Climate ensemble

A climate ensemble involves slightly different models of the climate system. There are at least five different types, to be described below. For the equivalent in numerical weather prediction, see ensemble forecasting. A climate ensemble involves slightly different models of the climate system. There are at least five different types, to be described below. For the equivalent in numerical weather prediction, see ensemble forecasting. The aim of running an ensemble is usually in order to be able to deal with uncertainties in the system. An ultimate aim may be to produce policy relevant information such as a probability distribution function of different outcomes. This is proving to be very difficult due to a number of problems. These include: Multi-model ensembles (MMEs) are widely used in IPCC assessments, and a comprehensive collection of climate models can be accessed in the Coupled Model Intercomparison Project. Members of a multi-model ensemble are developed by different organisations involved in climate change research and can differ substantially in their software design and programming approach, their handling of spatial discretisation and exact formulation of physical, chemical and biological processes. The benefits of using a multi-model ensemble are seen in 'the consistently better performance of the multi-model when considering all aspects of the predictions' . Perturbed physics ensembles (PPEs) form the main scientific focus of the ClimatePrediction project. 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 are subject to considerable uncertainty. We do not know the extent to which different choices of parameter-settings or schemes may provide equally realistic simulations of 20th century climate but different forecast 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. A more general approach is coined 'perturbed parameter ensemble' (also abbreviated as PPE), as apart from physical parameters other parameters, relating to the carbon cycle, atmospheric chemistry, land use etc. can be perturbed.