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The IPCC definition for an outcome or result when its likelihood is less than 33% probable. If not referring to IPCC conclusions, the meaning is not specified in terms of a .

In detail

In everyday language when the term likely is used to describe an event it suggests that something has a small chance of occurring. However when used to describe projections of future climate change likely has a particular meaning.

One of the criticisms of the IPCCs’ second assessment report in 1995 was that different scientists described the change or likelihood of an event happening or being caused by other events (such as climate change and human activity) in different ways. This made it difficult to develop a consensus across the report and so made it hard for readers to understand the relative degree of scientific certainty and uncertainty in statements or findings. To address this issue the IPCC Third Assessment Report aimed to be more consistent. Scientists developed a system for describing the relative degree of probability in the reports. This included both qualitative and quantitative descriptors to indicate the assessed likelihood, using expert judgement, of an outcome or a result. Unlikely specifically means a particular outcome or an event has a less than 33% chance of happening or occurring.  

Terminology Likelihood of the ocurrance/outcome
Virtually certain
> 99% probability of ocurrance
Very likely
> 90% probability 
> 66% probability
About as likely as not  > 33 to 66% probability
< 33% probability
Very unlikely
< 10% probability
Exceptionally unlikely
< 1% probability

The IPCC uses a different system to describe the degree of confidence that experts have their conclusions.

Find out more

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