Ensemble

A set of simulations (each one an ensemble member) made by either adjusting parameters within plausible limits in the model, or starting the model from different initial conditions. While many parameters are constrained by observations, some are subject to considerable uncertainty. The best way to investigate this uncertainty is to run an ensemble experiment in which each relevant parameter combination is investigated. This is known as a perturbed physics ensemble.

In detail

UKCIP02 was derived using an ensemble of three runs, which was generated by running the model with three different initial conditions, and averaging the output so that the user was presented with one projection.

UKCP09 generates a perturbed physics ensemble consisting of 280 model variants of the Hadley centre climate model. In total 31 parameter values were perturbed. In addition, UKCP09 also uses single projections from runs of other IPCC models, and this is the multi-model ensemble.

• Further explanation is provided in Box 1.1 of the UKCP09 Climate change projections report.

• 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 Chapter 3.2.12.

• A comparison of the UKCP09 methodology with an alternative method is described in Annex 2.3, and summarised in Box 4 of the UKCP09 Briefing report.