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The atmospheric gases most
responsible for this effect are water vapor (H2O), carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone
(O3), collectively known as greenhouse gases. These gases,
therefore act as effective global insulators. The degree of the
greenhouse effect is dependent primarily on the concentration of
greenhouse gases in the planetary atmosphere. Greenhouse gases
comprise less than 1% of the atmosphere. Their levels are
determined by a balance between sources and sinks. Sources and
sinks are processes that generate and destroy greenhouse gases
respectively. It so happens that heteronuclear diatomic
molecules and tri- (and more) atomic gases absorb at infrared
wavelengths but homonuclear diatomic molecules do not absorb
infrared light. This is why H2O and CO2 are greenhouse gases but
the major atmospheric constituents (N2 and O2) are not. For
example, while the planets Venus, Earth, and Mars have similar
amounts of incident solar radiation, the dense, carbon
dioxide-rich atmosphere of Venus causes a runaway greenhouse
effect with surface temperatures hot enough to melt lead, the
atmosphere of Earth causes a greenhouse effect of habitable
temperatures, and the thin atmosphere of Mars causes a minimal
greenhouse effect.
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Greenhouse gases leads to the
heating up of the earth by selectively transmitting infrared
waves trapping some and allowing some to pass through into
space. The greenhouse gases absorb these waves and reemits the
waves downward, causing the lower atmosphere to warm. There is a
natural balance between the energy coming into our atmosphere
from the sun and the energy escaping from the earth back into
space. The energy that comes in as relatively short wavelength
UV radiation and the energy that goes out as longer wavelength
IR radiation. Some of the ultraviolet light that comes from the
sun passes through the earth’s atmosphere and warms the earth.
As the earth cools, it emits infrared radiation. As the earth
cools down again, it releases energy as infrared radiation. The
greenhouse gases absorb some of the IR energy released as the
earth cools. When they re-emit it, some of the energy is sent
back toward the earth, rather than out into space. Thus, the
greenhouse gases trap some of the energy that would have
otherwise escaped. This trapped energy leads to temperatures on
the earth that are higher than they would be without the
greenhouse gases.
Features of greenhouse gases are
• Water vapor It is a major
contributor to greenhouse effect, and accounts for about 60% of
the total greenhouse effect. It is the most abundant greenhouse
gas. Its effects vary due to localized concentrations, mixture
with other gases, frequencies of light.
• Carbon dioxide(Co2) Carbon
Dioxide (CO2) is a colorless, odorless non-flammable gas and is
the most prominent Greenhouse gas in Earth's atmosphere. When
its atoms are bonded tightly together, the carbon dioxide
molecule can absorb infrared radiation and the molecule starts
to vibrate. Eventually, the vibrating molecule will emit the
radiation again, and it will likely be absorbed by yet another
greenhouse gas molecule. This absorption-emission-absorption
cycle serves to keep the heat near the surface, effectively
insulating the surface from the cold of space. It is recycled
through the atmosphere by the process photosynthesis, the
process of green plants and other organisms, by which light
energy is trapped and used to convert carbon dioxide, water, and
other minerals into oxygen and energy rich organic compounds.
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In addition to carbon dioxide and
water vapor (H2O), methane ( ), nitrous oxide ( ), are
greenhouse gases. They all are molecules composed of more than
two component atoms, bound loosely enough together to be able to
vibrate with the absorption of heat.
• Methane(CH4) Methane is a
colorless, odorless, flammable gas, and is an extremely
effective absorber of radiation. It is formed when plants decay
and where there is very little air. It stays in the atmosphere
for only 10 years, but traps 20 times more heat than carbon
dioxide.
• Ozone(O3) It is now
considered to be one of the most important greenhouse gas along
with carbon dioxide and methane. Ultraviolet radiation and
oxygen interact to form ozone in the stratosphere.
• Nitrous Oxide(N2O) Nitrous
oxide is another colorless greenhouse gas, however, it has a
sweet odor. This gas is released naturally from oceans and by
bacteria in soils.
