Rather than relying on the outcome of a single climate model, we run ensembles of thousands of models. Each version of the model within an ensemble is very slightly different from the others but still plausibly represents the real world, and so produces slightly different outcomes. We can then look at outputs, such as the average, over the entire ensemble to give us our modelling results. This allows us to understand the uncertainty in the models better.
Getting volunteers to run members of an ensemble on their home computer has allowed climateprediction.net to create much larger climate model ensembles than has been possible before.
There are several ways of creating the small variations between the models within an ensemble, each designed to look at a different question about the model: