Science & Technology
11:30 pm
Fri June 22, 2012

Methane the Potent

Methane comes from a lot of things, not just a gas well. It bubbles out of swamps, landfills, and rice paddies. Believe it or not, cows are a major source of methane. A recent study said it was up to 100 times more potent a greenhouse gas than carbon dioxide. To explain why, the story begins not on earth, but in space.

All energy on earth comes from the sun, mainly in the form of light.

"Light is energy," said Neil Donahue, an atmospheric chemist at Carnegie Mellon University. "The sun gives off about 400 watts — four 100-watt light bulbs — per square meter of surface area."

Visible light allows us to see things around us, and ranges along the red-green-blue spectrum — the colors of the rainbow. We interpret the different wavelengths of light by their frequencies. Violet is the shortest of these frequencies; red is longest. Frequencies longer than red are invisible to humans and are in what we call the infrared, or "below red," spectrum.

"For instance, [when] you put on night vision goggles, night vision goggles have sensors that can sense heat-light, heat-radiation, which has a longer wavelength, longer frequency," Donahue said.

This is important to remember when considering what happens to all the light and heat coming from the sun. Some of this light bounces back into space from clouds and snow. The rest of it heats the earth and is absorbed by plants, sunbathers, solar panels, and rocks. This heat eventually finds its way back into space.

"The heat that's coming off the earth is all kinds of different colors, all kinds of different frequencies, some shorter, some longer. Almost all of them we can't see. Almost all of them are in the infrared," Donahue said.

This heat trying to escape the earth occupies various frequencies in the infrared spectrum. This light comes in tiny particles called photons that travel in wave patterns and don't always go straight back into space. They can bump into molecules in the air, and when they do, that molecule can redirect the photon.

"Sometimes it spits it up, sometimes it spits it down, and light will make this random walk through the air, and it just doesn't go very far," Donahue said.

Air molecules called greenhouse gases block the photons, but each greenhouse gas molecule can only block certain photons, the ones that are tuned to a particular frequency on the infrared spectrum.

"They can only absorb light of some colors, and some frequencies, and some molecules absorb quite a lot of different colors and frequencies, and some have very specific frequencies they absorb," Donahue said. "What that means is that at different parts of the infrared spectrum … it's really easy for light to get all the way from the ground to space. Other places, it's almost impossible and light will only travel a few meters."

Two very well known molecules are largely responsible for this bottleneck: the celebrities of the greenhouse effect.

"The two gases that do most of the work of absorbing light in the air are carbon dioxide and water, and they cover big chunks of the infrared spectrum. So where they're present, heat doesn't get very far," Donahue said.

Carbon dioxide is the main greenhouse gas everyone talks about. Water isn't something humans emit, but it's part of what keeps the planet from becoming a cold rock in space. These are the molecules that are clogging the dial for infrared light trying to get out into space.

"And there [are] other areas, and we call them 'window regions,' because just like a window, light can go right on through. Where if I put a molecule in the atmosphere that absorbs in those windows regions, that one molecule or a few of those molecules can do an awful lot of work in catching photons and slowing down the heat transfer," Donahue said.

And one molecule that blocks out this so-called 'window' frequency?

"That's where methane is," Donahue said. "Methane absorption happens to sit in some of these areas where the atmosphere isn't doing a lot of absorption of energy as it goes."

One might wonder why certain molecules are tuned to different frequencies,a nd why methane can't block photons tuned to the CO2 station on the infrared dial. It has to do with methane itself. Those four hydrogen atoms connected to the carbon are held together by what amounts to a spring. Floating in the air, they bounce around. Think of a jack-in-the-box with four heads.

"If you pull it too far away, it snaps back together. If you push it too far in, it gets pushed back out. And it will do that, bouncing back and forth, with this characteristic frequency," Donahue said.

These atoms of the methane molecule bounce back and forth with the same frequency as any other methane molecule on earth. Same with water, same with CO2, or any molecule. Each has its own signature, its own music. That's how these molecules block out different parts of the infrared spectrum.

"The frequency of the vibration of the molecule, which is exactly the same as the frequency of the light, that's the thing that gets matched," Donahue said.

Further, CO2 molecules outnumber methane by between 200 and 400 to 1. There's much less methane than carbon dioxide in the atmosphere, which means the windows where methane sits are largely open for light to escape into space. You block the windows and light can't get through. You're trapping heat, just like you would in a greenhouse.

This story is part of "Gas Planet," a special presentation of the Allegheny Front.