out of 1


About as likely as not

Between 33 and 66 chances out of 100


Less than 33 chances out of 100

Very unlikely

Less than 10 chances out of 100

Extremely unlikely

Less than 5 chances out of 100

Exceptionally unlikely

Less than 1 chance out of 1CC

SOURCE: IPCC (2007a).


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