Module 02: Energy and Radiation


Climate is determined by a balance of energy flows into and out of the Earth at planetary scale. Most of the energy flow is accomplished by electromagnetic radiation: solar radiation heats the planet, infrared emission cools the planet. Because planets are surrounded by the vacuum of space, they can’t exchange energy with the rest of the universe by convection or conduction.

Earth’s atmosphere is selectively transparent to radiation of different wavelengths because of the presence of greenhouse gases (especially CO2 and H2O) with complex molecular structures. The selectively transparent atmosphere mediates energy flows, allowing most solar radiation to pass through but blocking the transmission of longer infrared waves emitted by the warm surface.

Learning Objectives

Upon completion of this module you should be able to: 

  1. Enumerate three mechanisms for heat transfer.
  2. Explain what determines the temperature of the Earth and how it changes.
  3. Describe the concept of blackbody radiation and its emission spectrum.
  4. Explain the relationship between the temperature of a physical object and the rate at which it radiates energy.
  5. Explain what is meant by selective transparency of different materials.

Review Questions

  1. What are three ways to transfer heat energy from place to place?
  2. Which has shorter waves: near infrared or microwaves? Ultraviolet or X-rays?
  3. Which color of light has more energy per photon: blue or red?
  4. About what percentage of the Sun’s radiation is visible light?
  5. About what fraction of the Sun’s energy that hits the Earth is absorbed at the surface?
  6. What parts of the Earth have high albedo?  What areas have low albedo?
  7. What is a “blackbody?” 
  8. If the Kelvin temperature of a blackbody doubles, by what multiple does the rate of energy emitted by the blackbody increase?


Discuss the role of molecular absorption and emission of radiation in determining the dependence of atmospheric transparency on wavelength. Why is the atmosphere nearly transparent to incoming solar radiation whereas it is nearly opaque to many wavelengths of outgoing radiation from the Earth? How can it be that trace gases such as carbon dioxide and water vapor can have such a large effect on the transparency of the atmosphere, and how can we know this?

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