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8/13/2019 03 Energy Balance
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Timescales
Geological time (big changes globally)
Glacial-interglacial cycles (really recent time)associated with shifting land masses and effects on
ocean circulation Human time (anthropocene)relationship to
geological forces
Rates of change
Timescales of movement of material and energy
Evolution versus extirpation versus extinction
Human-induced rates of change versus rates of naturalself-regulation
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The Earths Energy Balance
A budgeting exercise
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Over Time
Origin of earth (4.6 bybp)
Dating of extra-terrestrialmaterial
Hadean & Archaen (4.62.5 bybp)
unidirectional change inorganization ofmaterial/planetaryevolution; driven by energyfrom radioactive decay
Proterozoic &Phanerozoic (2.5 bybp to
present)
Solar energy more
important/recycling ofmaterials
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Major points
How do the various components of earth
system move energy and matter on the
earths surface?
Over long time scales the planet must be in
steady state (input = output)
In the present system, energy balance at theearths surface is driven by solar radiation
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Basic physics
Electromagnetic (EM) radiation if generally
propagated as a wave
But EM can often behave more like a
particle - photon
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c = ln
Fig. 3-2
Wave number (n) = 1/l(cycles/cm or cm-1)
(cm/cycle or cm)
Waves defined by their speed (c), wavelength l, and frequency n
Frequency and wavelength are inversely related
n= c/l
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Electromagnetic spectrum
E = hn = hc/l
At times, EM radiation behaves more like a particle - photon
High energy/high frequency/low Low energy/low frequency/high
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Flux
How energy (or any material) passes
through a unit surface area per unit time
Can think of energy as a particle
Units: some mass per unit area per unit time
mg/m2/hr (= mg m-2h-1)
mmol quanta/ m2
/hr
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Which passes through a larger unit area?
Which has the higher flux?
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Think about how that affects energy
flux with latitude on earth.
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Back to energy
Energy is expressed as Joules (J)measures heat,electricity and mechanical work
1 J = 0.239 calories
1 J = 2.7778 107 kilowatt hour
Power (the rate at which work is done or energy ismoved) is expressed in Watts (W)
1 W = 1 J/second
W/m2then is a unit of energy flux
=J/m2/s
Energy flux is important for global climate
Polar regions cooler due to lower energy flux(mass per unit area per unit time)
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Flux also depends on distance of an object or
observer from the object emitting the radiant
energy
Flux of solar energy
decreases with distance
from the sun.
Relationship is aninverse-square law
Double distance from the
sun and intensity of
radiation (or energy flux)decreases by a factor of
.
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Temperature
Measure of internal heat energy
Rate of motion of molecules in a substance
Faster movement = higher temperature
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Blackbody radiation
A body that emits radiation over all possiblefrequencies (remember inversely related to
wavelength) Wavelength distribution depends on the
temperature of the blackbody
The Planck function describes thewavelength distribution relative to radiationintensity
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Stefan-Boltzmann Law
F= sT4
Weins Law
The flux of radiation
emitted by a blackbody
reaches its peak value at
lmaxwhich depends inversely
On the bodys absolute temperature
The energy flux emitted by a blackbody
is related to the fourth power of thebodys absolute temperature
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UV Vis IR (heat)
Blackbody emission curve for sun and earthnote higher energy
shorter wavelength (higher frequency) emissions for the sun.
Energy flux and blackbody
radiation temp. are related.
Because it is hotter, sun emitsmore energy per unit area at all
wavelengths.
Changes in energy flux determine the
spectrum of the EM radiation emitted.
Shifts along the laxis.
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Energy output of the sun
Predicts a maximum of radiation emissionin the visible region (0.40.7 mm)
Large component of high energy UV(remember this is damaging)
Assume Earth is a blackbody radiator
The Earth System responds to both theamount of solar radiation and its EMspectrum
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Earths energy balance
If temperature is constant, the planet has to
be in radiative balance
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Fig. 3-19 Earths globally averaged atmospheric energy budget.
Energy input = energy output
Some incoming solar radiation is lost before reaching the earths surface (30%).
Some is returned to space as IR (70%)
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O3Plus CO2and H2O
Some incoming solar radiation is reflected by clouds
Some is absorbed in the atmosphere (ozone at low l; CO2and H2O at high l)
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UV Vis IR (heat)
O3 H2O, CO2
Fig. 3-8 Blackbody emission curves for the Sun and Earth.
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O3Plus CO2and H2O
Only 50% if incoming solar radiation gets through the atmosphere to the Earths
surface
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Blackbody radiation ofthe sun minus that lost
in the atmosphere.Solar spectrum clipped
at high and low endsEnergy loss shiftslmaxto lower wavelengthswhen energy is re-radiated by the Earth.Reradiated as IR
(heat).
Net effect of atmosphere on incoming and outgoing radiation
Shift in lmax
controlled
by Weins law
Assumes
Earth is a
blackbodyradiator.
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CO2andH2O
Outgoing IR also absorbed by greenhouse
gases.
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Greenhouse gas absorption
Major role in warming atmosphere
Traps heat
Leads to re-distribution of radiation before
it is re-radiated to space
Without greenhouse gases, planet would be
much colder (~ -20oC)
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Feedback loops
Feedback between lithosphere, hydrosphere,
atmosphere and biosphere interact to
maintain relatively constant, favorabletemperatures over most of geologic history
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Greenhouse effect
Planetary energy balance in the absence of agreenhouse
Simple model
Planet with no atmosphere and albedo that of Earth Energy emitted by Earth= energy absorbed by Earth
From distance of sun, Earth is a circle (projected area)
Energy absorbed = energy inputreflected light
Albedo is the average color of the planet Low albedo = dark color = absorbs heat (warms up)
High albedo = light color = high reflectance (cools down)
Assuming blackbody radiation, Stefan-Boltzman lawpredicts the temperature of the atmosphereless planet
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Energy absorbed=Energy inputReflected energy
=REarth2 SREarth
2 SA =REarth
2 S(1A)
Energy emitted= 4REarth2
sTe4
Energy emitted=Energy absorbed
sTe4=
S
4(1A)
Box Fig. 3-1
Where S is solar fluxA is earths albedo
We know how to
calculate area of a circle
Apply Stefan-Boltzman Law and simplify
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High albedo = high reflectivity
Low albedo = low reflectivity(light is absorbed)
Fig. 2-6
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Energy absorbed=Energy inputReflected energy
=REarth2 SREarth
2 SA =REarth
2 S(1A)
Energy emitted= 4REarth2
sTe4
Energy emitted=Energy absorbed
sTe4=
S
4(1A)
Box Fig. 3-1
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Energy absorbed=Energy inputReflected energy
=REarth2 SREarth
2 SA =REarth
2 S(1A)
Energy emitted= 4REarth2
sTe4
Energy emitted=Energy absorbed
sTe4=
S
4(1A)
Box Fig. 3-1
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Fig. 3-2 The greenhouse effect of a one-layer atmosphere.
sT4s= (S/4)*(1-A) + sT4e
sT4s= 2sT4e
sT4e= (S/4)*(1-A)
Provides 33o
C of surface warming
Treat atmosphere as black body
Energy balance
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Fig. 3-19 Earths globally averaged atmospheric energy budget.