The Greenhouse Effect

The Greenhouse Effect is the process by which certain gases in the Earth's atmosphere to trap heat from the surface.  This effect creates an insulating blanket around the earth that keeps the planet warm.  Without this mechanism, the Earth's temperature would range from about 33^o C (59^o F) to -19^o C (2^o F).   This climate would be too cold for most organisms to survive.  This process receives its name, The Greenhouse Effect, because of it's similarity to a greenhouse here on Earth.  The clear glass or plastic that makes up the walls and ceiling of a greenhouse allows the heat of the sun to enter the building and warm the plants, while effectively trapping the heat inside to keep the temperature inside of the building warmer than the air temperature outside of the greenhouse.  

   http://www.epa.gov/globalwarming/kids/greenhouse.html

How Does the Greenhouse Effect Work?

     As energy leaves the sun and travels towards the earth, it must pass through the Earth's atmosphere to reach the surface.  Upon reaching Earth's atmosphere, three things can happen to this energy.  First, approximately 25% of the energy is simply reflected back into space and becomes lost energy.  Another 20 to 25% of the energy is absorbed by the gases that make up the atmosphere.    The other 50% of the solar energy reaches the surface of earth.  Of the energy that reaches the surface of the earth, approximately 85% of it is absorbed by the soil, plants and oceans.  The rest of the energy is sent back up into the atmosphere by reflective surfaces such as snow, ice and deserts.  Some of the energy absorbed into the surface of the earth is released back into the atmosphere as radiant energy.  Various gases absorb different types of solar energy.  For example, Oxygen (O₂) and Ozone (O3) tend to absorb ultraviolet light that features a short wavelength.  Carbon Dioxide (CO₂) however absorbs the longer wavelength of infrared radiation.  The absorption of this energy creates some of the heat needed by earth to sustain life.

Diagram of the Greenhouse Effect

 

       PRESENT COMPOSITION OF 

       THE  ATMOSPHERE:

 

The atmosphere is comprised of gases which are considered to be permanent (gases which remain essentially constant by percent) and gases considered to be variable (gases which have changing concentrations over a finite period 
of time).

PERMANENT gases in the atmosphere by 
percent are:

Nitrogen    78.1%
Oxygen     20.9%

(Note that these two permanent gases 
together comprise 99% of the atmosphere)

Other permanent gases:

Argon	             0.9%
Neon	             0.002%
Helium	             0.0005%
Krypton                   0.0001%	
Hydrogen	             0.00005%

VARIABLE gases in the atmosphere and 
typical percentage values are:

Water vapor             0 to 4%
Carbon Dioxide	0.035%
Methane	                0.0002%
Ozone	                0.000004%

Source: EXPLORES! Atmospheric

http://www.met.fsu.edu/explores/
atmcomp.html

Why is there Concern about the Greenhouse Effect?

      For the past 4 billion years the the exchange of energy between the surface and atmosphere of the Earth and with Space has generally been in balance.  The carbon dioxide and other greenhouse gases that are released by plants, decomposition and human activities has been in balanced with the carbon dioxide absorbed by terrestrial vegetation and oceans. 

     However, there has been a  dramatic increase in the amount of these greenhouse gases since the early 1900's that can be linked to the industrial revolution.  What has changed over the past century is the burning of fossil fuels (oil, coal, wood) to generate electricity, run cars and factories, and heat or cool homes.  These fossil fuels are responsible for about 98% of the U.S. carbon dioxide, 24% of the methane and 18% of the nitrous oxide emissions.  In addition to the burning of fossil fuels, increasing agriculture, deforestation, landfills, industrial production and mining contributes to these emissions.    Since the beginning of the industrial revolution, scientists have noted a nearly 30% increase in the amount of carbon dioxide (CO₂), methane (CH₄)concentrations have more than doubled and nitrous oxide (NO_x) concentrations have increase about 15%.  These atmospheric gases are commonly known as greenhouse gases since they enhance the "heat-trapping" capability of the atmosphere. Overall, in 1997 the United States emitted about one-fifth of the world's greenhouse gases. Source: http://www.epa.gov/globalwarming/climate/index.html

     Scientists who have been studying the global temperatures of the Earth have noticed that the mean surface temperature has been increasing since the late 19th century and throughout the 20th century.  In fact, the 10 warmest years of the 20th century has occurred during the last 15 years, with 1998 posting the warmest year recorded.  Most scientist theorize that this dramatic increase is directly related to the greenhouse gas emissions that humans have been putting into the atmosphere during the past 100 years.

