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http://www.globalwarmingart.com/wiki/File:IPCC_Radiative_Forcings_gif

A climate modeling study's interpretation of how the evolution of the various forcing factors have affected climate during the last 100 years.Changes in radiative forcings between 1750 and 2000 based on estimates by the Intergovernmental Panel on Climate Change (IPCC 2001a, [1]). The words across the bottom indicate the IPCC's assessment of how well each factor is understood.

Understanding global warming requires understanding the changes in climate forcings that have occurred since the industrial revolution. These include positive forcing from increased man-made greenhouse gases, negative forcing from man-made sulphate aerosols, poorly constrained forcings from indirect aerosol feedbacks, and a variety of other minor contributions from natural and artificial sources such as solar variability. The poorly constrained aerosol effects result both from limited physical understanding of how aerosols interact with the atmosphere and limited knowledge of aerosol concentrations during the pre-industrial period. This is a significant source of uncertainty in comparing modern climate forcings to past states.

Contrary to the impression given by this figure, it is not possible to simply sum the radiative forcing contributions from all sources and obtain a total forcing. This is because different forcing terms can interact to either amplify or interfere with each other. For example, in the case of greenhouse gases, two different gases may share the same absorption bands thus partially limiting their effectiveness when taken in combination.

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