Radiative forcing is a measure of the influence that a particular factor has in altering the earth’s energy balance and is specified by energy rate (power) per area (W/m2). Some factors cause atmospheric warming (greenhouse gases) while some factors cause cooling (aerosols, ice cover, and clouds). The amount of forcing from the most important factors is shown in Figure A.2. Carbon dioxide is the largest positive radiative forcing agent, contributing 1.66 W/ m2, while aerosols are the largest negative forcing agent, though they have a

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Radiative Forcing (Wm-2)

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Radiative Forcing (Wm-2)

Figure A.2 Radiative forcing (RF) in watts per square meter for various anthropogenic and natural climate factors between the years of 1750 and 2005. Factors that are positive contribute to warming the earth; factors that are negative cool the earth. The bars represent 90% confidence intervals. LOSU is a designation for the level of scientific understanding of a particular factor.

source: Reproduced by permission from Climate Change 2007:The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Figure TS.5. Cambridge: Cambridge University Press, 2007.

large uncertainty. Solar irradiance contributes only a small radiative forcing. Overall, the radiative forcing is 1.6 (0.6 to 2.4) W/m2 , which is why global warming is occurring. The large degree of uncertainty is caused by the uncer­tainty in the aerosol effects.