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 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 ()
2 5 2c h FB ( ) ch / kT e 1
k = Boltzmann constant h = Plancks constant c = speed of light in a vacuum 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 Earths 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 4
(1 ) Earth as a blackbody 4 emits longwave F TEarth
radiationLFL TEarth (1 ) S 0 4
1/ 4 Greenhouse Effect Solve for no atmosphere TEarth = 255 K (-18C) Actual surface emission gives: TEarth = 288 K (15C)
Greenhouse Effect = ~ 33C Earths Atmosphere Nitrogen 78% Oxygen 21% Argon 1%
Carbon Dioxide 0.037% Greenhouse Gases in ppm H2O0.1-40,000 CO2 380 CH4 1.7 N2O0.3 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 Earths 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 Earths water didnt evaporate. Are there other possibilities? Change in albedo perhaps? Methane story isnt over Zahnle, et al., Geobiology (2006), 4, pp271-283
