The historical experiment is the model's attempt to reproduce the climate we have actually observed, from the mid-19th century up to the near-present. It is one of the cornerstone simulations every model contributes to CMIP.

What it does

Starting from a stable pre-industrial state, the model is driven forward using our best reconstruction of the real-world forcings over that period:

• rising greenhouse gas concentrations,
• volcanic eruptions and changes in solar output,
• aerosols and other pollutants,
• land-use change.

Everything else — the winds, ocean currents, the timing of El Niño — the model works out for itself.

Why we care

The historical run is how we check the model against reality. If it can reproduce the observed warming, the pattern of temperature change, and events like the cooling after major volcanic eruptions, we gain confidence that its projections of the future are worth taking seriously. It is also the baseline that future scenario runs branch off from.