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. 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.
The table below shows the phrases related to likelihood often stated in reports (like the IPCC reports) and the corresponding probability associated with them. 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.
|Likelihood Term||Likelihood of occurrence|
|Virtually certain||>99% probability|
|Extremely likely||>95% probability|
|Very likely||>90% probability|
|More likely than not||>50% probability|
|About as likely as not||33 to 66% probability|
|Very unlikely||<10% probability|
|Extremely unlikely||<5% probability|
|Exceptionally unlikely||<1% probability|
- 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.
- IPCC. (Accessed May 21, 2016). Thermal Stress (Heat Waves, Cold Spells) [Online], Available: http://www.ipcc.ch/ipccreports/tar/wg2/index.php?idp=353
- 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