A part of the Bayesian statistical framework that is used to provide a measure of how informative the climate model is about the real world. In the UKCP09 method, differences between different international climate models (that make up the multi-model ensemble (MME)) are taken as being representative of the difference between the Hadley Centre model and the real world. In other words, the MME members are used as a proxy for the real world or true climate. This discrepancy term is used to incorporate uncertainty arising from climate modelling structural error, and thus prevent the projections from being too heavily biased by structural errors present in the Hadley Centre models.
Different climate modelling groups develop their climate models in different but equally plausible ways, which results in different models producing different projections, even though they may be run under identical modelling conditions e.g. the same emissions scenario. These differences represent one part of the modelling uncertainty in UKCP09 which is termed the structural error component. In order to account for this source of uncertainty in the climate modelling, UKCP09 incorporates projections from twelve other international climate models (that make up the MME), and this is achieved by using a discrepancy term as part of the Bayesian statistical framework. This discrepancy term is a measure of how informative a model is about the real world. In UKCP09 each member of the MME is used as a proxy for the real world, and the difference between each and the nearest PPE member generated from the Hadley Centre model, is used to calculate the discrepancy term. The benefit of this approach is that, by incorporating projections from other climate models, it prevents the final projections from being too heavily biased by the structural error inherent in the Hadley Centre model.
- 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. Discrepancy is explained in more detail in Section 3.2.9.