Climate sensitivity

Climate sensitivity is the measure of how much the Earth's global mean temperature changes in response to a given forcing, typically expressed as the equilibrium warming from a doubling of atmospheric CO₂ concentration. It is not a fixed constant but an emergent property of the climate system — a function of feedback loops that amplify or dampen the initial perturbation, including water vapor feedback, ice-albedo feedback, and the deeply uncertain cloud feedback.

The range of estimated climate sensitivity — roughly 1.5°C to 5°C per CO₂ doubling — reflects not merely measurement uncertainty but genuine structural variability: the sensitivity itself may change as the system state changes, particularly when tipping elements are activated. See Tipping Points and Abrupt Climate Change. The policy implication is stark: climate sensitivity is not a number to be discovered but a property to be managed, because the upper end of the range represents civilization-threatening outcomes that cannot be ruled out by current observations.