Defra logo
  • Other defra.gov.uk sites
 
UK Climate Projections logo    
   

2.1 Comparing UKCIP02 values with UKCP09 distributions

The UKCP09 climate projections represent a significant advance in scientific understanding and modelling of climate systems, as well as the complexity of analysis of modelled outputs. There have been fundamental changes to the underlying modelling methodologies, and results generated, making it difficult to directly compare UKCP09 with earlier UKCIP02 projections.

Major changes adopted in UKCP09 projections include:

  • the explicit effects of land and ocean carbon cycle feedbacks, and the uncertainty in land carbon cycle feedback;

  • uncertainty due to natural variability;

  • modelling uncertainty: UKCIP02 was derived using one variant of one (Met Office) model, whereas UKCP09 is derived from ensembles of variants of Met Office models, together with an ensemble of other international models;

  • uncertainties associated with the statistical processing required to convert results from model ensembles into probabilistic projections;

  • observations were used to weight the projections towards climate model variants that were relatively good at simulating several large scale features of the mean climate;

  • a change spatial averaging and coordinate system (i.e. from 50 km to 25 km grid squares);

  • UKCIP02 used pattern scaling of an A2 scenario in 2080s to infer changes from B2 and A1FI in 2020s, 2050s and 2080s with no separate treatment for response to aerosol and greenhouse gas forcing;

  • a change in some of the emissions scenarios used.

Consequently, there will be a number of contributing reasons that explain how, for a specific variable/location, the UKCIP02 value compares to the UKCP09 distribution. The identification of these would be a major undertaking, and of questionable value, as UKCIP02 should not be seen as a benchmark against which subsequent work must be compared. If existing assessments/decisions are based on UKCIP02, as a first step in verifying their continued validity, it is useful to compare the two sets of projections relative to changes in climate of interest.

     
     
Figure 1: A comparison of UKCP09 cumulative distribution functions (CDF) (red line) and UKCIP02 scenario values (blue circle) for change in summer mean temperatures in the 2080s under a High Emissions scenario (A1FI). Two locations are shown: (a) Dorset and (b) Falkirk.
    
     
02-09_Fig1.jpg