"The farther backward you can look,
the farther forward you are likely to see."
- Winston Churchill

     Scientist have been able to look thousands of years into the past by studying core samples of polar and glacial ice, which contains bubbles of the air that was present at the time that the ice was formed. Additionally, by studying how temperature reacts to carbon dioxide, along with studying fossilized coral, plants, animals and trees scientist have been able to determine what the Earth's temperature was like during that same time period.  From these studies, we have been able to see the long term relationship between variations of carbon dioxide and temperature.  The graph to the right demonstrates that when carbon dioxide levels in the atmosphere increases then the global temperature also increases.  From the various studies that have been done, many scientists have concluded that the Earth is undergoing Global Warming.

Photos of ice coring and coral reef sampling taken by NOAA

The GISP2 Drill Dome.
Photo by Mark Twickler, GISP2 SMO, University of New Hampshire.

       Ice Core sample taken from drill.
       Photo by Lonnie Thompson,
       Byrd Polar Research Center,
       Ohio State University.

For more information on stories from
 the ice:
   http://www.pbs.org/wgbh/warming/stories/

      http://www.srv.cmdl.noaa.gov/ccgg/
figures/co2obs.gif

Reef flat, Palau Archipelago, Micronesia 
  containing various species of branching coral
  (Acropora).
   Photo by Jerry Wellington.

   Scientists take samples from the center of
    the coral.  Clipperton Atoll, 10°N, 109°W.
    Photo by Maris Kazmers.

http://www.ngdc.noaa.gov/paleo/globalwarming
/gallery.html

Why is there controversy about the Greenhouse Effect?

     Some people contend that carbon dioxide levels naturally go up and down and that we do not need to worry about the Greenhouse Effect or Global Warming.  The amount of carbon dioxide in the atmosphere varies throughout the year as we pass through the seasons.  The levels of carbon dioxide decreases during the growing seasons when plants take up CO₂ during photosynthesis.  Additionally, the level of CO₂ increases during the winter months when many of the plants and trees are dormant and humans are burning more fossil fuels for heat.
     The graph below shows that there is a natural fluctuation in the level of carbon dioxide, especially over the course of a year.  However, long term studies have shown that the overall trend of carbon dioxide levels have been steadily increasing all over the world.  

                                          http://wwwsrv.cmdl.noaa.gov/ccgg/figures/co2obs.gif

     According to the Environmental Protection Agency, the gradual increase in carbon dioxide and other greenhouse gases levels have caused global climatic changes.  During the past century the global sea level has risen 4 - 8 inches, along with a decrease in snow cover and floating ice.  The polar icecaps have begun to melt and break off chunks to form icebergs at an astonishing rate.  
     Increases in temperature causes an increase in the evaporation rate within the water cycle.  Because of this, precipitation has increase over the continents along with changing rain patterns.  High latitudes have been noting an increase in precipitation while the tropical areas that normally have high rainfall have been experiencing a decrease. 
     

     Overall, scientists have projected a global surface temperature increase by between 1.4 and 5.8 ^oC above 1990 levels by 2100. Source: http://interacademies.net/intracad/tokyo2000.nsf    While this number seems to be small, some scientists predict that global climatic changes will result in widespread weather pattern changes, resulting in prolonged drought or flooding along with sever storms.
     The extreme version of the greenhouse effect can be seen by studying the planet Venus.  The surface temperature of Venus is unbearable hot - about 462^o C or 864^o F!  The atmosphere of the planet is roughly 97% carbon dioxide and approximately 3% nitrogen.  There is only a trace of water and water vapor on the planet.  Many scientist theorize that Venus's closeness to the sun (it is the second planet from the sun) cause it to experience a runaway greenhouse effect with the oceans drying up!
For more information about Venus:

http://seds.lpl.arizona.edu/nineplanets/nineplanets
/venus.html

http://pds.jpl.nasa.gov/planets/welcome/venus.htm

                          http://www.soton.ac.uk/~engenvir/environment/air/greenhouse.problems.html

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