GW 3.5 Greenhouse Gases

For most solids and liquids, what is the emissivity and absorptivity like?

• Fairly smooth functions of wavelength

• They may be higher at some wavelengths and lower at others

• They usually don’t wiggle up and down much

• Usually fairly close to 1

What does absorption for gases depend on?

• Strongly depends on wavelength and the amount of gas

• In fact, gases can only absorb and emit electromagnetic radiation (light) at certain discrete energies

Why can only particular frequencies and wavelengths of light interact with particular gases?

• The energy of photons is proportional to their frequency

What causes the greenhouse effect?

• Gases in the atmosphere absorbing the radiation that Earth emits

• Earth mostly emits infrared light between 5-30 μm

  • The gases that absorb a lot of this light are the most imporatnt greenhouse gases

What is the most important greenhouse gas on Earth?

• H₂O water vapor

  • Concentration depends strongly on temperature (the hotter it is, the more humid it can be)

Why is CO2 a “very good” greenhouse gas?

• It absorbs a lot at wavelengths that Earth emits where H2O doesn’’t

What are “window” regions?

• Wavelengths where radiation emitted at Earth’s surface can make it all the way to space without being absorbed

  • This happens despite all of the greenhouse gases in Earth’s atmosphere