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

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

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

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!

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

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
H2O ,000
CO2 380
CH4 1.7
N2O 0.3
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, pp