Feedbacks and Climate Sensitivity

The climate is changing. The main reason is because of human-induced changes in atmospheric composition which produce warming from increased greenhouse gases. This is referred to as a forcing of the climate system. There are many other forcings, both natural and anthropogenic. The issue then is to determine the consequences in terms of the change in climate and its impacts. There is a direct response to just about any forcing, and in some cases that is the answer we seek. But in many or most cases, it is not so simple. Rather, the initial change provokes other responses, especially in the atmospheric and ocean circulation, that in turn cause other changes to occur. If the response amplifies the original change, then it is referred to as a positive feedback. Whereas if the response offsets and reduces the outcome, then it is a negative feedback. The size of some effects is quantified in Section 13.5.