Transcript Document
Faint Young Sun Paradox
Part I
September 9, 2008
Katye Altieri
History of earth Systems
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??
Habitable planets
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
Radiation cont.
The different types of radiation are distinguished by their
wavelength λ
a = long wavelength less Energy
b = short wavelength more Energy
Electromagnetic spectrum
Blackbody radiation
Monochromatic emissive power (or irradiance) of radiation
emitted by a blackbody is related to temperature (T) and
wavelength (λ)
2c h
FB ( ) ch / kT
e
1
2
k = Boltzmann constant
h = Planck’s constant
c = speed of light in a vacuum
5
Blackbody radiation cont.
Integrate over all wavelengths and the total emissive power
(FB in W m-2) of a blackbody is
FB FB ( )d T
4
0
= 5.671x10-8 W m-2 K-4, the Stefan-Boltzmann constant
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!
E T
4
Sun emits E as a blackbody at ~6000K
Total Energy
output of Sun
3.8x1026 Watts
Earth receives
1370 W m-2
S0 Solar
constant
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
At equilibrium, In=Out
Incoming solar energy at the surface of the Earth Fs
S0 ~1368 Wm-2
S0
FS (1 )
4
Earth as a blackbody emits longwave radiation FL
FL T
4
Earth
(1 ) S0
4
1/ 4
TEarth
Greenhouse Effect
Solve for no atmosphere
TEarth = 255 K (-18°C)
Actual surface emission gives:
TEarth = 288 K (15°C)
Greenhouse Effect = ~ 33°C
Earth’s Atmosphere
Nitrogen
78%
Oxygen
21%
Argon
1%
Carbon Dioxide 0.037%
Greenhouse Gases in ppm
H2O 0.1-40,000
CO2 380
CH4 1.7
N2O 0.3
O3 0.01
Greenhouse gases
Faint Young Sun Paradox
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
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
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