Forcings are the things which drive the climate system. We call them forcings because they force the system from the outside: if these things change, we expect the climate system to respond.

For instance, if the sun puts out more energy, we would expect the Earth to heat up. Examples of forcings include solar variability, sulphates from volcanic activity and greenhouse gases. These are all treated as external to the climate system whereas the chaotic variability we target in the initial condition ensemble is due to factors internal to the climate system.

We normally distinguish between natural forcings, such as volcanoes and solar activity, and human-induced forcings, such as greenhouse gases, aerosols or land use change.

The first experiment we normally run in a new project is one with present day forcings to investigate whether or not the model responds to the measured forcings in a similar way to the real climate system.

In most projects we want to analyse the sensitivity of the model to different forcing conditions. In the geoengineering projects, for example, we change the amount of sulphate aerosols in the aerosol forcing to a higher concentration, aiming to test how the climate system would react to such a measure.

In the projects that aim to identify the influence of anthropogenic forcing, mainly greenhouse gas forcing, on present day climate, we change the greenhouse gas forcing files to simulate a world that might have been without anthropogenic climate change. Most weather@home projects comprise this experiment.

All projects aiming at future projections need to include forcings for the future, which we obviously cannot measure but have to assume. Therefore we need a future forcings ensemble with variations of different forcings because we don’t know what the sun or the volcanoes are going to do over the next fifty years. We also don’t know how levels of greenhouse gases are going to change over that period. So we’re going to run a large number of different futures which seem to be plausible, in which we vary solar, sulphate and greenhouse forcing, to span what we hope will be the likely range.

In a lot of future experiments, however, we are interested in isolating the effect of a certain forcing, such as a doubling of the amount of carbon dioxide in the atmosphere. In such an experiment we keep solar and volcanic forcings constant, i.e. we assume there will be no major volcanic eruption in the future period of interest, and only change the greenhouse gas forcing. Applying this strategy it is possible to identify the influence of a single type of forcing if comparing the future simulations with present day simulations.