Why have the 10, 50 and 90th probability levels been chosen for the key findings?

This was a decision informed by stakeholder consultation. However, it should be noted that the project team are not advocating the use of only these probability levels when using the projections.

In detail

The IPCC uses the following probability levels to describe the proposed likelihood associated with a particular outcome (note that this is different from the amount of confidence that the IPCC places in the projection):

• Virtually Certain > 99%
• Very Likely > 90%
• Likely > 66%
• About as likely as not 33–66%
• Unlikely < 33%
• Very Unlikely < 10%
• Extremely Unlikely <1%

The PDFs are themselves uncertain, and get more uncertain at the extremes of the distributions. As a general guideline we suggest that users should be able to use the distribution from the 10% to the 90% probability levels, but not outside this range. The 10, 50 and 90% probability levels were chosen by the users in recognition that these represented a good spread of results and that they would be able to get changes associated with other probability levels from the UKCP09 User Interface.

Find out more

• IPCC Fouth Assessment report – Guidance note on uncertainty (pdf, 40 KB)