What do we mean by probability in the probabilistic climate projections?

The probabilities given in the probabilistic climate projections can be seen as the relative degree to which each possible climate outcome is supported by the evidence currently available, taking into account our understanding of climate science, observations and using expert judgement. They are constrained by the methodology used to produce them.

In detail

The probabilities are calculated based on a methodology described in more detail in Chapter 3 of the Climate change projections report, and they include the effects of natural variability. As with climate modelling, the methodology used to define the probabilities varies between different scientific institutions and thus the results will differ depending on the methodology used. However, the methodology developed for UKCP09 used a design process and new multiple model runs, rather than (as with other methodolgies) an ensemble of opportunity of existing climate models. While a number of methods for probabilistic climate projection have been published in the research literature, we are not aware of any that have been designed to sample uncertainties as comprehensively as is done in UKCP09 (for example there are several methods which sample uncertainties in physical climate system processes, but none which combines these with uncertainties in both carbon cycle processes and downscaling). Probabilities will also change over time as statistical methods change and improve, and new observations and models are developed and incorporated.