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) k = Boltzmann constant h = Planck’s 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  = 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!

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 Earth as a blackbody emits longwave radiation FL

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