When discussing the enhanced greenhouse effect and global warming, climate scientists often refer to a concept called "radiative forcing." This is a measure of the influence that an independent factor (ice albedo, aerosols, land use, carbon dioxide) has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system. It is also used as an index of the influence a factor has as a potential climate change mechanism. Radiative forcings can be positive or negative. A positive forcing (corresponding to more incoming energy) warms the climate system. A negative forcing (corresponding to more outgoing energy) cools the climate system.
As shown in the illustration, examples of positive forcings (those which warm the climate) include greenhouse gases, tropospheric ozone, water vapor, solar irradiance, and anthropogenic effects. Those that have a negative rating (they cool the climate) include stratospheric ozone and certain types of land use. Climatologists often use radiative forcing data to compare various cause-and-effect scenarios in climate systems where some type of change (warmer or cooler) has taken place. Radiative forcings and their effects on climate are also built into climate models in order to enable climatologists to determine the effects from multiple input factors on climate change. The greenhouse gases that remain in the atmosphere the longest will have the greatest impact on global warming, making it imperative that climatologists not only be able to identify them, but also understand them and how they react with the atmosphere.
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