The increase in temperature following a doubling of CO2 in the atmosphere.
More generally, climate sensitivity refers to the change in surface air temperature that arises some time after a significant change in radiative forcing.
The evaluation of climate sensitivity from models requires very long simulations with global climate models, or it may be deduced from observations.
Climate sensitivity measures the response of global average temperature to increased concentrations of greenhouse gases. The IPCC has estimated that a doubling of CO2 from the level of 280 ppm (parts per million) from before industrialization to 560 ppm at around year 2100 will raise the temperature about 3°C.
Most of the expected change is not due to the direct radiative forcing of CO2. At least half of the rise in temperature is due to positive feedbacks. Indeed, without any feedbacks, a doubling of CO2 would result only in a 1°C global temperature rise.
The uncertainty in climate sensitivity estimates is due entirely to feedbacks in the system, namely, the water vapor feedback (see last paragraph), the ice-albedo feedback and the cloud feedback.
Climate sensitivity can also be estimated for doublings in other greenhouse gases. Then the carbon dioxide equivalent for that particular gas is used.