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Greenhouse Effect and Earth-Atmosphere Energy Balance

This lecture reviews the concepts of heat transfer, latent heat, Wien's displacement law, and Stefan-Boltzmann law. It also discusses selective absorption and emission, as well as the energy balance between the Earth and the atmosphere.

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Greenhouse Effect and Earth-Atmosphere Energy Balance

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  1. NATS 101Lecture 6Greenhouse Effect and Earth-Atmo Energy Balance

  2. Review Items • Heat Transfer • Latent Heat • Wien’s Displacement Law Ramifications • Stefan-Boltzman Law Ramifications

  3. New Business • Selective Absorption and Emission • Earth-Atmo Energy Balance

  4. Modes of Heat Transfer Williams, p. 19 Latent Heat Convection Conduction Radiation Remember this thought experiment and the incandescent light bulb demo

  5. Latent Heat Take 2 Williams, p 63 Takes energy from environment Emits energy to environment

  6. General Laws of Radiation • All objects above 0 K emit radiant energy • Hotter objects radiate more energy per unit area than colder objects, result of Stefan-Boltzman Law • The hotter the radiating body, the shorter the wavelength of maximum radiation, result ofWien’s Displacement Law • Objects that are good absorbers of radiation are also good emitters…today’s lecture!

  7. Sun’s Radiation Spectrum Planck’s Law Ahrens, Fig. 2.7 Key concept: Radiation is spread unevenly across all wavelengths

  8. Sun - Earth Radiation Spectra Ahrens, Fig. 2.8 Planck’s Law Key concepts: Wien’s Law and Stefan-Boltzman Law

  9. What is Radiative Temperature of Sun if Max Emission Occurs at 0.5 m? • Apply Wien’s Displacement Law

  10. How Much More Energy is Emitted by the Sun than the Earth? • ApplyStefan-Boltzman Law

  11. Radiative Equilibrium • Radiation absorbed by an object increases the energy of the object. • Increased energy causes temperature to increase(warming). • Radiation emitted by an object decreases the energy of the object. • Decreased energy causes temperature to decrease(cooling).

  12. Radiative Equilibrium (cont.) • When the energy absorbed equals energy emitted, this is calledRadiative Equilibrium. • The corresponding temperature is theRadiative Equilibrium Temperature.

  13. Why Selective, Discrete Absorption/Emission? Life as we perceive it: A continuous world! Atomic perspective:A quantum world! Gedzelman 1980, p 103

  14. Energy States for Atoms Gedzelman 1980, p 104 Hydrogen Atom Electrons can orbit inonly permitted states A state corresponds tospecific energy level Onlyquantum jumpsbetween states Intervals correspond tospecific wavelengths

  15. Energy States for Molecules Molecules can alsorotate, vibrate, librate But only atspecific energy levels or frequencies Quantum intervals between modes correspond tospecific wavelengths Gedzelman 1980, p 105 H2O molecule H2O Bands

  16. Selective Absorption The Bottom Line Each molecule has aunique distribution of quantum states! Each molecule has a unique spectrum of absorption and emission frequencies of radiation! H2O molecule Williams, p 63

  17. Absorption Visible IR Visible (0.4-0.7 m) is absorbed very little O2 an O3 absorb UV (shorter than 0.3 m) Infrared (5-20 m) is selectively absorbed H2O & CO2 are strong absorbers of IR Little absorption of IR around 10 m – atmospheric window Ahrens, Fig. 2.9

  18. Visible radiation (0.4-0.7 m) is not absorbed Infrared radiation (5-20 m) is selectively absorbed, but there is an emission window at 10 m Total Atmospheric Absorption Ahrens, Fig. 2.9

  19. Simple Example of the Greenhouse Effect(0% Solar absorbed, 100% IR absorbed) Radiative Equilibrium 1 Unit Outgoing IR to Space 1 Unit Incoming Solar 1/2 1/4 1/8 1/16 ½ emitted to space ½ emitted to ground 1/16 1 1/2 1/4 1/8 2 Units IR Emitted by Ground Take Home Point: Surface is warmer with selectively absorbing atmosphere than it would be without it.

  20. Global Solar Radiation Balance (Not all Solar Radiation SR reaches the surface) 30% SR reflects back to space Albedo: percent of total SR reflected ~20% absorbed by atmosphere 70% SR absorbed by earth-atmosphere Ahrens, Fig. 2.13 ~50% SR absorbed by surface

  21. Atmosphere Heated from Below Ahrens, Fig. 2.11 old ed. Air above ground heats by convection and absorption of some IR from ground Net Effect: Atmosphere is Heated From Below Air contacting ground heats by conduction Ground heats further through absorption of IR from atmosphere Solar radiation heats the ground

  22. Global Atmo Energy Balance Ahrens, Fig. 2.14 Solar Atmosphere Ground

  23. Summary • Greenhouse Effect (A Misnomer) SFC Warmer than Rad. Equil. Temp Reason: selective absorption of air H2O and CO2 most absorbent of IR • Energy Balance Complex system has a delicate balance All modes of Heat Transfer are important

  24. Assignments for Next Lectures • Ahrens (next lecture) Pages 42-50 Problems 2.15, 2.16, 2.18

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