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The atmosphere

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					Earth’s Atmosphere

                             Composition
                           Chief constituents of
                           Earth’s atmosphere:
                             nitrogen (78%)
                              oxygen (21%)


plus smaller amounts of argon (almost 1%),
CO2, Ne, He, CH4, Kr, H, N2O, Xe,
ozone

plus water vapor (0-4%)
                                                   1
                                Layers of the Atmosphere


                             Home of the ozone ―layer‖ — a concentration
          Stratosphere       of ozone molecules (just a few parts per
                             million).
                                                                  the tropopause
           Troposphere
                             Where humans live; home of all weather.
NOT TO SCALE

               Contains ~80% of atmosphere’s mass (i.e., air molecules).
               Temperature drops ~3.5°F per 1000’ of elevation gain.
                 When it’s 30°F and snowing on Mt. Hamilton (4209’),
                 it’s 44°F and drizzling in downtown San José (~100’).


                                                                               2
       The ―composition‖ of solar radiation

2% ultraviolet (UV)   49% visible   49% infrared (IR)




                                                        3
   Solar radiation energizes Earth, which also
                     radiates.

                                             ―Insolation‖ is an
   Incoming solar radiation = Insolation       example of a
                                               portmanteau.

                                49% infrared
                                49% visible
                  Earth          2% ultraviolet




Some insolation is reflected by Earth’s surface or
atmosphere; the rest passes through the atmosphere to
strike Earth’s surface—imparting energy to Earth.
                                                                  4
Fortunately for humans and all life on Earth, most shortwave
radiation is absorbed or reflected in Earth’s atmosphere.
                                                               5
   Earth radiates its energy as infrared radiation.

Incoming solar radiation (insolation)



                 Earth               Earth radiates strictly at infrared
                                     wavelengths (remember that the Sun
                                     radiates at visible and UV wavelengths, too).

      Earth radiates 100% infrared



  Most atmospheric gas molecules are oblivious to the longer-
  wavelength radiation from Earth, just as they are to solar radiation.
  However, molecules of certain gases—called ―greenhouse gases‖—
  absorb part of Earth’s infrared radiation.

                                                                                     6
   If Earth emitted exactly as much energy as it
absorbs from the Sun (physicists call such an object
a blackbody), its equilibrium T would be -18°C (255
                      K) = 0 °F




                                                       7
But the average T of Earth’s surface is ~15 °C (278 K) =
59 °F.


     Why?


                        the ―Greenhouse Effect‖
             some IR absorbed
             in the atmosphere
                             IR      in the atmosphere
                           emitted                     IR
                                                     emitted




           So the ―greenhouse effect‖ is a good thing.
        It makes Earth’s surface warm enough for life.         8
           The ―Greenhouse Effect‖

1. Insolation absorbed by Earth’s surface.
2. Earth emits infrared radiation.
3. Greenhouse gases absorb some of Earth’s infrared
radiation.
4. Greenhouse gases emit infrared radiation in all
directions.
5. Earth absorbs downward-directed infrared radiation.


           Result: Warmer troposphere

                                                         9
                     emits                                       reaches
     The Sun                    solar radiation                            The Earth

                             infrared   visible    ultraviolet                 emits


water vapor, CO2,                       is selectively              Infrared radiation
CH4, N2O, ozone                         absorbed by
        such as     Greenhouse
                    gas
                                              subsequently
                    molecules                    have


                                              causes
                                                                 More kinetic energy
                  More collisions
                  of gas molecules
                  in the
                                                  cause
                  atmosphere                                Increased
                                                            temperature
                                                            of the atmosphere            10
        Let’s consider the fate of insolation
                                                     clouds
                  reflected
                              by
                                   atmosphere           gas molecules
  Solar     can
                  to space
radiation    be                                 aerosols
                              by
                  absorbed         Earth’s surface


                   albedo




                                                                        11
         Reflected radiation       often expressed
Albedo =                           as a percentage
         Incoming radiation

