The main greenhouse gases named in IB Physics B.2 are methane, water vapor, carbon dioxide, and nitrous oxide: , , , and . Each has both natural sources and sources created or increased by human activity.
Why They Produce the Greenhouse Effect
- Earth’s surface absorbs incoming solar radiation and emits energy mainly as infrared radiation.
- Greenhouse-gas molecules have quantized molecular energy levels. Infrared radiation at suitable frequencies can be absorbed by exciting molecular rotational and vibrational modes.
- The molecules subsequently emit infrared radiation in all directions. Some is directed back toward Earth’s surface, reducing the rate at which energy escapes to space.
- Earth’s surface and atmosphere therefore reach a higher equilibrium temperature than they would without these gases.
| Gas | Important sources and role |
|---|---|
| Methane, | Produced naturally in wetlands and by decomposition; human-related sources include livestock, rice farming, landfill, and fossil-fuel extraction. |
| Water vapor, | Enters the atmosphere mainly through evaporation. Its concentration generally increases as temperature rises, producing a positive feedback. |
| Carbon dioxide, |
A common misconception is that greenhouse gases mainly block incoming sunlight. In fact, their key role is absorbing and re-emitting Earth’s outgoing infrared radiation. Another misconception is that the greenhouse effect and ozone depletion are the same process; they are distinct.
IB Exam Technique
This is SL and HL content in B.2. For a State question, list the four gases and formulas. For an Explain question, link infrared absorption at specific molecular energy levels to re-emission in all directions and a higher equilibrium temperature. Distinguish the natural greenhouse effect from the enhanced greenhouse effect, which results from human activities.