Glossary
The English version of our glossary is currently under construction and will be regularly updated.
Probability
Scientific findings are often subject to various uncertainties, due e.g. to unanswered questions or to processes that have yet to be sufficiently explained. Consequently, IPCC Reports also include information on the uncertainty of findings and the respective probability of future events.
In IPCC Reports, the probability of an event, outcome or result is – provided that it can be effectively assessed – expressed using standard terms, which are defined in Footnote 6 of the Summary for Policymakers provided by Working Group I. See also Uncertainty.
Scientific findings are often subject to various uncertainties, due e.g. to unanswered questions or to processes that have yet to be sufficiently explained. Consequently, IPCC Reports also include information on the uncertainty of findings and the respective probability of future events.
In the IPCC’s Fourth Assessment Report (AR4), the three Working Groups expressed uncertainty in different ways. For example, Working Group I (The Physical Science Basis) primarily used a quantitative probability scale. On the basis of expert assessments, the probabilities for findings and events were expressed in the following categories:
- Virtually certain (> 99 percent probability of occurrence),
- Extremely likely (> 95 percent probability),
- Very likely (> 90 percent probability),
- Likely (> 66 percent probability),
- More likely than not (> 50 percent) probability,
- Unlikely (< 33 percent probability),
- Very unlikely (< 10 percent probability),
- Extremely unlikely (< 5 percent probability).