    Earth’s overall albedo: 0.30 = 30%

 Fresh snow      0.75–0.95     75–90%
 Thick clouds    0.6–0.9       60–90%
 Thin clouds     0.3–0.5       30–50%
 Ice             0.3–0.4       30–40%
 Water           0.1           10%
 Grass           0.1–0.3       10–30%
 Dirt            0.1–0.2       10–20%
 Forest          0.03–0.1      3–10%
                                                     12
The amount and distribution of energy in the Earth
system, and thus Earth’s climate, depend on three
variables:
  1) The amount of incoming solar radiation.
            depends on Sun’s output

  2) The fraction of solar radiation that is reflected (albedo).
              depends on Earth’s albedo
  3) The amount of Earth’s infrared radiation that escapes to
  space.      depends on composition of Earth’s atmosphere
              (GHGs)


These factors balance to produce stable climate
conditions.


                                                                   13
                       Earth’s Energy Balances
 342                                                          107 + 235
                                 =
  Energy entering top of atmosphere Energy leaving top of atmosphere
                                  =
        Energy gained by atmosphere Energy lost by atmosphere
67 + 350 + 78 + 24                                       165 + 324 + 30
                                    =
       Energy entering Earth’s surface Energy leaving Earth’s surface
  168 + 324                                               24 + 78 + 390




                                                                        14
 The energy balance       outgoing          incoming
 at Earth’s surface.
                 Earth’s surface

What if all greenhouse gases were removed from the
atmosphere?




                                                       15
The energy balance         outgoing              incoming
at Earth’s surface.
                  Earth’s surface

                                              ....incoming
So if all greenhouse                          energy
gases were removed                             would decrease
from the atmosphere....


Earth’s surface would
cool, and would emit less
infrared radiation until the
balance is restored.       the cooling would stop (at 0°F)
                                                             16
Problem
 Start with current conditions
 (Earth’s surface temperature
 =15°C; atmosphere w/greenhouse
 gases.
 Imagine that the concentrations of        incoming energy
 greenhouse gases increased by                would increase
 50%.
   1. Sketch two diagrams illustrating
   how the energy balance would
   change.

   2. How would Earth’s average
   surface temperature change?

Earth’s surface would heat and emit more
infrared radiation until balance is restored.
            final T = ??
                                                           17
Let’s compare Earth to Venus.




                                18
Based solely on proximity to the sun, which
planet will be warmer?
                                              Venus




                       Earth



               Venus




                                                      19
 Completely covered
                           Clouds, ocean, land


  However
   by thick clouds


 albedo = 0.70 (70%)       albedo = 0.30 (30%)

     In these units,     If insolation = 100 units,
  193 x 0.3 = 58 units       100 x 0.7 = 70 units
are absorbed by Venus.    are absorbed by Earth.

                                                      20
 So…. If a planet’s surface temperature depended
solely on the rate at which it absorbs solar radiation,
                       cooler
        Venus would be ______ than Earth.




  However
Earth’s average surface temperature: 15 °C
 Venus’ average surface temperature: ___
                     °C
                                  480

  astrology.com: ―Venus is all about pleasure….we
appear attractive thanks to Venus’ energy….beauty is
 also strongly associated with Venus….inextricably
   linked to refinement, culture, charm and grace‖


                                                          21
The reason: Different atmospheres




                                 ―runaway
                                greenhouse
                                   effect‖
                                             22
  Earth’s Climate System: The Major Players

Three Overarching Variables (ECS B2, B3)
 1) the amount of incoming solar radiation
 2) the fraction of solar radiation that is reflected (albedo)
 3) the amount of Earth’s IR radiation that escapes to space


Regional Controls of Climate (ECS C1)
 Altitude, latitude, mountains, proximity to oceans (geography)
 Axial tilt
 Pressure systems


Global Factors That Affect Climate (ECS C2 ff.)
 External and internal factors
 Factors that humans can and cannot influence

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