There are many greenhouse gases, including carbon dioxide (CO2), methane (CH4). and vaporized water (H2O). These and other greenhouse gases warm the Earth primarily by:

Question

BlackTom AI · Accepted Answer

Greenhouse gases warm the Earth primarily by absorbing some of the infrared radiation emitted from the Earth and re-emitting it, including back toward the surface.

Relevant concepts:
- Solar radiation that reaches Earth is mostly shortwave (visible/UV). The surface absorbs this energy and warms.
- A warmed surface emits longwave infrared (IR) radiation. Greenhouse gases (CO2, CH4, H2O vapor) have molecular vibrational modes that absorb IR at specific wavelengths.
- Absorbed IR is re-emitted isotropically; some goes upward to space, some back downward, increasing the net heat retained near the surface.

Step-by-step reasoning:
1. Incoming solar power per unit area averaged over the planet is S(1 − A)/4. With S ≈ 1361 W/m² and albedo A ≈ 0.3, the average absorbed is ≈ 240 W/m².
2. A blackbody in equilibrium emits F = σT^4. Setting σT^4 = 240 W/m² gives T ≈ (240/5.67×10^−8)^(1/4) ≈ 255 K (−18 °C). This is the effective radiating temperature of Earth as seen from space.
3. Measured mean surface temperature is ≈ 288 K (15 °C). The ~33 K difference is explained by greenhouse gases: they absorb some of the surface’s IR and re-radiate part back downward, so the surface must be warmer to reach radiative equilibrium.
4. Therefore the primary warming mechanism is radiative trapping via absorption and re-emission of IR by greenhouse gases.

Why the correct option is correct:
- It matches the radiative physics: GHGs selectively absorb outgoing longwave IR and increase the downward IR flux, raising the surface temperature until energy balance is restored.

Why the other options are incorrect:
- Raising atmospheric pressure: Local compression can heat air, but global warming is not caused by a net increase in atmospheric pressure; pressure changes are not the mechanism of the greenhouse effect.
- Making more clouds: Clouds affect climate (both reflect sunlight and trap IR), but clouds are not the fundamental cause; greenhouse gases directly absorb IR regardless of cloudiness.
- Preventing convection like a greenhouse: A glass greenhouse reduces convective heat loss; the atmosphere still convects. The atmospheric greenhouse effect is primarily radiative, not just suppression of convection.
- Making politicians mad: Humorous and irrelevant.

Conclusion: The correct choice is “Absorbing some of the infrared radiation emitted from the Earth,” which matches the provided answer.

There are many greenhouse gases, including carbon dioxide (CO2), methane (CH4). and vaporized water (H2O). These and other greenhouse gases warm the Earth primarily by:单项选择题

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