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Section 3.5 The greenhouse effect

As with the radiative budget, we also need to think about what passage through the atmosphere does to the incoming and outgoing radiation, as different molecules in the atmosphere absorb different wavelengths of light.

  • The atmosphere is mostly transparent to visible wavelengths. By time sunlight reaches the surface, most ultraviolet and much infrared light has been absorbed.

  • This incoming visible light is absorbed by the surface, leading to heating of the surface.

  • As the surface heats, it emits more radiation at infrared wavelengths.

  • This infrared (longwave) radiation from the surface is emitted upward into the atmosphere. However, the atmosphere is not transparent to infrared light: molecules such as H\(_2\)O and CO\(_2\) absorb at these wavelengths.

  • This absorption leads to increased heating and re-radiation of energy back toward the surface.

This selective absorption leads to the greenhouse effect: the surface absorbs shortwave radiation but emits at longwave (infrared) wavelengths which are absorbed by "greenhouse" gases in the overlying atmosphere. The surface temperature is thus higher than would be present without an atmosphere. In fact, without this warming effect, the equilibrium surface temperature of the Earth would be closer to 255 K (-18 C), below the freezing point of water. The greenhouse effect thus appears to be necessary for life to exist (as we know it) on planet Earth.

This behavior also provides the physical basis for our understanding anthropogenic climate change and global warming. After H\(_2\)O vapor, the most important greenhouse gas is CO\(_2\text{.}\) Increased CO\(_2\) concentrations from human activities have thus influenced the energy budget of incoming and outgoing solar radiation in the Earth's atmosphere.