Oxygen (O2) and nitrogen (N2) do not have much greenhouse effect, but several trace gases including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and various chlorofluorocarbons are important greenhouse gases. The primary mechanism by which these greenhouse gases warm the Earth is:

Question

BlackTom AI · Accepted Answer

Greenhouse gases warm the planet primarily by absorbing outgoing infrared (longwave) radiation emitted by Earth's surface and re-emitting it both upward and downward, increasing the net downward radiative flux to the surface.

1) Relevant knowledge points and concepts
- Electromagnetic spectrum: sunlight arrives mainly as shortwave (visible/UV); Earth emits longwave infrared because it is much cooler than the Sun.
- Molecular absorption: gases with vibrational modes that change the dipole moment (CO2, CH4, N2O, CFCs) absorb IR efficiently; O2 and N2 lack these IR-active modes.
- Radiative balance and the greenhouse effect: absorption and re-emission of IR leads to a higher surface temperature than would occur for a bare radiating Earth.

2) Complete reasoning process step by step
- The Sun’s shortwave radiation penetrates the atmosphere and warms the surface.
- The surface emits longwave IR according to its temperature.
- Greenhouse gas molecules absorb portions of this outgoing IR at specific wavelengths, then re-emit radiation in all directions.
- Some re-emitted IR travels back to the surface, reducing net cooling and forcing the surface to warm until outgoing energy balances incoming solar energy again.

3) Calculations and intermediate steps (energy-balance example)
- Solar constant S0 ≈ 1361 W/m^2. Average over Earth’s surface: S0/4 ≈ 340 W/m^2.
- With albedo ≈ 0.3, absorbed flux ≈ 0.7 × 340 = 238 W/m^2.
- For a blackbody, σT^4 = 238 W/m^2, with σ = 5.67×10^−8 W·m^−2·K^−4.
- T = (238 / 5.67e-8)^(1/4) ≈ 255 K (−18 °C) — this would be Earth’s surface if it radiated directly to space.
- Actual mean surface ≈ 288 K (15 °C), about 33 °C warmer due to greenhouse gases radiatively trapping heat.

4) Why the correct answer is correct
Absorbing outgoing IR explains the additional downward radiation and the observed warmer surface temperature; it matches molecular physics (IR-active vibrational modes) and the radiative energy-balance explanation above.

5) Why other options are incorrect
- Raising atmospheric pressure: adiabatic compression warms air only locally; pressure changes are not the primary planetary-scale greenhouse mechanism.
- Chemical reactions: such reactions do not provide the sustained global radiative trapping required.
- Preventing convection like glass: a glass greenhouse limits convection, but the atmospheric greenhouse effect is primarily radiative, not a convection barrier.
- Making politicians mad: facetious and irrelevant.

6) Conclusion
The correct choice is “Absorbing some of the infrared radiation emitted from the Earth.” This matches the provided answer.

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