Greenhouse gases absorb specific wavelengths of outgoing infrared radiation and re-emit the energy in all directions, including toward Earth’s surface. Some aerosols also absorb and emit radiation, but their net climatic effect depends on composition.
This is core SL and HL content from 6.1 Introduction to the atmosphere.
The Mechanism
- The Sun supplies mainly short-wave radiation, much of which passes through the atmosphere and warms Earth’s surface.
- The warmed surface emits long-wave infrared radiation.
- Greenhouse gases, including carbon dioxide, methane, water vapour and nitrous oxide, absorb particular infrared wavelengths, increasing molecular vibration or rotation.
- Excited molecules transfer energy through collisions or re-emit infrared randomly. Some travels toward the surface, reducing the rate at which energy escapes to space.
- This maintains the natural greenhouse effect, which keeps Earth warmer than it would otherwise be. Higher greenhouse gas concentrations strengthen the process, producing the enhanced greenhouse effect and global warming.
| Atmospheric component | Interaction with radiation | Typical climatic effect |
|---|---|---|
| Greenhouse gases | Selectively absorb and re-emit outgoing infrared radiation | Warming |
| Black carbon aerosol | Absorbs radiation, heats the atmosphere and emits thermal infrared | Usually warming |
| Sulfate aerosol | Mainly scatters and reflects incoming solar radiation | Usually cooling |
Aerosols therefore do not all behave like greenhouse gases. Their effects vary with composition, altitude, concentration and cloud interactions. For example, sulfate aerosols from the 1991 Mount Pinatubo eruption reflected sunlight and temporarily cooled global temperatures.
A common misconception is that greenhouse gases “reflect” infrared like a mirror. They instead absorb and re-emit it. Another misconception is that the greenhouse effect is inherently harmful. The natural effect supports life, while anthropogenic enhancement drives warming and creates unequal impacts on vulnerable societies.
Exam Technique
For an explain question, present the complete causal sequence and distinguish the natural from enhanced greenhouse effect. Do not state that all aerosols warm.