The Greenhouse Effect

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The Greenhouse Effect
Lisa Goddard goddard_at_iri.columbia.edu
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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 directions. (From Classical
Electrodynamics, by J. D. Jackson)
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Main Points

• Energy balance InOut (in equilibrium)
• Greenhouse Effect Difference betweensurface
  temperature/radiation Earths effective
  temperature/radiation

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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 RadiationCheck out how Planck
distributions evolve with temperature

• Planck Function, spectrum, and color
• http//cs.clark.edu/mac/physlets/BlackBody/blackb
  ody.htm
• BlackBody, The Game!
• http//csep10.phys.utk.edu/guidry/java/blackbody/b
  lackbody.html
• Planck Law Radiation Distributions
• http//csep10.phys.utk.edu/guidry/java/planck/plan
  ck.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 latitude,
reducing the energy per unit area from equator to
pole. What happens at the pole?
The effect of the tilting earth surface is
equivalent to the tilting of the light source
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Blackbody Equilibrium(Energy Conservation)

• Energy In Energy Out

EmittedEarthlight 4pR2Earth x SEarth
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Why is Earth visible from space?
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Blackbody Equilibrium(Energy Conservation)

• Energy In Energy Out

Consider albedo ?
EmittedEarthlight 4pR2Earth x SEarth
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Reflection of Solar Radiation The Earths Albedo

• 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.

Components of the Earths albedo and their value
in and the processes that affect incoming solar
radiation in the Earths atmosphere
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Blackbody Equilibrium

• Whats missing is the atmosphere

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Greenhouse Effect
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Absorption of Infrared (Longwave) Radiation in
Earths 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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1st 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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1st Law of Thermodynamics

• dEint dQ dW

Earths atmosphere (1) Constant volume W0
(in equilibrium) (2) Sun is approx. constant
dQ 0 (although Q gt 0) (3) Therefore
dEint 0
If Earths 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 ?gt5µm)
Molecule Lifetime(years) Concentration(ppbv) Spectral Range (µm) Relative Forcing
CO2(Carbon Dioxide) 2 3.39 x 103 13.5-16.5 (center _at_ 15)also 5.2, 9.4, 10.4 1
O3 (Ozone) 0.1-0.3 variable 9.0 9.6also 5.75, 14.1
N2O(Nitrous Oxide) 120 300 7.8 17.0 206
CH4(Methane) 5-10 1700 7.7 21
CFCl3 (CFC11) 65 0.26 8 - 12 12,400
CF2Cl2 (CFC12) 110 0.54 10.5 11.4 15,800
CF3Cl (CFC13) 400 0.007 8.9 - 9.3
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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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Earths 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 Earths 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 absorbed solar radiation F(?) outgoing
infrared flux (outgoing longwave radiation,
OLR) Fah horizontal energy flux in atmos. Foh
horizontal energy flux in ocean Fv(0) atmos.
to ocean energy flux
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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
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Incoming Solar Flux (Shortwave) at TOA(TOA Top
Of Atmosphere)
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Incoming Solar Flux (Shortwave) at TOA
320
330
340
350
360 (W/m2)
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.
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Reflected Solar at TOA
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Planetary Albedo
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Earths Surface Properties as seen from Space
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Global Rainfall - a Proxy for Clouds
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Net Shortwave (Solar) Radiation(Includes albedo)
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Outgoing Longwave Radiation (OLR) at TOA
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Net Incoming Radiation
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Surface vs. TOA Longwave

• 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....

Annual mean surface outgoing IR
emissivity Natural surfaces are not perfect
black bodies. They absorb and emit only some of
the amount predicted by the Stefan-Boltzman Law.
The ratio between actual and predicted emission
is the emissivity.
Annual mean TOA outgoing IR
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Greenhouse Effect

• The difference between the longwave radiation
  from the Earths 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.

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Websites   http//yosemite.epa.gov/oar/globalwarm
ing.nsf/content/Emissions.html http//gaw.kishou.
go.jp/wdcgg.html http//www.ncdc.noaa.gov/oa/clim
ate/globalwarming.html http//icp.giss.nasa.gov/e
ducation/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)