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Friday, January 1, 2016

The Greenhouse Effect

Much has been written about the green house effect without really explaining what it is and how it works.  I thought I would chip in my two cents worth and hopefully make it make more sense.  I'll start with the Sun which has the mother of all green house effects.  I'll use the sun because it is pure physics with no emotion involved.  Then, when we are in the proper scientific mind set we can look at the earth.

So,,,, the sun started out as a cloud of elements with by far the greatest part being hydrogen.  It collapsed under its own gravity and if you have ever pumped up a bicycle tire, you know that the end where the hose is, warms up.  A diesel engine works on the same principle.  You compress the charge of air about 20 times and it becomes so hot that when you spray in the mist of diesel oil, the oil ignites.  Of course, with a cloud of gas heavy enough to produce the sun, the compression was far greater and the hydrogen at the middle was very hot and very compressed.  It was so hot and so compressed that the hydrogen atoms began to fuse together and produce Helium, releasing large quantities of energy as photons (electromagnetic energy).

This heated up the nearby hydrogen which was hot enough to fuse but not compressed enough.  What is interesting  is that very hot hydrogen absorbs and re-emits the photons (light) produced by the nuclear furnace.  And it re-emits them  in all directions.  Some are sent back toward the core, some sideways and some toward the surface.  All are again and again absorbed, emitted, absorbed, and emitted, over and over again.  There are various estimates  for the time it takes  the energy of a photon to reach the surface of the sun but all are in the thousands of years.

Compare this to what the situation would be if each photon created in the center of the sun was not absorbed by the overlying hydrogen.  As quickly as the energy was produced it would travel through the overlying hydrogen and leave the sun.  Remember that it only takes 8 minutes for the energy of the sun to reach us once it has reached the surface and we are 93 million miles from the sun.  Instead of staying in the sun, the energy would leave the sun in about 1/2 X 8minutes / 93 X 60 = about three seconds.  The sun would cool down and probably the thermo-nuclear reaction at the center would stop.

This is the green house effect. On the Earth, the atmosphere is transparent to most* of the light energy coming from the sun so the atmosphere is heated from below as this radiation is absorbed by the earth.  The warmed earth radiates long wave radiation.   If this long wave (infra-red) energy was radiated at the speed of light straight back into space, the planet would cool rapidly.  If on the other hand, some of the gases in the atmosphere absorb and re-emit the energy coming from the planetary surface in all directions, the planet warms up. It is a fact of nature that the wave lengths of energy coming off the earth, mainly in the infra red spectrum is absorbed and re-emitted by the gases we call green house gases.  They include water vapor, methane, nitrous oxide and a bunch of others in addition to Carbon dioxide.

*Some short wave lengths are absorbed in the upper atmosphere

On earth we have the same phenomenon as on the sun but vastly reduced.  Green House gases in the atmosphere  absorb photons in the infra-red part of the spectrum and re-emit them in every direction.  More of these molecules then do the same.  This delays the escape of heat from the atmosphere and causes it to warm.  The greater the proportion of green house gases in the atmosphere, the longer it takes for the energy coming off the earth to reach space and head out at the speed of light.

As the earth (or any body)  warms up, it radiates more energy.  Eventually we reach equilibrium with the same amount of energy being radiated as is being absorbed but the Earth is now warmer.