In Module 3, we learned about equilibrium climates in which the rate of energy input from the Sun is balanced by the rate of energy emitted back to space by the Earth. Now we consider what happens when these flows of energy become unbalanced.
Radiative forcing (measured in Watts per square meter) of climate is defined as the instantaneous net difference between the rate of energy inflow and the rate of energy outflow before the temperature has time to adjust to the imbalance. The response to radiative forcing is the resulting change in global mean surface temperature (in Kelvin) that is required to restore radiation balance. The strength of this response is called Climate Sensitivity: it’s the ratio of climate response to radiative forcing measured in Kelvin per (Watt per square meter).
Climate Sensitivity is determined by feedback processes in the Earth’s climate system. Positive feedbacks increase the response to forcing and make the climate more sensitive. Negative feedbacks reduce the response to forcing and make the climate less sensitive. Without any feedback the Earth’s climate would warm about 0.27 Kelvin for each Watt/square meter of radiative forcing. Including feedbacks associated with water vapor, albedo, clouds, and vertical mixing the Earth’s overall climate sensitivity is about 0.8 Kelvin for each Watt/square meter. The radiative forcing of atmospheric CO2 is about 3.7 Watts per square meter per doubling of CO2, so the climate sensitivity can be written as 3 Celsius per doubling of CO2.