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The
Greenhouse Effect
The
greenhouse effect is a natural occurrence that maintains Earth's average
temperature at approximately 60 degrees Fahrenheit.
The greenhouse effect is a
necessary phenomenon that keeps all Earth's heat from escaping to the outer
atmosphere. Without the natural greenhouse effect it is certain that life on
Earth would be difficult to sustain.
Temperatures on Earth would be much
lower than they are now, and the existence of life on this planet would not be
possible. The global average temperature would drop precipitously 33 degrees
from its current 15° to -18°C. The Earth would become an ice planet.
However, too many greenhouse gases in Earth's atmosphere could
increase the greenhouse effect.
This could result in an increase in mean global
temperatures as well as changes in precipitation patterns.
The Earth's atmosphere, a thin blanket of gases, protects
the planet from the harshest of the sun's ultraviolet radiation. The
atmosphere, by trapping the Earth's warmth, keeps rivers and oceans from freezing.
Carbon dioxide and water vapor are the most important gases in creating
the insulating or "greenhouse effect" of the atmosphere.
Some
heating of the atmosphere occurs when molecules, aerosols, and particles in
the air absorb radiation from the sun.
Some heating occurs from contact
with Earth’s surface, which absorbs most of the sun’s energy and emits
long-wave radiation back into the atmosphere.
Under
normal conditions some of the sun's heat is radiated back into space
The
'Greenhouse Effect'
occurs when heat is trapped in the atmosphere by gases
The Sun powers Earth’s climate,
radiating energy at very short wavelengths, predominately in the visible or
near-visible (e.g., ultraviolet) part of the spectrum.
Here are the
different types of radiation in the EM spectrum, in order from lowest energy to
highest:
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Radio: Yes, this is
the same kind of energy that radio stations emit into the air for
your boom box to capture and turn into your favorite Mozart,
Madonna, or Justin Timberlake tunes. But radio waves are also
emitted by other things ... such as stars and gases in space. You
may not be able to dance to what these objects emit, but you can
use it to learn what they are made of.
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Microwaves: They
will cook your popcorn in just a few minutes! Microwaves in space
are used by astronomers to learn about the structure of nearby
galaxies, and our own Milky Way!
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Infrared: Our skin
emits infrared light, which is why we can be seen in the dark by
someone using night vision goggles. In space, IR light maps the
dust between stars. Visible: Yes, this is the part that our eyes
see. Visible radiation is emitted by everything from fireflies to
light bulbs to stars ... also by fast-moving particles hitting
other particles. Ultraviolet: We know that the Sun is a source of
ultraviolet (or UV) radiation, because it is the UV rays that
cause our skin to burn! Stars and other "hot" objects in
space emit UV radiation. |
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X-rays: Your doctor
uses them to look at your bones and your dentist to look at your
teeth. Hot gases in the Universe also emit X-rays .
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Gamma-rays:
Radioactive materials (some natural and others made by man in
things like nuclear power plants) can emit gamma-rays. Big
particle accelerators that scientists use to help them understand
what matter is made of can sometimes generate gamma-rays. But the
biggest gamma-ray generator of all is the Universe! It makes gamma
radiation in all kinds of ways. |
NASA
Graphic
Roughly one-third of the solar
energy that reaches the top of Earth’s atmosphere is reflected directly back
to space. The remaining two-thirds is absorbed by the surface and, to a lesser
extent, by the atmosphere. To balance the absorbed incoming energy, the Earth
must, on average, radiate the same amount of energy back to space. Because the
Earth is much colder than the Sun, it radiates at much longer wavelengths,
primarily in the infrared part of the spectrum . Much of this thermal radiation
emitted by the land and ocean is absorbed by the atmosphere, including clouds,
and reradiated back to Earth. This is called the greenhouse effect. The glass
walls in a greenhouse reduce airflow and increase the temperature of the air
inside. Analogously, but through a different physical process, the Earth’s
greenhouse effect warms the surface of the planet. Without the natural
greenhouse effect, the average temperature at Earth’s surface would be below
the freezing point of water. Thus, Earth’s natural greenhouse effect makes
life as we know it possible. However, human activities, primarily the burning of
fossil fuels and clearing of forests, have greatly intensified the natural
greenhouse effect, causing global warming.
The two most abundant gases in the
atmosphere, nitrogen (comprising 78% of the dry atmosphere) and oxygen
(comprising 21%), exert almost no greenhouse effect. Instead, the greenhouse
effect comes from molecules that are more complex and much less common. Water
vapor is the most important greenhouse gas, and carbon dioxide (CO2) is the
second-most important one. Methane, nitrous oxide, ozone and several other gases
present in the atmosphere in small amounts also contribute to the greenhouse
effect. In the humid equatorial regions, where there is so much water vapor in
the air that the greenhouse effect is very large, adding a small additional
amount of CO2 or water vapor has only a small direct impact on downward infrared
radiation. However, in the cold, dry polar regions, the effect of a small
increase in CO2 or water vapor is much greater. The same is true for the cold,
dry upper atmosphere where a small increase in water vapor has a greater
influence on the greenhouse effect than the same change in water vapor would
have near the surface.
EPA Diagram
Several components of the climate
system, notably the oceans and living things, affect atmospheric concentrations
of greenhouse gases. A prime example of this is plants taking CO2 out of the
atmosphere and converting it (and water) into carbohydrates via photosynthesis.
In the industrial era, human activities have added greenhouse gases to the
atmosphere, primarily through the burning of fossil fuels and clearing of
forests.
Adding more of a greenhouse gas,
such as CO2, to the atmosphere intensifies the greenhouse effect, thus warming
Earth’s climate. The amount of warming depends on various feedback mechanisms.
For example, as the atmosphere warms due to rising levels of greenhouse gases,
its concentration of water vapor increases, further intensifying the greenhouse
effect. This in turn causes more warming, which causes an additional increase in
water vapor, in a self-reinforcing cycle. This water vapor feedback may be
strong enough to approximately double the increase in the greenhouse effect due
to the added CO2 alone.
Additional important feedback
mechanisms involve clouds. Clouds are effective at absorbing infrared radiation
and therefore exert a large greenhouse effect, thus warming the Earth. Clouds
are also effective at reflecting away incoming solar radiation, thus cooling the
Earth. A change in almost any aspect of clouds, such as their type, location,
water content, cloud altitude, particle size and shape, or lifetimes, affects
the degree to which clouds warm or cool the Earth. Some changes amplify warming
while others diminish it. Much research is in progress to better understand how
clouds change in response to climate warming, and how these changes affect
climate through various feedback mechanisms.
Credit: GRACE Goddard Space Flight
Center NASA, UNEP, EPA, Woods Hole Oceanographic Institute, NOAA,
University of Colorado, CIA, U.S. Department of Energy,The Smithsonian,IPCC,
2007: Climate Change 2007: The Physical Science Basis
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