The amount of CO2 that is released to (or removed from) the atmosphere by temperature change.
1. With gamma, the climates system's temperature is the forcing and CO2 is the response. This makes it the inverse of climate sensitivity, where CO2 is the forcing and temperature is the response.
2. Click here to read a Real Climate article about a study that tries to predict what the earth's gamma will be in response to global warming over the next century.