Search site

Perturbed physics ensemble (PPE)

A perturbed physics ensemble (PPE) is an ensemble of model runs from different model variants, produced by varying the values of parameters in a given climate model configuration.

In detail

A perturbed physics ensemble (PPE) is generated by varying the values of parameters in a given climate model, thus creating different variants of that model, and making simulations using each variant. The production of PPEs in UKCP09 takes into account parameter errors, which arise as a result of incomplete or imprecise knowledge of the actual values of the climate model parameters that are used to represent processes within the climate model. By perturbing (varying) climate model physical parameters within plausible ranges to create different variants, and making a projection of climate change for each variant, it is possible to quantify the impact of this source of uncertainty in resultant climate projections.

Ideally, the full range of variability of each model parameter would be explored simultaneously, and a unique climate projection generated. To do so is currently computationally prohibitive. In UKCP09 therefore, the atmosphere PPE is generated by running 280 variants of the Met Office Hadley Centre climate model HadSM3 (the slab-ocean model version of HadCM3). The resulting PPE is used to train a statistical emulator with allows a larger range of variability in the model parameters to be explored and quantified.

  • An overview of the uncertainties associated with climate modelling is described in of the UKCP09 Briefing report.

  • A comparison of the UKCP09 methodology with an alternative method is described in , and summarised in of the UKCP09 Briefing report.

  • More details about uncertainties are provided in of the UKCP09 Climate change projections report. A discussion of sources of uncertainty in the UKCP09 probabilistic projections is available in .

  • An overview of how probabilistic projections are created and presented in UKCP09 is described in 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 , and discussion about the interpretation of UKCP09 probabilistic projections in .

  • Parameter error was evaluated in multiple, and in some cases less rigorous, ways in the preparation of marine and coastal projections for UKCP09. of the UKCP09 Marine & coastal projections report summarises how uncertainty is considered and illustrated in the various marine and coastal projections.