Transcript Document

The Greenhouse Effect
Lisa Goddard
[email protected]
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Electromagnetic Spectrum
Sensitivity of human eyes to EM radiation
 Definition of visible spectrum
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Absorption Profile of Liquid Water
Absorption coefficient for liquid water as a function of linear frequency.
The visible region of the frequency spectrum is indicated by the vertical dashed lines.
Note that
the scales are logarithmic in both
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(From Classical Electrodynamics, by J. D. Jackson)
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Main Points
• Energy balance: In=Out (in equilibrium)
• Greenhouse Effect: Difference between
surface temperature/radiation &
Earth’s effective temperature/radiation
OUTLINE
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•
Blackbody Radiation
Planetary energy balance
Greenhouse Effect
Modelling energy balance
A view of Earth’s radiation balance from space
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Blackbody: Definition
A blackbody is a hypothetical body made up of molecules that
absorb and emit electromagnetic radiation in all parts of the
spectrum
– All incident radiation is absorbed (hence the term black), and
– The maximum possible emission is realized in all wavelength
bands and in all directions
In other words…
A blackbody is a perfect absorber and perfect
emitter of radiation with 100% efficiency at all
wavelengths
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Planck Function &
Blackbody Radiation
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Note logarithmic
scale
Blackbody emission curves for the Sun and Earth.
The Sun emits more energy at all wavelengths.
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Fun with BB Radiation
Check out how Planck distributions evolve with
temperature
• Planck Function, spectrum, and color
• http://cs.clark.edu/~mac/physlets/BlackBody/blackbody.htm
• BlackBody, The Game!
• http://csep10.phys.utk.edu/guidry/java/blackbody/blackbody.html
• Planck Law Radiation Distributions
• http://csep10.phys.utk.edu/guidry/java/planck/planck.html
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Blackbody Equilibrium
(Energy Conservation)
Energy In
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Effect of latitude on solar flux
2
1
The solar flux of beam 1 is equal to that of beam 2.
However, when beam 2 reaches the Earth it spreads over
an area larger than that of beam 1. The ratio between the
areas (see figure above) varies like the inverse cosine of The effect of the tilting earth surface
latitude, reducing the energy per unit area from equator is equivalent to the tilting of the
light source
to pole. What happens at the pole?
Blackbody Equilibrium
(Energy Conservation)
Energy In = Energy Out
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Emitted
“Earthlight” 11
4πR2Earth x SEarth
Why is Earth visible from space?
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Blackbody Equilibrium
(Energy Conservation)
Energy In = Energy Out
Consider albedo

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Emitted
“Earthlight” 13
4πR2Earth x SEarth
Reflection of Solar Radiation:
The Earth’s Albedo
Components of the Earth’s albedo and their value in % and the
processes that affect incoming solar radiation in the Earth’s
atmosphere
•The ratio between
incoming and
reflected radiation
at the top of the
atmosphere (TOA)
is referred to as the
planetary albedo.
•The albedo varies
between 0 and 1.
Blackbody Equilibrium
• What’s missing is the atmosphere
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Greenhouse Effect
Incoming
solar radiation
Reflection
Emission from atmos.
Emission from atmos.
Transmission
Emission from surface
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Absorption of Infrared (Longwave)
Radiation in Earth’s Atmosphere
Absorption of 100% means that no radiation penetrates the atmosphere. The nearly complete absorption
of radiation longer than 13 micrometers is caused by absorption by CO2 and H2O. Both of these gases
also absorb solar radiation in the near infrared (wavelengths between about 0.7 μm and 5 μm).
The absorption feature at 9.6 micrometers is caused by ozone.
(From data originally from R. M. Goody and Y. L. Yung, Atmospheric Radiation, 2nd ed., New York: Oxford University Press, 1989, Figure 1.1.)
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st
1
Law of Thermodynamics
dEint = dQ – dW
The internal energy Eint of a system tends to increase if
energy is added as heat Q and tends to decrease if energy
is lost as work W done by the system.
The First Law of Thermodynamics: Four Special Cases
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st
1
Law of Thermodynamics
dEint = dQ – dW
Earth’s atmosphere:
(in equilibrium)
(1) Constant volume: W=0
(2) Sun is approx. constant
dQ = 0 (although Q > 0)
(3) Therefore: dEint = 0
If Earth’s [effective] temperature is constant (dE = 0) then how
does surface temperature increase?
