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11-member RCM

All modelling methods incorporate assumptions. Although assumptions are part of UKCP09, it presents a range of projections that show some of those previously hidden, and addresses others, e.g. sampling uncertainty in the carbon cycle, aerosol chemistry and ocean dynamics.

  • That sources of uncertainty represented by the other IPCC models and internal structural processes will not contribute much extra uncertainty

Unlike the UKCP09 probabilistic projections, the 11-member RCM only represents projections from the Hadley Centre model and does not take into account any other IPCC model projections. This is a primary limitation of the 11-member RCM information in comparison to the UKCP09 probabilistic projections, and should be kept in mind when using the 11-member RCM information.

There are also structural processes and uncertainties that are not taken into account in the HadRM3 model, which are taken into account in the UKCP09 probabilistic projections. The RCM does not include a full ocean or a carbon cycle.

  • That each of the 11 runs of the RCM are equally plausible

Unlike the UKCP09 probabilistic projections, there has been no weighting placed on the model projections in order to provide a probabilistic distribution.

Because of this assumption, it is recommended that RCM data be checked as to where they lie within the UKCP09 probabilistic distribution. Such an assessment can provide an evaluation of the proportion of the full probabilistic range that is explored by the RCM data.

  • That the medium emissions scenario is adequate for decision making

As the 11-member RCM information was created for the specific purpose of  downscaling the UKCP09 probabilistic projections and not for the purpose of decision-making, it only contains information for one emissions scenario. The use of only one emissions scenario may not result in a robust decision-making process, and users should be aware of this before using the 11-member RCM output.