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Question
The time, our featured question comes from Tasmia Zaman, who asks:

“Explain why and how the molecular structure of Methane enables it to act as a greenhouse gas."

Great question, and our answer comes from John Edwards, a Met Office research scientist.

Answer
“To exert a greenhouse effect a gas must be able to absorb infra-red radiation in the atmosphere.

A gas does not absorb radiation of all wavelengths equally and which wavelengths are absorbed strongly, weakly, or not at all depends on the molecular structure of the gas, with each gas having a different absorption spectrum. When a molecule absorbs radiation its energy changes, but a molecule can possess only certain discrete amounts of energy (this is known as quantization) and only radiation of the right wavelength to trigger a transition between two such energy levels can be absorbed; shorter wavelengths correspond to higher energies.

A molecule of methane consists of a central carbon atom and four hydrogen atoms arranged tetrahedrally around it. The bonds between the carbon and the hydrogen atoms may undergo stretching or bending vibrations and radiation which excites these vibrations can be absorbed.

For methane, the dominant vibrational transition corresponds to the absorption of radiation with a wavelength of about 7.6 microns. Other atmospheric gases do not absorb especially strongly around this wavelength; so increasing the amount of methane in the atmosphere increases the absorption, or trapping, of infra-red radiation, enhancing the greenhouse effect.”

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