INTRODUCTION
The greenhouse effect refers to circumstances where the short wavelengths
of visible light from the sun pass through a transparent medium and are
absorbed, but the longer wavelengths of the infrared re-radiation from the
heated objects are unable to pass through that medium. The trapping of the long
wavelength radiation leads to more heating and a higher resultant temperature.
Besides the heating of an automobile by sunlight through the windshield and the
namesake example of heating the greenhouse by sunlight passing through sealed,
transparent windows, the greenhouse effect has been widely used to describe the
trapping of excess heat by the rising concentration of carbon dioxide in the
atmosphere. The carbon dioxide strongly absorbs infrared and does not allow as
much of it to escape into space.
GREEN HOUSE EFFECT
The greenhouse effect is the process by which radiation from a planet's
atmosphere warms the planet's surface to a temperature above what it would be
in the absence of its atmosphere. If a planet's atmosphere contains radiatively
active gases (i.e., greenhouse gases) the atmosphere radiates energy in all
directions. Part of this radiation is directed towards the surface, warming it
.On Earth, solar radiation at the frequencies of visible light largely passes
through the atmosphere to warm the planetary surface. The surface itself emits
energy at the lower frequencies of infrared thermal radiation. Infrared
radiation is absorbed by greenhouse gases in the atmosphere. These gases also
radiate energy, some of which is directed to the surface and lower atmosphere.
The mechanism is named after the effect of solar radiation passing through
glass and warming a greenhouse, but the way it retains heat is fundamentally
different as a greenhouse works by reducing airflow, isolating the warm air
inside the structure so that heat is not lost by convection.
HISTORY
The existence of the greenhouse effect was argued for by Joseph Fourier in
1824. The argument and the evidence was further strengthened by Claude Pouillet
in 1827 and 1838, and reasoned from experimental observations by John Tyndall
in 1859. The effect was more fully quantified by Svante Arrhenius in 1896.
However, the term "greenhouse" wasn't used to describe the effect by
any of these scientists; the term was first used in this way by Nils Gustaf
Ekholm in 1901.In 1917 Alexander Graham Bell wrote "[The unchecked burning
of fossil fuels] would have a sort of greenhouse effect", and "The
net result is the greenhouse becomes a sort of hot-house." Bell went on to
also advocate the use of alternate energy sources, such as solar energy.
MECHANISM
The Earth receives energy from the Sun in the form UV, visible, and near IR
radiation, most of which passes through the atmosphere without being absorbed
or reflected. Of the total amount of energy available at the top of the
atmosphere (TOA), about 26% is reflected back out to space by the atmosphere
and clouds and 19% is absorbed by the atmosphere and clouds. Most of the
remaining energy is absorbed at the Earth's surface. Because it is warm, the
surface radiates far IR thermal radiation that consists of wavelengths that are
much longer than the wavelengths that were absorbed (the overlap between the
incident solar spectrum and the terrestrial thermal spectrum is small enough to
be neglected for most purposes). Most of this thermal radiation is absorbed by
the atmosphere, thereby warming it (in addition to sensible and latent heat
fluxes from the surface). The atmosphere radiates energy both upwards and
downwards; the part radiated downwards is absorbed by the Earth's surface. This
leads to a higher equilibrium temperature than if the atmosphere were absent.
GREEN HOUSE GASES
By their percentage contribution to the greenhouse effect on Earth the four
major gases are
water vapor, 36–70%
carbon dioxide, 9–26%
methane, 4–9%
ozone, 3–7%
It is not physically realistic to
assign a specific percentage to each gas because the absorption and emission
bands of the gases overlap (hence the ranges given above). The major non-gas
contributor to the Earth's greenhouse effect, clouds, also absorb and emit
infrared radiation and thus have an effect on radiative properties of the
atmosphere.
REAL GREEN HOUSES
The "greenhouse effect" of the atmosphere is named by analogy to
greenhouses which get warmer in sunlight. The explanation given in most sources
for the warmer temperature in an actual greenhouse is that incident solar
radiation in the visible, long-wavelength ultraviolet, and short-wavelength
infrared range of the spectrum passes through the glass roof and walls and is
absorbed by the floor, earth, and contents, which become warmer and re-emit the
energy as longer-wavelength infrared radiation. Glass and other materials used
for greenhouse walls do not transmit infrared radiation, so the infrared cannot
escape via radiative transfer. As the structure is not open to the atmosphere,
heat also cannot escape via convection, so the temperature inside the
greenhouse rises.The greenhouse effect, due to infrared-opaque "greenhouse
gases" including carbon dioxide and methane instead of glass, also affects
the earth as a whole; there is no convective cooling as air does not escape
from the earth.
CONCLUSION
The greenhouse effect is a natural process that warms the Earth’s surface.
When the Sun’s energy reaches the Earth’s atmosphere, some of it is reflected
back to space and the rest is absorbed and re-radiated by greenhouse gases
.Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide,
ozone and some artificial chemicals such as chlorofluorocarbons(CFCs).The
absorbed energy warms the atmosphere and the surface of the Earth. This process
maintains the Earth’s temperature at around 33 degrees Celsius warmer than it
would otherwise be, allowing life on Earth to exist.
REFERENCE
