Module 03: How Climate Works


Within the Earth system, vertical layering is critical to understanding the energy flows that define our climate. The surface is warmed by absorbing sunshine during daytime, yet receives much more downward radiation from the overlying warm air during both day and night. The surface cools mostly by emission of infrared radiation, and to a lesser extent by evaporation of surface water and through direct warming of surface air. The atmosphere is warmed from below by absorbing the upwelling infrared form the surface and also by condensing water as cloud droplets. A small amount of atmospheric warming is contributed by rising “thermals:” updrafts driven by buoyancy. 

Tropical regions receive much more radiant energy from the Sun and atmosphere than they emit and the poles receive much less than they emit. Lateral motions of the air (wind) and oceans (currents) transport excess energy as heat from the tropics to the poles. These transports account for nearly all weather on Earth and maintain a habitable climate almost everywhere. The Earth’s surface climate is determined by a combination of all these vertical and lateral energy flows.

Learning Objectives

Upon completion of this module you should be able to: 

  1. Apply the principle of energy conservation to planets to estimate their effective radiative temperature, including atmosphere.
  2. Describe how molecular structure gives rise to selective transparency in planetary atmospheres.
  3. Explain the role of selective transparency of gases in partitioning the upwelling and downwelling of radiation in Earth’s atmosphere.
  4. Enumerate the vertical flows of energy among the solid Earth, atmosphere, and outer space.
  5. Describe geographic variations of absorbed solar radiation, outgoing longwave radiation, and net radiation in the annual mean.
  6. Explain the role of air and ocean circulation in carrying energy across geographic regions.
  7. Enumerate the components of Earth’s surface energy budget, including their relative magnitudes and geographic variations.


Slide Deck / Lecture

Review Questions

  1. Explain the greenhouse effect and how it works in the Earth’s climate system.
  2. We discussed a “layer model” of the greenhouse effect in which visible light passes through glass to warm a dark surface. When extra glass layers are added, the surface warms up. Why?
  3. Use the difference between reflection and emission to explain surface albedo vs emission of thermal infrared radiation from the surface. 
  4. Name two important “greenhouse gases” in Earth’s atmosphere.  What makes a gas a strong greenhouse gas? 
  5. What makes the Earth’s atmosphere “selectively transparent” to different wavelengths of radiation? Over what broad range of wavelengths is the air transparent? Over what broad ranges of wavelengths is it more opaque?
  6. The outgoing infrared radiation above the atmosphere at 14 micron wavelength is about the same that would be emitted by a blackbody at 220 Kelvin (around -50 Celsius). Why?
  7. Explain how the greenhouse effect depends on the variation of temperature with altitude in Earth’s atmosphere.
  8. Explain how vertical mixing of the air by thermal convection affects the strength of the greenhouse effect.
  9. What are the main ways that Earth’s surface gains heat energy (warms up)? What are the main ways Earth’s surface loses energy (cools off)?
  10. During the last Ice Age, the concentration of atmospheric CO2 was only about 200 parts per million (ppm). Now it’s about 400 ppm. This means the atmosphere absorbs 3.7 Watts m-2 more outgoing radiation than it did then. At what concentration would it absorb an additional 3.7 Watts m-2 compared to now?
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