Definition of the greenhouse effect

Summary

The greenhouse effect is the process by which certain atmospheric constituents absorb and re-emit infrared (longwave) radiation, reducing the rate at which heat escapes from a planet’s surface to space. On Earth, sunlight enters the atmosphere as shortwave radiation and warms the surface; the surface then emits longwave infrared radiation, which is partly absorbed by greenhouse gases, clouds, and some aerosols. Because this absorbed energy is re-emitted in all directions, less outgoing longwave radiation reaches space, leading to a higher average surface temperature than would occur without the effect. Formally, the greenhouse effect is defined as the infrared radiative effect of all infrared-absorbing constituents in the atmosphere: greenhouse gases, clouds, and some aerosols absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere. The “enhanced greenhouse effect” refers to the strengthening of this natural process due to increased concentrations of greenhouse gases from human activities, which increases radiative forcing and contributes to additional warming.

Key Points

• 1

  Greenhouse gases, clouds, and some aerosols absorb Earth’s outgoing infrared radiation and re-emit it, decreasing heat loss to space.

• 2

  Without the greenhouse effect, Earth’s average surface temperature would be much colder (about −18 °C instead of about 15 °C).

• 3

  The enhanced greenhouse effect describes how human-driven increases in greenhouse gas concentrations strengthen the natural greenhouse effect.