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MET 112 Global Climate Change - Lecture 2
The Earth’s Energy Balance
Dr. Eugene Cordero
San Jose State University
Outline




Energy
Solar and terrestrial radiation
Absorption and scattering
Global energy balance
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Earth’s Energy Balance
 Energy entering top of atmosphere
= Energy leaving top of atmosphere
 Energy entering the Earth’s surface
= Energy leaving Earth’s surface
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Energy transmission
There are three modes of energy transmission
in the atmosphere.
 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.
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Convection
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Electromagnetic radiation

 Radiation is the transfer of energy by rapid oscillations
of electromagnetic fields.
 The most important general characteristic is its
Defined as the crest-to-crest distance
wavelength (), ____________________________.
 Radiation travels through space at the speed of light (3
x 108 m s-1).
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MET 112 Global Climate Change
Radiation
 What emits radiation?
– All objects with a temperature greater than 0°K 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.
(Wien’s Law)
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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?
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Review questions
 Considering the previous discussion
– Which object would emit more (intensity) radiation:
Earth or Sun?
Sun
– If you were examining the radiation emitted by both
the Sun and Earth, which would have a longer
wavelength?
Earth
– What wavelength radiation are you emitting right
now?
infrared
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MET 112 Global Climate Change
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
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Terrestrial or Longwave Radiation
 Planets mainly emit
infrared radiation
 Radiation emitted by
planets occurs
mainly at
longer
wavelengths _____
than those
contained in solar
radiation
Solar Radiation
(“Shortwave”)
Terrestrial
Radiation
(“Longwave”)
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MET 112 Global Climate Change
Solar vs. Terrestrial Radiation
 The sun is much
hotter than planets;
therefore, sunlight
consists of shorter
wavelengths than
planetary radiation;
 Thus …
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Energy from the Sun
 Obviously, the Sun provides the Earth with it’s energy.
The question is, how much of the Sun’s energy does
the Earth get?
 Sun’s energy is either
– Scattered (reflected away) or
– Absorbed
 Scattering happens by bouncing off
– Particles in the atmosphere
– Earth’s surface
 Absorption happens when certain gases absorb the
energy
– The reality is the only certain gases absorb certain
wavelengths. MET 112 Global Climate Change
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Absorption of radiation
 Absorption of shortwave radiation by
atmospheric gas molecules is fairly weak;
– most absorption of shortwave radiation
occurs at the Earth’s surface.
 Most gases do not interact strongly with
longwave radiation, however
– Greenhouse gas molecules absorb certain
wavelengths of longwave radiation.
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MET 112 Global Climate Change
Absorption of
Radiation in the
Earth’s
Atmosphere
Incoming solar radiation
 Each ‘beam’ of incoming sunlight can be either:
– Reflected back to space: Albedo
 Clouds
 Atmosphere
 Surface
– Or absorbed; either by atmosphere (e.g.
clouds or ozone) or Earth’s surface.
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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.
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MET 112 Global Climate Change
Recap
or solar radiation comes from the sun
Shortwave
______________
and is composed of both ultraviolet and visible
radiation
Longwave, terrestrial or infrared
 __________________ 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.
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MET 112 Global Climate Change
Some surface
radiation
escapes to
space
Most
outgoing
longwave is
absorbed in
atmosphere
(by
greenhouse
gases)
Longwave radiation is
emitted from surface.
Some
atmospheric
radiation
escapes to space
Greenhouse
gases emit
longwave
upward and
downward
Some
atmospheric
radiation is
absorbed at the
surface
Greenhouse Effect
Sequence of steps:
1. Solar radiation absorbed by earth’s surface.
2. Earth gives off infrared radiation.
3. Greenhouse gases absorb some of the Earth’s infrared
radiation.
4. Greenhouse gases (water and CO2) give off infrared
radiation in all directions.
5. Earth absorbs downward directed infrared radiation
Result: warmer surface temperature
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MET 112 Global Climate Change
Energy Balance
 Assume that the
Earth’s 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 = 15°C
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MET 112 Global Climate Change
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.
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MET 112 Global Climate Change
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.
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MET 112 Global Climate Change
Sudden Removal of all Greenhouse
Gases
As surface cools, emission
of radiation decreases until
balance is restored. At this
point, cooling stops
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MET 112 Global Climate Change
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 = -18°C
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MET 112 Global Climate Change
Question
 Start with the following diagram and
assume the earth’s 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 = 15°C
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MET 112 Global Climate Change
Earth’s Greenhouse Effect
 Without the greenhouse effect, the surface
temperature of Earth would be
– Way Cold (-18°C)
 Greenhouse gases play an important role in
shaping climate.
– More GHGs – warmer climate
– Less GHGs – cooler climate
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Activity 2
1. What percentage of the sun’s radiation is
a) absorbed by the Earth’s surface?
b) absorbed by the atmosphere
c) reflected out to space?
2. What percentage of the energy received by the earth’s surface
comes directly from greenhouse gas emissions?
3. If the sun’s radiation was to increase by 10%, how would the
following energy units change (increase, decrease or stay the
same)
a) Energy gained by the Earth’s surface.
b) Energy lost by the Earth’s surface.
c) Energy emitted by greenhouse gases.
d) Energy lost to space.
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MET 112 Global Climate Change
Activity
Draw a diagram showing how the
earth is heated by the Sun.
Include arrows and/or lines to
indicate the ‘Energy Balance” of the
earth.
Energy Balance – Energy coming in
and energy going out.
In your diagram, include how clouds
absorb, reflect and emit energy.
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