Surface temperature of a planet: distance from the Sun and its albedo

Mean surface temperature of the earth

• Blackbody temperature of the earth : 5° C
• Albedo of the earth: ~ 30%
• Surface temp should be, considering the albedo effect : -25° C
• Actual mean temperature of the earth's surface: +15° C. Why?

Natural Greenhouse effect: today primarily by carbon dioxide and water vapor

Energy from the Sun:: Faint Young Sun Paradox- Problem #1

• 25-30% dimmer Sun at the beginning of planet formation
• Positive feed back effects - frozen earth
• Surface temperature of the earth would be below freezing until 2 b.y. ago
• Geologic evidence of liquid water as far back as 3.8 billion years ago

What saved the earth?: Possible solutions

• Albedo lower in the past (practically 0!)
• Greenhouse effect larger in the past
• Solar evolution models might be wrong

Early Greenhouse?

• Possible greenhouse gas: carbon dioxide as an early greenhouse gas
• Needed amount is ~1000 times that today (0.3 bar vs. 3.5 x 10^-4 bar today)
• Do we have enough carbon in the earth?

Carbon reservoirs in the earth

• Most of earth’s carbon is locked up in rocks
• Maybe this was available to the atmosphere to increase greenhouse effect
• Vocanic emissions today and in the past

What happens when the Sun gets brighter - Problem #2

• Positive feedback effect-boiling ocean

The record of climate in the earth - always had water, but -----oscillations

• Strong record of extensive glaciations in the last million years
• Ice cover over a substantial portion of the N. American continent
  • Evidence from ancient glaciations
• Strong warm climate in the Cretaceous
  • No ice caps at poles-temperate climate all the way to the poles
• Why did it not go into a frozen or steamhouse mode?
  • The presence of thermostat

Long term stability of the earth's climate: The Earth's Thermostat

• Continental crust made up of K, Na, Mg, Fe, Ca silicates
• Chemical erosion of land : H₂O, CO₂ , and reactions with Ca-silicate rocks
• Transport of Ca²⁺, HCO₃^-, SiO₂ ions into the ocean
• Inorganic and organic precipitation of CaCO₃ and SiO₂ as skeletal parts
• Cycling of products due to plate-tectonics
• Formation of Ca-silicate (rocks) and CO₂ and introduction into the atmosphere
• Influences the CO₂ content of the atmosphere
  • Controlling mechanisms
    • plate-tectonics for input of CO₂
    • Supply of Ca to oceans

Checks and balances - equilibrium

• consequences of input faster than removal as carbonate
  • increase in atmospheric CO₂
  • warmer planet
  • wetter planet as warm air holds more water vapor
  • more CO₂ in soils due to increased plant cover
  • more soil mineral dissolution
  • increase in rate of input of Ca into oceans
  • Removal of CO₂ as Calcium carbonate
  • less CO₂ in the atmosphere and cooling of the planet
• consequences of input lower than removal as carbonate
  • decrease of atmosphercic CO₂
  • cooler planet
  • less chemical erosion and less Ca and HCO₃^- to oceans
  • lessened removal as carbonate
  • Build up of CO₂ in the atmosphere to warm up the planet
  
  
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