Probabilistic climate change projections

A projection of future change in climate (relative to a baseline period) that assigns probability levels to different climate change outcomes. This projection provides a change value for the future climate, expressed as the difference from the (modelled) 1961–1990 baseline climate (as opposed to the absolute value expressed in probabilistic climate projections).

In detail

The Met Office Hadley Centre have developed a methodology to provide probabilistic climate change projections to describe the changes in future climate (relative change) for the each 25 x 25 km grid square in the UKCP09 25 km grid and for certain pre-defined aggregated areas. 

A probabilistic climate change projection is a measure of strength of evidence in different future climate outcomes. This measure is dependent on the method used, is based on the current evidence available and encapsulates some, but not all, of the uncertainty associated with projecting future climate.

• Section 1.1.1 of the UKCP09 Climate change projections report explains what is meant by probability in UKCP09: further explanation is provided in Box 1.1. It is also recommended users read the UKCP09 Guiding Principles.

• An overview of how probabilistic projections are created and presented in UKCP09 is described in Box 3 of the UKCP09 Briefing report.

• More details are provided in Chapter 3 of the UKCP09 Climate change projections report, including an overview of the methodology in Section 3.2.12, and discussion about the interpretation of UKCP09 probabilistic projections in Section 3.3.

• Some worked examples were developed prior to the launch of UKCP09 that used dummy outputs to examine how the UKCP09 probabilistic climate change projections might be used. The worked examples should not be interpreted as a best practice method for using UKCP09, but as one way of using the outputs.

• A selection of inappropriate uses of the UKCP09 probabilistic projections is also provided.

• There are some variables for which probabilistic projections could not be produced for UKCP09. These are summarised in Chapter 5 of the UKCP09 Briefing report with further detail in Section 4.8 of the UKCP09 Climate change projections report.

• A discussion of sources of uncertainty in the UKCP09 probabilistic projections is available in Annex 2.4.

• The UKCP09 User Interface allows the UKCP09 probabilistic projections to be provided as either future absolute climate or as change from the 1961–1990 baseline climate.