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Thursday, June 25, 2009

Greenhouse Effect

We all know a microwave works. We don't question the science behind it. We put our food in, pick a time, press Start, and some time later take out our hot meal or drink.

The same basic science that allows a microwave to work explains how carbon dioxide absorbs infrared radiation, a.k.a. heat. Really.

Does everyone understand what "electromagnetic radiation" is? Probably not. I got an Electrical Engineering degree several years ago, so it's a subject I studied for three years... it felt like forever back then.

So, breaking it down, and I apologize in advance if I seem to be too simplistic, here is a visualization exercise that might help you understand:

This involves a jump rope and a fence. Tie one end to the fence.... Now, shake one end up and down and watch the "hump" move toward the fence. That's a decent way to visualize light and other "electromagnetic energy" moving away from its source...

If you were actually holding magnet in your hand while you shook it, you'd be generating an electromagnetic wave. Here's why: A changing magnetic field induces a changing electric field and visa versa. That's just wave physics. We've all seen and perhaps done the grade school science experiment where we put iron filings on a sheet of paper and hold a magnet under the paper and watch the filings line up... That's a physical representation of the magnetic field... move that with a regular frequency up and down, and there will also be a corresponding electric field emanating from your hand as a result.

Now, back to the rope. Imagine shaking it back and forth sideways so that you have two or three humps. Have two friends slowly walk into the valleys... the "wave" passes them right by. The wave might be three feet from one peak to the next peak, and your friends are only two feet or less apart, but the wave passes right through them because of how your friends are spaced apart.

Now, have your friends in this imaginary experiment stand where the peaks appear. As the rope hits them, your wave is "absorbed" and your rope collapses, no matter how hard you shake. This is how polarized lenses in your glasses block some of the light... Thin wires spaced at very precise distances from each other are suspended in the lens, blocking light waves that are travelling in a side-to-side wave. Light energy travels in waves just like your magnet-shaking experiment... only the "frequency" is really high (wicked fast) and the waves are wicked small.

Now, let's once more go back to your rope. Imagine tying a piece of thread to the two points on the rope that form the peaks when you shake it back and forth at one speed. If you could focus on the thread, you'd watch it twist back and forth. This is very much like how "microwaves" force water molecules to twist back and forth. Water molecules are the exact size to be like the string tied at the two perfect points on the rope, and the microwaves shake them at precise speeds (very fast). The friction between the twisting (oscillating) water molecules generates heat.

Now let's get back to carbon dioxide... It has three atoms arranged in such a way that it absorbs two frequencies in the "infrared" frequency range. You can think of two frequencies in a range just like two colors of the whole visible light "spectrum."

Infrared energy is heat energy. When you feel heat coming from a hot stove, your hand is absorbing infrared radiation.

So, just like water is the perfect size to be rotated by microwave radiation, carbon dioxide is arranged in such a way as a molecule to absorb two frequencies, or two specific wavelengths, of infrared energy.

What happens when CO2 absorbs infrared? It heats up. When it heats up, it radiates heat in all directions.... just like a pot taken from a hot stove.

When the earth absorbs a lot of solar radiation all day, it heats up. The heated surface radiates heat, or infrared radiation, in every direction, including back toward "space." Carbon dioxide suspended in the atmosphere absorbs some of this radiation, and when radiating heat in all directions, reflects heat back toward the earth's surface, where it is absorbed by water vapor in the atmosphere and the surface.

If you put a thin aluminum pan and a cast iron skillet on the same temperature stove heating element, the cast iron skillet will absorb more heat. Because it has more metal molecules, it has more "mass" and can store more heat energy. Therefore, when you remove both from the stove at the same time, the heavier skillet will be hotter longer than the thin aluminum pan. If you apply this line of thought to the carbon dioxide in the atmosphere, more CO2 molecules will absorb more heat, and therefore reflect more heat back to the earth's surface for a longer period of time at night when the surface has stopped absorbing light energy from the sun. This leads to a slight increase in average temperature over a day.

These slight increases will not even be perceptible by people, only evidenced by upward trends in average temperatures. Even if one city or region has a record cold winter in the near future, the trend for the planet's average temperature is increasing. Even though the changes are very small, the overall trend is known as global warming. A very slight increase at the edge of the freezing point has been turning glaciers and polar ice back to water... the difference in color between the white ice of the glacier and the dark blue ocean is significant as the sun shines on it. (Follow the link!)

This trend is not new at all. In fact, I found a section on it in my textbook published in 1977, over 32 years ago! Follow the 'greenhouse effect' link in the sidebar or the title.

I hope you enjoyed this!

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I'm always interested in discussing issues if you're actually interested in exploring the subject.