Radiative Forcing

The presence of greenhouse gases naturally affects the climatic equilibrium of the Earth. The ability of these gases to absorb IR radiation coming from the Earth (which would otherwise be lost to space) is known as radiative forcing (of climate change). The atmospheric concentration of greenhouse gases is a primary parameter of greenhouse effect and it changes continuously. As it increases, the outgoing long-wave IR radiation decreases. The concept of radiative forcing is used to describe this reduction of IR radiation due to release of a unit of a greenhouse gas into the atmosphere and by how much this change can alter the balance of energy transfers between the atmosphere, space, land, and oceans. In general, all other things being equal, if greenhouse gas concentrations increase, radiative forcing (energy per unit area - watt per square meter) is positive. Radiative forcing depends on several factors that affect the climatic effects of increases in atmospheric concentrations:

1. The present atmospheric concentration. Changes in concentration imply effects on radiative forcing but if a gas which is already very concentrated is released, this does not greatly increase radiative absorption.

2. The wavelengths at which the gas molecules absorb. An increase in CH4 concentration has a large effect on IR absorption because it absorbs in a region of the spectrum not absorbed by CO2 (which is much more abundant).

3. The absorption per molecule. A small increase in concentration of a strongly absorbing gas can have a very large effect.

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