Greenhouse Gases Online
Greenhouse gases are components of the atmosphere that contribute to the
greenhouse effect. Some greenhouse gases occur naturally in the atmosphere,
while others result from human activities. Naturally occurring greenhouse gases
include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain
human activities, however, add to the levels of most of these naturally
occurring gases.
The "Greenhouse effect"
Main article: Greenhouse effect
When sunlight reaches the surface of earth, some of it is absorbed and warms the
earth. Because the Earth's surface is much cooler than the sun, it radiates
energy at much longer wavelengths than the sun (see black body radiation and
Wien's displacement law). Some energy in these longer wavelengths is absorbed by
greenhouse gases in the atmosphere before it can be lost to space. The
absorption of this longwave radiant energy warms the atmosphere (the atmosphere
also is warmed by transfer of sensible and latent heat from the surface).
Greenhouse gases also emit longwave radiation both upward to space and downward
to the surface. The downward part of this longwave radiation emitted by the
atmosphere is the "greenhouse effect." The term is in fact a misnomer, as this
process is not the primary mechanism that warms greenhouses.
The major natural greenhouse gases are water vapor, which causes about 36-70% of
the greenhouse effect on Earth (not including clouds); carbon dioxide, which
causes 9-26%; methane, which causes 4-9%, and ozone, which causes 3-7%. It is
not possible to state that a certain gas causes a certain percentage of the
greenhouse effect, because the influences of the various gases are not additive.
(The higher ends of the ranges quoted are for the gas alone; the lower ends, for
the gas counting overlaps.) Other greenhouse gases include, but are not limited
to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and
chlorofluorocarbons (see IPCC list of greenhouse gases).
The major atmospheric constituents (nitrogen, N2 and oxygen, O2) are not
greenhouse gases. This is because homonuclear diatomic molecules such as N2 and
O2 neither absorb nor emit infrared radiation, as there is no net change in the
dipole moment of these molecules.
