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Title: MET%20112%20Global%20Climate%20Change%20-%20Lecture%202


1
MET 112 Global Climate Change - Lecture 2
  • The Earths Energy Balance
  • Dr. Craig Clements
  • San José State University
  • Outline
  • Energy
  • Solar and terrestrial radiation
  • Absorption and scattering
  • Global energy balance

2
Review Why is CO2 So Important?
  • Carbon Dioxide is a greenhouse gas.
  • Greenhouse gases are those gases that cause the
    greenhouse effect.
  • The greenhouse effect makes a planets surface
    temperature warmer than it would otherwise be.
  • The stronger the greenhouse effect, the warmer
    the surface (other factors being equal).
  • Consider the blanket analogy

3
Earths Energy Balance
  • Energy entering top of atmosphere
  • Energy entering the Earths surface

Energy leaving top of atmosphere
Energy leaving Earths surface
Conservation of Energy
4
There are three modes of energy transmission in
the atmosphere.
Energy transmission
  • Conduction the transfer of energy in a
    substance by means of molecular excitation
    without any net external motion.
  • Convection the transfer of energy by mass
    motions within a fluid or gas, resulting in
    actual transport of energy.
  • Radiation the propagation of electromagnetic
    waves through space.

5
Conduction
6
Convection
7
Electromagnetic radiation
l
  • Radiation is the transfer of energy by rapid
    oscillations of electromagnetic fields.
  • The most important general characteristic is its
    wavelength (?), ______________________________.
  • Radiation travels through space at the speed of
    light
  • (3 x 108 m s-1).

Defined as the crest-to-crest distance
8
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9
Radiation
  • What emits radiation?
  • All objects with a temperature greater than 0K
    emit some type of radiation (energy)
  • Examples
  • Radiation laws
  • Warmer objects emit more intensely than cold
    objects. (Stefan-Boltzmann Law)
  • Warmer objects emit a higher proportion of their
    energy at short wavelengths than cold objects.
    (Wiens Law)

10
Review questions
  • Considering the previous discussion
  • Which object would emit more (intensity)
    radiation Earth or Sun?
  • If you were examining the radiation emitted by
    both the Sun and Earth, which would have a longer
    wavelength?
  • What wavelength radiation are you emitting right
    now?

Sun
Earth
infrared
11
Solar Radiation (Sunlight)
  • Sunlight is primarily made up of the following
  • Visible Light (44)
  • Infrared Radiation (48)
  • Ultraviolet Radiation (7)

Unit 1 ?m 0.000001 m
12
Terrestrial or Longwave Radiation
  • Planets mainly emit
  • infrared radiation
  • Radiation emitted by planets occurs mainly at
    wavelengths _____
  • than those contained in solar radiation

Terrestrial Radiation (Longwave)
Solar Radiation (Shortwave)
longer
13
Solar vs. Terrestrial Radiation
  • The sun is much hotter than planets therefore,
    sunlight consists of shorter wavelengths than
    planetary radiation
  • Thus

14
Energy from the Sun
  • Obviously, the Sun provides the Earth with its
    energy. The question is, how much of the Suns
    energy does the Earth get?
  • Suns energy is either
  • Scattered (reflected away) or
  • Absorbed
  • Scattering happens by bouncing off
  • Particles in the atmosphere
  • Earths surface
  • Absorption happens when certain gases absorb the
    energy
  • The reality is the only certain gases absorb
    certain wavelengths.

15
Absorption of radiation
  • Absorption of shortwave radiation by atmospheric
    gas molecules is fairly weak
  • most absorption of shortwave radiation occurs at
    the Earths surface.
  • Most gases do not interact strongly with longwave
    radiation, however
  • Greenhouse gas molecules absorb certain
    wavelengths of longwave radiation.

16
Absorption of Radiation in the Earths Atmosphere
17
Incoming solar radiation
  • Each beam of incoming sunlight can be either
  • Reflected back to space
  • Clouds
  • Atmosphere
  • Surface
  • Or absorbed either by atmosphere (e.g. clouds or
    ozone) or Earths surface.

Albedo
18
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19
Recap
  • ______________ radiation comes from the sun and
    is composed of both ultraviolet and visible
    radiation
  • __________________ radiation comes from the Earth
    and is composed of infrared radiation
  • Recall that everything (above a temperature of
    0K) emits some type of radiation (energy) with a
    particular wavelength.

20
Recap
Shortwave or solar
  • ______________ radiation comes from the sun and
    is composed of both ultraviolet and visible
    radiation
  • __________________ radiation comes from the Earth
    and is composed of infrared radiation
  • Recall that everything (above a temperature of
    0K) emits some type of radiation (energy) with a
    particular wavelength.

Longwave, terrestrial or infrared
21
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22
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23
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24
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25
Review - sensors that measure radiation
Pyranometer
  • A _________________ measures solar radiation.
  • A__________________ measures infrared radiation
    (terrestrial) that comes from the Earth.

Pyrgeometer
26
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27
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28
Some atmospheric radiation escapes to space
Some surface radiation escapes to space
Greenhouse gases emit longwave upward and downward
Most outgoing longwave is absorbed in atmosphere
(by greenhouse gases)
Some atmospheric radiation is absorbed at the
surface
Longwave radiation is emitted from surface.
29
Greenhouse Effect
  • Sequence of steps
  • Solar radiation absorbed by earths surface.
  • Earth gives off infrared radiation.
  • Greenhouse gases absorb some of the Earths
    infrared radiation.
  • Greenhouse gases (water and CO2) give off
    infrared radiation in all directions.
  • Earth absorbs downward directed infrared
    radiation
  • Result warmer surface temperature

30
Energy Balance
  • Assume that the Earths surface is in
    thermodynamic equilibrium
  • Thermodynamic Equilibrium
  • The flow of energy away the surface equals the
    flow of energy toward the surface

Surface
Average surface temperature 15C
31
Sudden Removal of all Greenhouse Gases
Removal of greenhouse gases would decrease
downward flow of energy now energy away from
surface is greater than energy toward surface.
32
Sudden Removal of all Greenhouse Gases
Removal of greenhouse gases would decrease
downward flow of energy now energy away from
surface is greater than energy toward surface.
Thus, average surface temperature starts to
decrease.
33
Sudden Removal of all Greenhouse Gases
As surface cools, emission of radiation decreases
until balance is restored. At this point, cooling
stops
34
Result A Very Cold Planet!
As surface cools, emission of radiation decreases
until balance is restored. At this point, cooling
stops and equilibrium is restored.
Average surface temperature -18C
35
Earths Greenhouse Effect
  • Without the greenhouse effect, the surface
    temperature of Earth would be
  • Way Cold (-18C)
  • Greenhouse gases play an important role in
    shaping climate.
  • More GHGs warmer climate
  • Less GHGs cooler climate

36
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37
Classwork 1-1
  • Start with the following diagram and assume the
    earths surface temperature is 15C and that the
    atmosphere has greenhouse gases.
  • Imagine that the concentrations of greenhouse
    gases were to increase by 50. 1. Draw two more
    diagrams illustrating (with arrows) how the
    energy balance would change with the increase in
    greenhouse gases and explain why. 2. How would
    the average surface temperature change?

Surface
Average surface temperature 15C
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