An ensemble is a group of climate model simulations used to understand a problem by representing many different possible future scenarios.

  • A single climate model will provide an outcome depending on how it is configured using assumptions and simplifications of some climate processes (parameterisations), initial conditions, or the inclusion/exclusion of specific processes.

Due to uncertainties in the parametisations and internal climate variability, single model runs can produce different outcomes which can help understand elements of our climate. Using a single model can show how changed initial conditions affect an outcome.

  • Many climate models are run to form an “ensemble of opportunity”. An ensemble allows comparison between different models and how the outcomes are affected.

The most well-known ensemble of opportunity is the Coupled Model Intercomparison Project (CMIP) directed by the World Climate Research Program. CMIP involves different modelling groups from all around the world running climate models using the same set of inputs and scenarios. Internationally, there are climate modelling groups in many other countries. They create models with different ways of representing physical processes. CMIP, currently in Phase 6, provides a source of data for the Intergovernmental Panel on Climate Change (IPCC) assessment reports.

It is important to understand that ensembles of opportunity like CMIP do not have constraints on the modelling groups that take part. CMIP is designed to aid scientific understanding not to provide the best set of data for decision makers to adapt to climate change. Users of CMIP data need to thoroughly understand the data being used, which models these data come from and whether these models are fit-for-purpose.