One of the main advances in UKCP09 is that it provides probabilistic projections. This means that different future climate outcomes described in probabilistic terms, based on the strength of evidence associated with them. As such, probability levels associated with a given change should be interpreted as indicating the relative likelihood of the projected change being at or less than the given change.
For example, if a projected temperature change of +4.5°C is associated with the 90% at a particular location in the 2080s for the UKCP09 medium emission scenario, this should be interpreted as it is projected that there is a 90% likelihood that temperatures at that location will be equal to or less than 4.5°C warmer than temperatures in the 1961–1990 baseline period. Conversely, there is a 10% likelihood that those temperatures will be greater than 4.5°C warmer than the baseline period.
- Section 1.1.1 of the UKCP09 Climate change projections report explains what is meant by probability in UKCP09.