Structural error or uncertainty
A source of modelling uncertainty which reflects the way in which different climate models represent the physical processes involved. In UKCP09 this source of modelling uncertainty is addressed through the use of 12 different global climate models that were used in the IPCC Fourth Assessment Report.
In developing any global climate model, various choices about how the model is constructed will be made. This might include the spatial resolution of the climate grid, the numerical integration scheme used and the physical processes represented. In addition, some physical processes represented in the model (e.g. cloud cover) will occur at spatial scales too fine to be represented explicitly, such that statistical relationships between coarse and finer scale (sub-grid scale) processes will be developed to model these sub-grid scale processes, in a parameterisation scheme.
This almost inevitably means that different climate modelling centres around the world implement different but equally plausible solutions. This means that different climate models are structured differently, which leads to differences in their respective climate change projections. It is these differences which are referred to as structural error, and in the UKCP09 method, structural error provides the discrepancy term in a Bayesian statistical framework.
In UKCP09, this source of uncertainty has been addressed by incorporating single projections from 12 international climate models (the multi-model ensemble, MME), into the discrepancy term in the Bayesian statistical framework.
- An overview of the uncertainties associated with climate modelling is described in Section 3.1 of the UKCP09 Briefing report.
- More details about uncertainties are provided in Chapter 2 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 12 climate models used to evaluate structural errors in UKCP09 is provided in Table 3.1.
- 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.
- Structural error was evaluated in multiple, and in some cases less rigorous, ways in the preparation of marine and coastal projections for UKCP09. Section 1.2 of the UKCP09 Marine & coastal projections report summarises how uncertainty is considered and illustrated in the various marine and coastal projections.