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Environmental Science
Climate Change
How Greenhouse Gases Trap Heat
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Greenhouse gases are a natural part of Earth’s atmosphere, and they help keep the planet warm enough for liquid water and life. Sunlight mostly passes through the atmosphere and warms the surface, while the warm surface gives off infrared radiation. Gases such as carbon dioxide, methane, nitrous oxide, and water vapor absorb some of this outgoing infrared energy. Climate change occurs when human activities increase the amount of heat-trapping gases, shifting Earth’s energy balance.
Key Facts
- Incoming solar radiation is mostly shortwave light, while outgoing Earth radiation is mostly longwave infrared radiation.
- Greenhouse gases absorb and re-emit infrared radiation, sending some energy back toward Earth’s surface.
- Energy balance condition: incoming solar energy = outgoing infrared energy.
- Radiative forcing measures a change in Earth’s energy balance and is often given in W/m².
- Carbon dioxide concentration has risen from about 280 ppm before industrialization to over 420 ppm today.
- Stefan-Boltzmann law: P = σAT^4, meaning hotter objects radiate much more energy.
Vocabulary
- Greenhouse gas
- A gas in the atmosphere that absorbs and re-emits infrared radiation, helping trap heat near Earth’s surface.
- Infrared radiation
- Electromagnetic radiation emitted by warm objects, including Earth’s surface, with wavelengths longer than visible light.
- Radiative forcing
- A measure of how much a factor changes the balance between incoming solar energy and outgoing heat energy.
- Albedo
- The fraction of incoming sunlight that a surface reflects back into space.
- Carbon cycle
- The movement of carbon among the atmosphere, oceans, living things, soils, and rocks.
Common Mistakes to Avoid
- Thinking greenhouse gases block sunlight from entering, which is wrong because most incoming visible sunlight passes through the atmosphere and warms the surface.
- Confusing weather with climate, which is wrong because weather describes short-term conditions while climate describes long-term patterns over decades or more.
- Assuming carbon dioxide is unimportant because it is a small fraction of air, which is wrong because even trace gases can strongly absorb specific infrared wavelengths.
- Treating all greenhouse gases as equally powerful, which is wrong because gases differ in infrared absorption strength, atmospheric lifetime, and concentration.
Practice Questions
- 1 A surface receives 340 W/m² of average incoming solar energy and reflects 30 percent of it. How much solar energy is absorbed per square meter?
- 2 Carbon dioxide rises from 280 ppm to 420 ppm. What is the percent increase in concentration?
- 3 Explain why adding more greenhouse gases can warm Earth even though the amount of incoming sunlight stays nearly the same.