This Week: The Greenhouse Effect
• Reading: Continue Chapter 3
• Problem Set 2 Due in Discussion Fri
This Week: The Greenhouse Effect (GHE)
• Atmospheric structure, composition, and
absorptivity
• Which gases contribute to the GHE and
why are some better than others?
• What are the major sources of GHG to
the atmosphere?
1-Layer Model of the Greenhouse Effect
(So/4)A
So/4
(1-) FsfOUT
FatmOUT
Atmosphere Tatm
FsfIN
FsfOUT
Surface Tsf
FatmOUT
The Greenhouse Effect
Ttrue – T”bare rock”
289 K – 256 K = 33 K
Definition: Absorption of terrestrial longwave radiation by the atmosphere, causing
the surface T to be larger than the planet’s
emission T (as determined from absorbed
solar radiation flux).
The Greenhouse Effect is a
1. Human-induced
environmental problem
2. A natural phenomenon
present on many planets
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The physics of the Greenhouse Effect can
best be described by analogy to
1. A greenhouse
2. A solar powered water
heater
3. Eggshells and orange
peels in Earth’s energy
drain
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Earth’s Atmosphere
Measures of Composition
Physical Characteristics
•Pressure
•Temperature
•Regions
Earth’s Atmosphere
•Thin collection of mainly gases
and some condensed phases
•Extends from Earth’s surface to
about 100 Km.
•Primary components (% by volume)
•N2 (78%)
•O2 (21%)
•Argon (0.9%)
•H2O vapor (0.00001 – 4%)
•CO2 (0.038%)
•Many trace and ultra-trace
components
Measuring Atmospheric Pressure
Patm at pt A and B is the same.
Height of fluid related to balance
between gravity and Patm
vacuum
h
A
B
Barometric Law—”Hydrostatic Equation”
Pressure Gradient Force
P(z2)
P(z1)
The atmosphere’s tendency
to be pulled into space is
balanced by gravity.
Gravity
Are these two forces always in balance?
Announcements
• Office Hours Today
– 4-5pm in 506 ATG
– 5-6pm in 406 ATG
• Go to Focus the Nation on Thursday
• JISAO lectures (see course website)
Today
• Review Pressure vs Altitude
• Temperature vs Altitude
• Atmospheric Absorptivity, key players
Pressure Decreases Exponentially w/Altitude
Gases (air) are compressible
fluids unlike liquids.
altitude
height
P
An exponential decay is an
exponential growth in reverse
“Compressible” bricks of
air stacked on each other
Vertical Profiles of Pressure
Mean values for 30oN, March
Pressure decreases exponentially
with increasing altitude.
-”air gets thinner as you go up”
1 hPa = 1 mbar ~ 0.001 atm
ln(P) is a straight line when
plotted vs. altitude.
What fraction of the atmosphere’s mass is
below 15 km?
1. 30%
2. 60%
3. 90%
73%
15 km
17%
%
90
%
60
30
%
10%
Vertical Profiles of Temperature
Mean values for 30oN, March
Regions of lower
atmosphere separated by
behavior of T with altitude
Altitude (km)
Temperature structure of
atmosphere is complex.
Atmospheric Structure and Composition
Key Points
• The atmosphere is a collection of ideal gases 
P = RT
• Pressure is force/area; difference in air
pressure will cause motion
• Air pressure and  decrease exponentially with
altitude (“air gets thinner”)
• T decreases from 0 – 15 km (troposphere),
increases from 15 – 50 km (stratosphere),
decreases again from 50 – 80 km (mesosphere)
The Greenhouse Effect (GHE)
• What gases contribute to the G.H.E.?
• What’s special about these “greenhouse
gases” (G.H.G)?
• How does adding a GHG to the
atmosphere warms the surface?
• What makes one GHG “better” than
another?
Solar and Terrestrial Emission Spectra
Assuming black bodies
What Gases are Greenhouse Gases (GHG’s)?
Greenhouse gases absorb terrestrial outgoing longwave radiation
I.e. they absorb infrared (IR) radiation
Several different gases give rise to the overall
Greenhouse Effect.
Why are only some gases GHG?
The answer lies in our analogy to charges on springs
interacting with EM radiation.
IR radiation carries enough energy to make molecules
vibrate and rotate.
Announcements
• Office Hours Today
– 4-5pm in 506 ATG
• Go to Focus the Nation on Thursday
• JISAO lectures (see course website)
Greenhouse Gases Absorb IR Radiation
Kirchoff’s law: to absorb radiation, the molecules must
be able to emit that radiation.
For gas to absorb IR radiation: must generate
oscillations in E&M fields when vibrate and rotate
+
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OC
+
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- + O C O
O
Earth Atmosphere’s Absorptivity
Absorption Spectrum
Indicates the absorptivity we
assumed in our 1-layer model
Emission Spectrum Taken From Space
Emission from cold atmosphere and warm surface
“Atmospheric Window”
Spectrum taken
over Niger valley,
N Africa
Addition of a GHG Absorbing at 11 m
1. Initial state
Addition of a GHG Absorbing at 11 m
2. Emission at 11 m
decreases (cold atmosphere)
Addition of a GHG Absorbing at 11 m
3. New equilibrium:
total emission must be same
emission at other ’s must
increase
Earth surface must heat!
Because H2O vapor absorbs the larger fraction of
OLR, reducing CO2 concentrations will not reduce
the Greenhouse Effect
1. True
2. False
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GHG Ranking Factors
1. Amount: more there is, more radiation can
be potentially absorbed
2. Ability: depends on the wavelength
3. Location: both where in the atmosphere and
where () in the outgoing radiation spectrum
Fraction absorbed
ability to absorb
Band Saturation
maximum possible


1

Intrinsic to GHG,
doesn’t depend on [GHG]

Simulated effect of
increasing [GHG] on 
“Emission Height”
Altitude (z)
Tb4
b
Ta4
Emission to space from
z = a carries much more
energy than from z = b
a
Temperature
Ts