Changes in the radiative balance of the Earth—including the enhanced greenhouse effect associated with rising atmospheric CO2 concentrations—are referred to as climate forcings (NRC, 2005d). Climate forcings are estimated by performing detailed calculations of how the presence of a forcing agent, such as excess CO2 from human activities, affects the transfer of radiation through the Earth's atmosphere.2 Climate forcings are typically expressed in Watts per square meter (W/m2, or energy per unit area), with positive forcings representing warming, and are typically reported as the change in forcing since the start of the Industrial Revolution (usually taken to be the year 1750). Figure 6.4 provides a graphical depiction of the estimated globally averaged strength of the most important forcing agents for recent climate change. Each of these forcing agents are discussed below.
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