abrupt-4xCO2 is an idealised stress-test. Starting from the pre-industrial control climate, the carbon dioxide concentration is instantly quadrupled and then held there while the model runs on.

What it does

Nothing about this is meant to be realistic — no real scenario quadruples CO2 overnight. That is the point. By applying one large, clean, well-defined push, we can watch the climate system respond without the clutter of volcanoes, aerosols and gradually changing emissions getting in the way.

Why we care

This experiment is how we measure some of the most important numbers in climate science:

• Climate sensitivity — how much the planet ultimately warms for a given increase in CO2.
• Feedbacks — how clouds, water vapour, ice and the like amplify or damp that warming.
• How quickly the ocean takes up heat, which sets the pace of warming.

Because every model runs exactly the same idealised experiment, abrupt-4xCO2 gives us a clean way to compare one model against another.