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A Comparison of Systematic Uncertainties in Perturbed Physics and IPCC AR4 Multi-Model Ensembles

A Comparison of Systematic Uncertainties in "Perturbed Physics" and IPCC AR4 Multi-Model Ensembles

Mat Collins, Ben Booth, Glen Harris, James Murphy, David Sexton and Mark
Webb

Hadley Centre for Climate Change, Met Office, UK.

While the AR4 archive provides a large gene-pool of possible models of long-term climate change, the perturbed physics approach (in which perturbations are made to key uncertain parameters in a single modelling framework allows us to build a more controllable system for quantifying uncertainties and expressing climate change in terms of the probability of different outcomes (see Sexton et al. abstract). Yet there is a potential for the perturbed physics approach to be "structurally constrained" thus limiting the range of responses that may be explored.

Here we compare and contrast sources of uncertainty in perturbed physics and multi-model ensembles. Focus will be on:

1. Global mean change, in which perturbations are made to parameters in the atmosphere component, the ocean component and the sulphur-cycle component in turn. By isolating the effects of uncertainty in feedbacks associated with each component, it is possible to contrast the relative impacts of uncertainties in different feedbacks. Comparison with the AR4 archive will be performed to reveal the overlap in global mean response.
2. Large-scale regional patterns, which will be compared between the different ensembles to assess the extent to which (i) the perturbed physics approach can mimic the behaviour of the AR4 ensemble and (ii) the sign and magnitude of the response can be linked to systematic uncertainties in the simulation of present-day climate and climate change.

In addition, plans for future work to produce a perturbed physics ensemble with HadCM3 coupled to a carbon cycle model will be outlined together with a plan to produce an ensemble in which parameters are perturbed in all components simultaneously. Comparisons with similar perturbed physics ensembles run to predict seasonal to decadal variations in climate will be explored by Murphy et al. in their abstract.