# Likelihood

Likelihood is a probabilistic estimate of the occurrence of a single event or of an outcome. Likelihood may be based on statistical or modeling analyses, expert views, or other quantitative analyses.[1] For example, a meteorologist might say "it is likely to rain", and assign a probability of this to occur, say, 70%.

Likelihood is often used in climate modeling and projections to communicate the probability of certain events occurring in the climate's future. For instance, heat waves are very likely to increase in both frequency and intensity under the effects of climate change.[2]

The table below shows the phrases related to likelihood often stated in reports (like the IPCC reports) and the corresponding probability associated with them.[3] Often the specific probabilities are the best guess of a single expert or a committee of experts, which makes the terms more useful than specific numbers.

Table 1. Relationship between likelihood terms and probability of occurrence
Likelihood Term Likelihood of occurrence
Virtually certain >99% probability
Extremely likely >95% probability
Very likely >90% probability
Likely >66% probability
More likely than not >50% probability
About as likely as not 33 to 66% probability
Unlikely <33% probability
Very unlikely <10% probability
Extremely unlikely <5% probability
Exceptionally unlikely <1% probability

## References

1. IPCC, 2012: Glossary of terms. In: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK, and New York, NY, USA, pp. 555-564.
2. IPCC. (Accessed May 21, 2016). Thermal Stress (Heat Waves, Cold Spells) [Online], Available: http://www.ipcc.ch/ipccreports/tar/wg2/index.php?idp=353
3. IPCC. (Accessed June 6, 2016). The IPCC Assessments of Climate Change and Uncertainties [Online], Available: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch1s1-6.html