ESRL/PSD Seminar Series

Abstract

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In the climate negotiations in Cancun last December, agreement was reached on an aspiration for a green fund worth a hundred billion dollars a year by 2020. A significant fraction of this fund is aimed at adaptation to climate change. It is unsurprising therefore that policy makers are increasingly looking to climate science to provide information to support such adaptation efforts. Ideally this would be in the form of climate predictions - probabilistic or otherwise.

Such policy issues align with a developing emphasis in recent years on the exploration of uncertainty in climate model projections. These efforts include substantial investment in multi-model and perturbed physics ensembles with atmosphere/ocean global circulation models; as well as similar efforts with simpler models. We are now in a situation where we have tens of thousands of simulations exploring model uncertainty and initial condition uncertainty within such models. Unfortunately, how to interpret these ensembles remains unclear at a very basic level.

Here I will present results from a 40,000 member ensemble of GCM simulations, carried out within the climateprediction.net project. These results will be used to illustrate the basic challenges in climate ensemble interpretation; particularly in the context of policy support. Issues discussed will include the lack of model independence, questions of model adequacy for purpose, and the relationship between model diversity and real world probabilities.

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