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Between the absorptions of water
vapor and those of carbon dioxide, there is an atmospheric
window. Compounds such as perflurocarbons (CF4, C2F6 etc.),
chlorofluorocarbons, halons and SF6 absorb very strongly in this
window. This means that they are extremely potent greenhouse
gases, especially given the absence of natural sinks to remove
them. The CFCs are gaseous compounds that contains molecules
with carbon atoms bonded exclusively to fluorine or chlorine
have perhaps the greatest potential among trace gases to induce
global warming, since they are both very persistent and absorb
strongly in the 8-13 µm window region. Due to this persistence,
and to their high efficiency in absorbing thermal IRE in the
window region, each CFCs molecule has the potential to cause the
same amount of global warming as do tens & thousands of CO2
molecules. They are released into Earth's atmosphere by,
cleaning solvents used in manufacturing electronics, aerosol
sprays, which can use CFCs as a propellant, refrigerators and
air conditioners, which use CFCs as coolant.
Consequences of greenhouse effect
Natural greenhouse effect prevents incoming heat from the Sun
from escaping straight back into space and keeps the global mean
surface temperature at around 15 degrees Celsius, due to the
presence of natural greenhouse gases. Without the greenhouse
effect, the Earth would be too cold for life to survive.
Concentrations of greenhouse gases are building up in the
atmosphere and this is preventing more heat from escaping,
called the “enhanced greenhouse effect”, leading to an increase
in the Earths temperature, causing what is known as “global
warming”. This is affecting ecosystems either directly, by
melting ice in the arctic regions, which leads to loss of
habitat for wildlife; or indirectly by changing weather
patterns, which can disrupt ecosystems which are used to certain
climatic conditions. Therefore global warming and overall
climatic change are adjustments made to deal with the extra
accumulation of heat due to enhanced greenhouse effect
The prevailing scientific opinion on the global change is that
most of the warming observed over the last 50 years is
attributable to human activities. The increased amounts of
carbon dioxide (CO2) and other greenhouse gases are the primary
human-induced. Carbon Dioxide is emitted into the air as humans
exhale, burn fossil fuels for energy, and deforest the planet.
Fossil Fuels were created chiefly by the decay of plants from
millions of years ago. These fossil fuels contain carbon, and
when they are burned, they combine with oxygen, forming carbon
dioxide. Deforestation is another main producer of carbon
dioxide. The causes of deforestation are logging for lumber,
pulpwood, and fuel wood. Also contributing to deforestation are
clearing new land for farming and pastures used for animals. As
we burn them down, carbon is released into the air and the
carbon bonds with oxygen to form carbon dioxide, adding to the
greenhouse effect. Methane may be released into the atmosphere
when it leaks from natural gas wells, pipelines, and coal mines,
or during burning of garbage, wood. Nitrous oxide (N20) is
released into Earth's atmosphere when nitrogen fertilizers are
used in agriculture, or soils are disturbed by erosion, plowing,
and other activities. All these human induced activities leads
to an overall increase in temperature. An increase in global
temperatures can in turn cause a number of changes, example,
rising sea levels, altered patterns of agriculture, increased
extreme weather events, and the expansion of the range of
tropical diseases.
• Sea level rise Global
warming could raise sea level by expanding ocean water, melting
mountain glaciers, and eventually causing polar ice sheets to
slide into the oceans. Although the timing and magnitude of
future sea level rise is uncertain, there is an emerging
scientific consensus that a significant rise is likely. In fact,
Scientists believe that, globally, sea level has risen 4-8
inches over the past century.
• Altered climate Global
warming refers to an average increase in the Earth's
temperature, which in turn causes changes in climate. As the
climate grows warmer, evaporation will increase, which will
increase average global precipitation. Soil moisture is likely
to decline in many regions, and intense rainstorms are likely to
become more frequent.
• Altered agricultural patterns
It seems obvious that any significant change in climate on a
global scale should impact local agriculture, and therefore
affect the world's food supply. Long term fluctuations in
weather patterns have extreme impacts on agricultural production
slashing crop yields, forcing farmers to adopt new agricultural
practices.
• Effects on ecosystem
Rising temperatures are beginning to have a noticeable impact on
ecosystems. Many ecosystems are very delicate, and the slightest
change can kill off several species as well as any other species
that depend on them, most ecosystems are interconnected.
Example, species dependent on cold weather conditions, will be
affected most. Oceans absorb carbon dioxide and forms carbonic
acid. Increased carbon dioxide content thus leads to
acidification of oceans, have a particularly detrimental effect
on corals and other marine organisms, may also directly affect
the growth and reproduction of fish as well as the plankton on
which they rely on for food.
• Effects on health Global
warming is expected to extend the favorable zones for vectors
conveying infectious disease such as malaria.
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