The natural greenhouse effect is the normal warming of Earth caused by atmospheric gases absorbing and re-emitting outgoing infrared radiation. The enhanced greenhouse effect is the additional warming caused by human activities increasing greenhouse-gas concentrations.
How the Mechanism Works
This content is part of IB Physics B.2 Greenhouse effect and applies to both SL and HL.
- Earth receives mainly short-wavelength radiation from the Sun. Some is reflected according to Earth's albedo, while the rest is absorbed by the surface and atmosphere.
- The warmed surface emits longer-wavelength infrared radiation.
- Greenhouse gases, including water vapour, carbon dioxide, methane and nitrous oxide, absorb particular infrared wavelengths because these match changes between molecular energy levels. This can also be described using a resonance model.
- The molecules re-emit infrared radiation in all directions. Some returns toward Earth's surface, reducing the net rate at which Earth loses energy to space.
- Earth warms until outgoing radiative power again balances absorbed incoming power.
| Natural greenhouse effect | Enhanced greenhouse effect |
|---|---|
| Occurs without human intervention and keeps Earth warm enough for liquid water and life. | Is the augmentation of the natural effect due to human activities. |
| Depends on naturally occurring greenhouse gases, especially water vapour and carbon dioxide. | Results from rising concentrations of gases such as carbon dioxide from fossil-fuel combustion and methane from agriculture. |
| Is necessary for Earth's habitable average temperature. | Produces an additional increase in Earth's equilibrium temperature and drives current climate change. |
A common misconception is that the greenhouse effect is caused by ozone depletion. This is incorrect: the greenhouse effect concerns the absorption and re-emission of outgoing infrared radiation, whereas ozone depletion concerns reduced absorption of ultraviolet radiation in the stratosphere.
Exam Technique
For an Explain question, link each stage causally: increased greenhouse-gas concentration causes greater infrared absorption and re-emission, reducing net energy loss and raising temperature until a new energy balance is established. Do not simply state that greenhouse gases “trap heat”; identify the radiation and energy-transfer mechanism.
