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Reader questions – Part 3 of 4

I enjoy emails from readers of this site.  My contact information is in the About page and the comments and encouragement I receive makes this site a joy to run.  I recently received an email from Brittany C asking four questions.  Brittany is allowing me to publish her questions and my answers.  These answers are a combination of scientific fact with conjecture and opinion from me. This is her 3rd question of the 4.

Question 3: Greenhouse gases trap in the heat from the sun’s rays.  Why do these gases trap in the heat?  Why don’t these gases keep the sun’s heat and rays from entering the earth’s atmosphere in the first place?  I would think that if the gases keep heat from escaping, they would also keep it from entering the atmosphere and reaching the earth’s surface?

In this question, I think we may be a victim of an over simplification of the explanation of greenhouse gases and a misunderstanding of how we receive energy from the sun.  Greenhouse gases are not a blanket covering the globe.  A blanket works as an insulator with a particular R value that impedes the flow of heat. Greenhouse gases are more of a filter in the way they actually act in our atmosphere and the way they balance the temperature of Earth.

The first step in this discussion requires one to understand that we do not directly receive heat from the sun. The sun is extremely hot at 5,500 C. However the space between the sun and Earth is extremely cold.  To be more precise, space has no temperature since it is a vacuum but something in space that was not receiving solar radiation would be approximately absolute zero or a few degrees above. If we did receive “heat energy” from the sun then CO2 and other greenhouse gases would slow this energy down, just like the blanket referred to above. Since the energy comes in a different form, CO2 has no effect on the inbound energy.

We receive different types of radiation but for this discussion we are receiving light energy.  Light has a fairly short wavelength (see a chart here) and carbon dioxide, as with most atmospheric gases, does a fairly poor job of blocking light energy since it is essentially invisible even at high concentrations. Once light hits something that that is fairly solid, such as the ground, it is partially absorbed by that solid. That absorption of light energy will be converted into heat within the object which is then radiated out.  Heat, in the form of infrared radiation, has a relatively longer wavelength.

Greenhouse gases, such as CO2, do a pretty good job of absorbing the wavelength of infrared. I won’t get into all of the physics behind how this works but CO2, water vapor, and methane will all absorb the infrared wavelength. As the CO2 in the atmosphere increases, the theory is that more of this heat is slowed down on its trip out of our atmosphere.  There is some contention that there is an upper limit to how much of this particular type of wavelength of energy is getting absorbed today but that is the subject for a different article.

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