Factors that affect climate change are usefully separated into forcings and feedbacks. The conceptual diagram of Figure 1-2 illustrates the connections between climate forcings, responses, and feedbacks as defined in this report. A climate forcing is an energy imbalance imposed on the climate system either externally or by human activities. Examples include changes in solar energy output, volcanic emissions, deliberate land modification, or anthropogenic emissions of greenhouse gases, aerosols, and their precursors. A climate feedback is an internal climate process that amplifies or dampens the climate response to an initial forcing. An example is the increase in atmospheric water vapor that is triggered by an initial warming due to rising carbon dioxide (CO2) concentrations, which then acts to amplify the warming through the greenhouse properties of water vapor. Climate change feedbacks are the subject of a recent report of the National Research Council (NRC, 2003).
Climate forcings can be classified as radiative (direct or indirect) or nonradiative. Direct radiative forcings affect the radiative budget of the Earth directly; for example, added CO2 absorbs and emits infrared (IR) radiation. Indirect radiative forcings create a radiative imbalance by first