What does 'climate sensitivity' mean?

Answer 1

Climate sensitivity determines how sensitive our planet is to radiative forcing, that is, to heat added to or removed from the atmosphere. It is usually expressed as the amount of global warming for doubled carbon dioxide (ie, from 280 ppm to 560 ppm). Many studies have shown this to be 3.7 Watts per square metre (W/m2) (IPCC AR4 Section 2.3.1). If there were no positive or negative feedbacks the global rise in temperature if carbon dioxide (CO2) doubled would be about 1° Celsius.

However, our climate has feedbacks, both positive and negative. The strongest positive feedback is water vapour. As the atmosphere warms, it holds more water vapour, a greenhouse gas, which increases the warming. This increased warming means more water vapour, and so on. Water vapour also has negative feedbacks - more clouds can have a cooling effect (reflecting radiation) and a warming effect (preventing heat loss to space). (Other feedbacks include the ice-albedo feedback and the lapse rate feedback.)

So to determine the net feedback, studies have looked at climate models, climate data from instruments and satellites, volcanic eruptions and palæoclimate models from the past. The studies all yield a broadly consistent range of climate sensitivity with a most likely value of 3°C for a doubling of CO2.