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This guidance note for lead authors of the IPCC Fourth Assessment Report emphasizes the importance of addressing uncertainties in climate science. It presents a simple typology of uncertainties, offers recommendations to authors on handling uncertainty, and outlines levels of confidence and likelihood terminology.
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Guidance Notes for Lead Authors of theIPCC Fourth Assessment Report on Addressing Uncertainties • Challenge: • Differences between social & natural sciences • Information used • Analyses • Indicators of change
Table 1. A simple typology of uncertainties Often underestimated by experts!
Some Recommendations to IPCC Authors • Treat uncertainty • Review information available on uncertainty • Make expert judgments - but watch for “group think” • Use appropriate precision: - sign change? - trend? - order of magnitude (powers of 10) - range of values? - probability distribution?
Levels of Confidence Uncertainty based on expert judgment Terminology Degree of confidence (in being correct) Very High confidence At least 9 out of 10 chance 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 1 out of 10 chance
Likelihood Probability assessment of an outcome (past or future) Terminology Likelihood of the occurrence/ outcome Virtually certain > 99% probability of occurrence Very likely > 90% probability Likely > 66% probability About as likely as not 33 to 66% probability Unlikely < 33% probability Very unlikely < 10% probability Exceptionally unlikely < 1% probability
