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Confidence is described as the degree of certainty in a particular outcome or prediction being correct. In the context of climate modelling, a claim to 90% confidence indicates an outcome that only happens one time in ten or less.

In detail

The IPCC refer to levels of confidence which can be used to characterise uncertainty that is based on expert judgment as to the correctness of a model, an analysis or a statement. The last two terms in this scale should be reserved for areas of major concern that need to be considered from a risk or opportunity perspective, and the reason for their use should be carefully explained.

TerminologyDegree of confidence in being correct
Very high confidenceAt least 9 out of 10 chance of being correct
High confidenceAbout 8 out of 10 chance
Medium confidenceAbout 5 out of 10 chance
Low confidence About 2 out of 10 chance
Very low confidenceLess than 1 out of 10 chance of being correct

This is subtly different from the IPCC's concept of likelihood, which refers to a probabilistic assessment of some well defined outcome having occurred or occurring in the future.

The UKCP09 probabilistic projections, to a certain extent, combine the IPCC concepts of confidence and likelihood. They are a measure of the degree to which a particular level of future climate change is consistent with the information used in the modelling process. Analysis of the results of UKCP09 reveals greater confidence in the projected changes within the 10-90 probability-level range.

For this reason anything outside of the 10-90 probability-level range can be taken into account but may not be considered robust in decision-making.

  • in the UKCP09 Briefing report provides an overview of confidence in climate projections. 

Find out more

  • Guidance Notes (pdf, 42 KB) for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties.