This Week The Greenhouse Effect

							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           (So/4)A    (1-ε) FsfOUT
                                              εFatmOUT


                   Atmosphere Tatm

         FsfIN                       FsfOUT    εFatmOUT

                    Surface Tsf
         The Greenhouse Effect

              Ttrue – T”bare rock”

           289 K – 256 K = 33 K

Definition: Absorption of terrestrial long-
wave 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
                                                         87%
   environmental problem
2. A natural phenomenon
   present on many planets
                                        13%




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The physics of the Greenhouse Effect can
    best be described by analogy to

1. A greenhouse
                                           53%
2. A solar powered water
   heater
3. Eggshells and orange                               24% 24%
   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.

vacuum       Height of fluid related to balance
             between gravity and Patm


     h
         A     B
Barometric Law—”Hydrostatic Equation”

Pressure Gradient Force

                  P(z2)
                            The atmosphere’s tendency
                            to be pulled into space is
                            balanced by gravity.
                  P(z1)

      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.
 P




                                          height
          altitude
     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%
                                           73%
 3. 90%



      15 km                          17%
                              10%
                          %




                                     %




                                            %
                         30




                                    60




                                           90
      Vertical Profiles of Temperature
                                              Mean values for 30oN, March




Temperature structure of
atmosphere is complex.




                              Altitude (km)
Regions of lower
atmosphere separated by
behavior of T with altitude
Atmospheric Structure and Composition
                    Key Points
• The atmosphere is a collection of ideal gases 
  P = ρRT

• Pressure is force/area; difference in air
  pressure will cause motion

• Air pressure and ρ 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 long-
  wave 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


                                   δ-         δ+
                                              H         δ+
                                                        H
                                   O
  δ- δ+ δ-                H             H          O
  O C O                   δ+            δ+         δ-
    δ- δ+    δ-
    OC       O
                                        H
                         δ-             δ+         δ+
                                                   H
                                                             δ+
                                                             H
                         O                   O
                                             δ-
                                                        O
                    H         H                         δ-
                    δ+        δ+
                                        H
                                        δ+
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 µm


                   1. Initial state
Addition of a GHG Absorbing at 11 µm


               2. Emission at 11 µm
               decreases (cold atmosphere)
Addition of a GHG Absorbing at 11 µm
                 3. New equilibrium:
                 total emission must be same
                 emission at other λ’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
                                       95%
 2. False




                                  5%
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                                       Fa
         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 (λ) in the outgoing radiation spectrum
                         Band Saturation
                                                       maximum possible
ability to absorb




                                 ε Fraction absorbed
                                                  1
φ




                    λ                                          λ
           Intrinsic to GHG,                           Simulated effect of
           doesn’t depend on [GHG]                     increasing [GHG] on ε
               “Emission Height”


               σTb4            σTa4
Altitude (z)




                b               Emission to space from
                                z = a carries much more
                                energy than from z = b



                                 a

                 Temperature     Ts