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1. September 9, 2008
Katye Altieri
History of earth Systems
Faint Young Sun Paradox
Part I

2. Sun
 Middle sized, middle aged, normal star
 Solar heating determines energy balance of Earth
 Core produces energy through nuclear reactions
 4 H atoms fuse  1 He atom
 Energy is transferred by electromagnetic
radiation
 Earth ~150 million km from the sun = perfect
distance
 Not too hot, not too cold, but why is that??

4. Habitable planets

5. Radiation
 Electromagnetic waves move through space at a
constant speed
 c = ~ 3x108 m s-1
 Sunlight, microwaves, heat from a fire, radio waves,
ultraviolet rays, x rays gamma rays

6. Radiation cont.
 The different types of radiation are distinguished
by their wavelength λ
 a = long wavelength less Energy
 b = short wavelength more Energy

7. Electromagnetic spectrum

8. Blackbody radiation
 Monochromatic emissive power (or irradiance) of
radiation emitted by a blackbody is related to
temperature (T) and wavelength (λ)
 k = Boltzmann constant
 h = Planck’s constant
 c = speed of light in a vacuum
1
2
)
( /
5
2



T
k
ch
B
e
h
c
F 




9. Blackbody radiation cont.
 Integrate over all wavelengths and the total
emissive power (FB in W m-2) of a blackbody is
  = 5.671x10-8 W m-2 K-4, the Stefan-Boltzmann
constant




0
4
)
( T
d
F
F B
B 



10. Blackbody radiation cont.
 E=total amount of radiation emitted by an object
per square meter (Watts m-2)
  is a constant
 T is the temperature of the object in K
 Simple relationship!
4
T
E 


11. Sun emits E as a blackbody at
~6000K

13. Total Energy
output of Sun
3.8x1026 Watts
Earth receives
1370 W m-2
S0 Solar
constant

14. Albedo
 Earth receives both short and longwave radiation from
the Sun
 Some radiation is reflected back to space
 Albedo-global mean planetary reflectance
 Clouds, air molecules, particles, surface reflection
 Earth’s albedo  ~ 0.3
 30% of the incoming solar flux is
reflected back to space

16. At equilibrium, In=Out
 Incoming solar energy at the surface of the Earth
Fs
 S0 ~1368 Wm-2
 Earth as a blackbody emits longwave radiation FL
4
Earth
L T
F 

)
1
(
4
0



S
FS
4
/
1
0
4
)
1
(





 


 S
TEarth

17. Greenhouse Effect
 Solve for no atmosphere
 TEarth = 255 K (-18°C)
 Actual surface emission gives:
 TEarth = 288 K (15°C)
Greenhouse Effect =  ~ 33°C

18. Earth’s Atmosphere
 Nitrogen 78%
 Oxygen 21%
 Argon 1%
 Carbon Dioxide 0.037%
Greenhouse Gases in ppm
H2O0.1-40,000
CO2380
CH4 1.7
N2O0.3
O3 0.01

19. Greenhouse gases

21. Faint Young Sun Paradox

24. Early Earth Atmosphere
 Methane and ammonia are even better GHG than
carbon dioxide
 There could be early volcanic sources of methane
and ammonia, but modern volcanic gases are
primarily CO2 and N2
 Without volcanic methane and ammonia, you are
left with “weakly reduced” atmosphere that leads
to a warm Earth

25. Methane vs. Carbon dioxide
CH4
 Currently, very short atmospheric lifetime ~ 10
years
 With O2 present, methane is oxidized to CO2
 In the absence of O2, CH4 lifetime can reach
~50,000 years
 No obvious large sources of methane pre-life
CO2
 Negative feedback: changes in the rate of
consumption by silicate weathering

26. Summary
 During Earth’s history somehow the amount of
greenhouse gases adjusted relative to the
amount of change in the radiative forcing. As the
sun has warmed, the amount of the greenhouse
effect has declined so that Earth’s water didn’t
evaporate.
 Are there other possibilities? Change in albedo
perhaps?
 Methane story isn’t over…
 Zahnle, et al., Geobiology (2006), 4, pp271-283