Energy Balance

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Transcript Energy Balance

MET 112 Global Climate Change - Lecture 2
The Earth’s Energy Balance
Dr. Craig Clements
San José State University
Outline




Energy
Solar and terrestrial radiation
Absorption and scattering
Global energy balance
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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 planet’s
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
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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
Conservation of Energy
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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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Conduction
Convection
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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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?
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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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
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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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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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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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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Review - sensors that measure radiation
Pyranometer
 A _________________
measures solar
radiation.
Pyrgeometer
 A__________________
measures infrared
radiation (terrestrial) that comes from the Earth.
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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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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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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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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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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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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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Homework 1-1
 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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Homework 1-2
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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