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Some general properties of absorption
by greenhouse gases (for λ>5μm)
Molecule
Lifetime
(years)
Concentration
Spectral Range
(ppbv)
(μm)
Relative
Forcing*
CO2
2
3.39 x 103
13.5-16.5 (center @ 15)
also 5.2, 9.4, 10.4
O3
0.1-0.3
variable
9.0 & 9.6
also 5.75, 14.1
N2O
120
300
7.8 & 17.0
206
CH4
5-10
1700
7.7
21
CFCl3
65
0.26
8 - 12
12,400
CF2Cl2
110
0.54
10.5 – 11.4
15,800
CF3Cl
400
0.007
8.9 - 9.3
(Carbon Dioxide)
(Ozone)
(Nitrous Oxide)
(Methane)
(CFC11)
(CFC12)
(CFC13)
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Radiative Transfer Processes
Visible (incoming solar radiation)
– absorption by air molecules
– absorption by the earth's surface
– scattering by clouds and earth's surface
Infrared (outgoing terrestrial radiation)
– absorption/emission by air molecules
– absorption/emission by clouds
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Earth’s Globally Averaged
Atmospheric Energy Budget
All fluxes are normalized relative to 100 arbitrary units of incident radiation.
Values are approximate.
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Modeling the Earth’s
Energy Balance
• Energy balance models (Global) – Figure
3-19 from Kump et al. is essentially
schematic for global EBM
• Radiative-convective models (1-D or 2-D)
or single-column models (1-D)
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Example: Energy budget of column
of atmosphere-ocean system
S
F+(z=)
(=net solar in)
S = absorbed solar radiation
Atmosphere
F+() = outgoing infrared flux
(outgoing longwave radiation, OLR)
Fah
Fah = horizontal energy flux in atmos.
Foh = horizontal energy flux in ocean
Ocean
Fv(z=0)
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Fv(0) = atmos. to ocean energy flux
Foh
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Radiation Balance
The annual mean, average around latitude circles, of the balance between the solar radiation absorbed
at the ground (in blue) and the outgoing infrared radiation from Earth into space (in red).
The two curves must balance completely over the entire globe, but not at every single latitude.
In the tropics, there is an access of radiation (solar radiation absorbed acceeds outgoing terrastrial radiation)
in middle and high latitudes all the way to the poles, there is a deficit (Earth is radiating into space more
than it receives from the sun). The atmosphere and ocean systems are forced to move about by this
imbalance, and bring heat by convection and advection from equator to the poles.
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Earth Radiation Budget
from Space: the Spatial
Pattern
Incoming Solar Flux (Shortwave) at TOA
(TOA = Top Of Atmosphere)
December
March
June
September
Incoming Solar Flux (Shortwave) at TOA
320
330
January
April
July
October
December
The globally-averaged, monthly values
of incoming solar radiation at the top of
the atmosphere showing the changes
due to the change in the distance
between the Earth and the Sun.
340
350
360 (W/m2)
Reflected Solar at TOA
December
March
June
September
Planetary Albedo
December
March
June
September
Earth’s Surface Properties as seen from
Space
Global Rainfall - a Proxy for Clouds
Net Shortwave (Solar) Radiation
(Includes albedo)
December
March
June
September
Outgoing Longwave Radiation (OLR) at TOA
December
March
June
September
Net Incoming Radiation
December
March
June
September
Surface vs. TOA Longwave
Annual mean surface outgoing IR
•From surface temperature
data we can calculate the
surface outgoing longwave
radiation by using the
Stefan-Boltzmann law and
by assuming emissivity* of
0.95
•Compare this with the
outgoing logwave radiation
at the top of the
atmosphere....
* emissivity: Natural surfaces are not perfect
black bodies. They absorb and emit only some
of the amount predicted by the StefanBoltzman Law. The ratio between actual and
predicted emission is the emissivity.
Annual mean TOA outgoing IR
Greenhouse Effect
The difference between the longwave radiation from the Earth’s surface and OLR
is the greenhouse effect. Note the strong GH effect in areas which are dominated
by deep tropical clouds that precipitate a lot (above). These clouds reach high
into the atmosphere (more than 10 Km) where the temperature is low, thus the
radiative longwave flux from their tops is relatively small. At the same time the
surface underneath is warm and the surface emitted longwave radiation is
almost entirely trapped in the cloudy atmosphere.
Websites:
http://yosemite.epa.gov/oar/globalwarming.nsf/content/Emissions.html
http://gaw.kishou.go.jp/wdcgg.html
http://www.ncdc.noaa.gov/oa/climate/globalwarming.html
http://icp.giss.nasa.gov/education/methane/intro/greenhouse.html
http://www.rmi.org/sitepages/pid340.php
http://www.agu.org/eos_elec/99148e.html (Vol. 80, No. 39, September 28, 1999, p.
453)
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