Questions: Greenhouse Gas Absorption and Emission Spectra

Question 1

Why are nitrogen (N₂) and oxygen (O₂) not greenhouse gases, despite making up 99% of the atmosphere?

• A. They absorb ultraviolet radiation instead of infrared, so their effect occurs in the upper atmosphere only
• B. They are greenhouse gases, but their concentrations are so high their absorption bands are already completely saturated
• C. They have no vibrational modes that produce a changing dipole moment, making them infrared-inactive
• D. Their absorption bands fall outside the wavelength range of Earth's emitted infrared radiation

Answer: (C)

Interaction with infrared radiation requires a molecule to have a dipole moment that changes during vibration. Symmetric diatomic molecules like N₂ and O₂ have no permanent dipole, and their symmetric stretching mode produces no change in dipole — there is no oscillating electric field for the infrared photon to couple with. This is a quantum mechanical selection rule, not a matter of concentration or saturation. Even if N₂ and O₂ were present in trace amounts, they would still be infrared-inactive.

Question 2

CO₂ concentration has increased from 280 ppm (pre-industrial) to 420 ppm today, an increase of 50%. If CO₂ concentration were to double again (from 420 to 840 ppm), how would the additional radiative forcing compare to what occurred going from 280 to 560 ppm?

• A. The forcing from 420→840 ppm would be roughly double the forcing from 280→560 ppm, since more molecules absorb more radiation
• B. The forcing from 420→840 ppm would be roughly the same as from 280→560 ppm, because each doubling adds approximately the same increment of forcing
• C. The forcing from 420→840 ppm would be less, because CO₂'s absorption band is becoming saturated and additional molecules have diminishing effect per doubling
• D. The forcing from 420→840 ppm would be larger, because more CO₂ means more absorption across more atmospheric levels

Answer: (B)

The relationship between CO₂ concentration and radiative forcing is logarithmic: each doubling of CO₂ concentration adds approximately the same increment of forcing (roughly 3.7 W/m²). This means 280→560 ppm adds ~3.7 W/m², 560→1120 ppm adds another ~3.7 W/m², and so on. The reason is that CO₂'s 15 μm core absorption band is already largely saturated — additional CO₂ matters at the band edges and in the upper atmosphere, but these contributions diminish as a fraction of the total with each doubling.

Question 3

The greenhouse effect works by greenhouse gases absorbing incoming solar radiation and preventing it from reaching Earth's surface, which heats the atmosphere.

• T. True
• F. False

Answer: False

This is a common misconception. Greenhouse gases are largely transparent to incoming solar radiation (mostly visible and near-infrared wavelengths). The greenhouse effect operates on *outgoing* thermal infrared radiation emitted by Earth's surface and lower atmosphere. Greenhouse gases absorb this outgoing radiation and re-emit it in all directions, including back toward the surface. The net effect is that energy that would otherwise escape to space is retained in the lower atmosphere, warming the surface. Solar radiation that is absorbed gets absorbed by the surface and clouds, not primarily by greenhouse gases.

Question 4

CO₂'s dominant absorption band near 15 μm is climatically important partly because it coincides with the peak wavelength range of Earth's outgoing infrared emission at typical surface temperatures.

• T. True
• F. False

Answer: True

Earth's surface emits thermal radiation as an approximate blackbody at ~288 K (15°C), with emission peaking near 10–15 μm. CO₂'s bending mode absorption band is centered near 15 μm, placing it squarely where Earth is radiating most strongly. This spectral coincidence is why CO₂ is so climatically effective despite its relatively low atmospheric concentration (~0.04%). A gas absorbing at wavelengths where Earth emits strongly intercepts a large fraction of the outgoing energy flux.

Question 5

Explain why the relationship between CO₂ concentration and radiative forcing is logarithmic rather than linear, and what this implies about the climate impact of each successive doubling.

Model answer: The relationship is logarithmic because CO₂'s core absorption band near 15 μm is already largely saturated at current concentrations — the atmosphere is already nearly opaque at that wavelength. Additional CO₂ cannot meaningfully increase absorption at the band center; instead, it widens the absorption at the band edges where the atmosphere is still partially transparent, and increases absorption in the upper atmosphere where air is thinner. Each doubling of CO₂ adds a roughly constant increment of forcing (~3.7 W/m²) because it provides a constant fractional expansion of the absorbing band. This means that, while each doubling has the same warming effect, successive absolute increases in CO₂ (in ppm) have a decreasing marginal effect.

The practical implication is that going from 280 to 560 ppm has the same forcing as going from 560 to 1120 ppm — each doubling adds ~3.7 W/m². This is why climate sensitivity is expressed per doubling, not per ppm increase. It also means early emissions had a larger per-molecule effect than current emissions, since the band was less saturated at lower concentrations.