The answer is the forcing mechanism. It is the process that would alter the balance of the climate system that includes solar irradiance,the volcanic eruptions as well as the enhancement of the greenhouse effect through emission of the carbon dioxide.
The Forcing Mechanism is a process which changes the balance of Energy in the Ecosystem. The alteration occurs when there is no relative balance between the arriving radiation from the sun and the leaving infrared radiation from the Earth. Factor that can result to this situation are events of volcanic eruption, increase of natural greenhouse effects and when solar irradiance is altered.
