The natural greenhouse effect is the warming of Earth caused when atmospheric gases absorb and re-emit infrared radiation from the surface. It keeps Earth warm enough for liquid water and life by reducing the rate at which energy escapes to space.
This is SL and HL content from Subtopic 6.1, Introduction to the atmosphere.
The Mechanism
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The Sun emits mainly short-wave radiation. Much of this passes through the atmosphere and is absorbed by Earth’s surface.
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The warmed surface emits energy as infrared radiation, also called long-wave radiation.
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Greenhouse gases (GHGs), including water vapour, carbon dioxide, methane and nitrous oxide, absorb some wavelengths of this outgoing radiation.
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GHG molecules re-emit infrared radiation in different directions. Some returns toward the surface and lower atmosphere, slowing their loss of energy to space.
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Earth becomes warmer until incoming solar energy is balanced by outgoing radiation. This higher-temperature energy balance makes the planet habitable.
Greenhouse gases do not form a solid layer or permanently trap heat. Energy still escapes to space, but it does so more slowly than it would without these gases.
| Natural greenhouse effect | Enhanced greenhouse effect |
|---|---|
| A natural process essential for life | Additional warming caused by increased GHG concentrations |
| Maintains Earth’s habitable temperature | Driven mainly by human activities such as fossil-fuel combustion and deforestation |
| Has operated throughout Earth’s history | Has intensified rapidly since industrialization |
A common misconception is that the greenhouse effect itself is harmful. The environmental problem is the enhanced greenhouse effect, not the natural process. Atmospheric carbon dioxide measurements from the Mauna Loa Observatory provide evidence of the recent increase that strengthens this enhanced effect. Stratospheric ozone depletion is a separate environmental issue and does not cause global warming.
Exam Technique
For an explain question, present the full causal sequence: incoming short-wave radiation, surface absorption, outgoing infrared radiation, GHG absorption and re-emission, then warming. A labelled energy-flow diagram can gain credit, but distinguish clearly between the natural and enhanced greenhouse effects.