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What makes methane a more potent greenhouse gas than carbon dioxide?
Methane has more bonds between atoms than CO2, and that means it can twist and vibrate in more ways that absorb infrared light on its way out of the Earth’s atmosphere.
December 7, 2023
When scientists talk about greenhouse gases “trapping heat” in the atmosphere, they’re actually talking about how these gases interact with infrared light. That’s because energy from the sun enters the Earth as a mix of visible, ultraviolet and infrared light, but it leaves the Earth almost entirely in the infrared.
Some molecules—the greenhouse gases, like carbon dioxide (CO2) and methane (CH4)—can grab infrared light on its way out.1 “So what happens is that infrared energy gets absorbed by the molecule,” says Desiree Plata, associate professor of civil and environmental engineering and director of the MIT Methane Network. “And that molecule enters an excited state. As it's relaxing back down, it releases some of that energy as heat, and that heat can go to outer space or can come back to planet Earth.”
This is the reason the buildup of greenhouse gases in the atmosphere is warming our planet. But it doesn’t explain why some molecules are more potent greenhouse gases than others. Methane, for instance, traps around 120 times as much heat as CO2 does moment to moment.2
To understand why, we have to look closer at these molecules’ structures.
Molecules are made of atoms constantly moving around each other, getting closer and further apart and vibrating in three-dimensional space. Depending which atoms the molecule is made of, it will move in different ways. “If you think about these geometries, it influences what I like to call the ‘dance moves’ that the molecule has access to,” says Plata. “Scientists call those dance moves ‘vibrational and rotational modes.’”
Some molecules, as they bend and twist on the dance floor, can warp themselves into an asymmetrical shape. CO2, for example, is usually a straight line, with one carbon atom in the middle and an oxygen atom on either side. But sometimes it bends, and the two oxygens stick out from the carbon in the same direction, like a dancer stretching their arms forward.
In that position, the molecule’s electrons will be pulled more strongly in one direction than another, and the molecule will develop an electric field.
Light, too, consists of vibrating electric and magnetic fields. So when our CO2 molecule is pulling off its bendy dance moves, it can find itself vibrating with an energy that matches the frequency of a passing wave of infrared light. The two will be in “resonance.”
When that happens, the two electric fields interact, and the light is absorbed by the molecule. The CO2 ends up with extra energy it has to get rid of as heat, and this is the heat that warms our planet.
And it’s important that this only happens at certain, very precise wavelengths, when the light and the molecule are in resonance. It means that greenhouse gases like CO2 can leave some wavelengths of light alone—like the visible and ultraviolet light entering the Earth—but absorb specific wavelengths of infrared light on its way out.
And a molecule with more “dance moves” has more wavelengths it can absorb light at.
“Now here's where the differences between methane and CO2 really come into play,” says Plata. “Methane is a central carbon atom surrounded by four hydrogen atoms. CO2, in contrast, is a central carbon atom with just two oxygen atoms attached to it. So if you think about the ways that those molecules can vibrate and move, methane has got many more dance moves.
“And because there are so many more vibrational and rotational modes associated with the methane molecule—a simple consequence of its geometry—it makes it a much more potent greenhouse gas.”
This also explains the power of even stronger greenhouse gases. Some human-made gases, like chlorofluorocarbons and sulfur hexafluoride, are yet more geometrically complex and may trap thousands of times as much heat as CO2.
And conveniently, it also explains why some molecules trap no heat at all. Nitrogen and oxygen together make up around 99% of our atmosphere, and if they were greenhouse gases the world would be very hot indeed. But these molecules have just two atoms apiece, and no matter how they “dance,” they are always symmetrical. They can’t make an electric field and can’t absorb infrared light at any wavelength.
Thank you to several readers for sending in related questions, including JoAnn Layton of Lincroft, New Jersey, and Abhigna Yella of Frisco, Texas.
1 For more detail on this process, see our answer to the question, “How do greenhouse gases trap heat in the atmosphere?”
2 For complicated reasons related to their lifetimes in the atmosphere, this does not mean that methane is 120 times as potent a greenhouse gas as CO2. Depending how you measure, it’s usually counted as between 28 and 80 times as powerful as CO2. For more detail, see our answer to the question, “Why do we compare methane to carbon dioxide over a 100-year timeframe? Are we underrating the importance of methane emissions?”