Forcing, Feedback and Climate Sensitivity
A change to the energy balance is a forcing, and the system's response depends on feedbacks that amplify or damp it. Sensitivity is the number that summarises all of them, and it is uncertain mainly because clouds can feed back either way.
What a learner can do afterwards
- Distinguish a forcing from a feedback using a worked example of each
- Explain the water vapour and ice-albedo feedbacks and say which direction each pushes
- State what climate sensitivity means and why its uncertainty range is dominated by clouds
1 · Read
Energy arrives from the Sun and leaves back to space, and anything shifting that balance starts a chain of replies. A forcing is the push from outside the system, like added greenhouse gases or volcanic particles blocking sunlight. A feedback is the system's own response to that push. Mixing the two up is the most common error, so always ask first: did this start the change, or answer it.
Two feedbacks amplify warming. Warmer air holds more water vapour, and vapour traps heat, so warming breeds more warming. Melting ice works the same way: bright ice reflects sunlight, but the darker surface left behind absorbs more of it, so warming breeds more warming again. Both push in the same direction as the original change.
A big eruption is the cleanest forcing you can study. Particles thrown high block sunlight and cool the planet for a while, then fade, and the system replies and recovers. A retreating glacier shows the harder case: human-added gases supply the forcing while reflectivity changes feed back, and reading the ice margin means separating the two.
Climate sensitivity is the number summarizing all feedbacks: how much warming a given push causes once replies play out. Models test it by switching feedbacks on and off. Clouds set the wide range, since low clouds cool by reflecting sunlight while high clouds warm by trapping heat.
Forcings start the change, feedbacks reply, and sensitivity totals the outcome.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.