Confidence and Likelihood in the IPCC Fifth Assessment Report
FACT SHEET
What does the IPCC mean when they use ‘confidence’ and ‘likelihood’ ?
When the IPCC scientists use the terms ‘confidence’ and ‘likelihood,’ each term has a different and very specific meaning relating to levels of certainty.
When scientists mention ‘confidence’, they are referring to the degree of confidence in being correct. In the case of the IPCC where scientists are analysing global climate, issues such as lack of observational data in certain regions, such as Africa, will affect scientists’ confidence in their findings. When scientists mention ‘likelihood’ they are referring to the probability of an event or outcome occurring.
Confidence
The IPCC Fifth Assessment Report (AR5) confidence levels are based on the evidence (robust, medium and limited) and the degree of scientific agreement (high, medium and low). The combined evidence and agreement results in five levels of confidence (very high, high, medium, low and very low), as shown in the figure below.
If an event is given a very high confidence level, there is a combination of high agreement and robust evidence that it will occur.
To understand the use of these terms, consider the following statement based on the AR5’s Summary for Policymakers: