Perturbed Physics Ensembles

Perturbed physics ensembles form a major scientific focus of the whole project, especially using the coupled model. Modern climate models do a good job of simulating many large-scale features of present-day climate. However, these models contain large numbers of adjustable parameters which are known, individually, to have a significant impact on simulated climate. While many of these are well constrained by observations, there are many which do not directly relate to observed quantities and are subject to considerable uncertainty as they are obtained in a trial-and-error way, trying to best match observations.

We do not know the extent to which different choices of parameter-settings may provide equally realistic simulations of 20th century climate but different forecasts for the 21st century. The most thorough way to investigate this uncertainty is to run a massive ensemble experiment in which each relevant parameter combination is investigated. Thus the perturbed physics ensemble is a central feature of the project.

The knowledge we gain from this experiment about model sensitivity will enable the scientific community to design better models in the future. By perturbing parameters which control the model’s physical processes (such as cloud formation) it is also possible to see different realisations of climate change.

As in the initial condition experiment everybody’s model will be unique because each will have a different combination of parameters.