Saturday, September 14, 2013

Difference between ´greenhouse effect´ and ´runaway greenhouse effect´?

The basic greenhouse effect works like this:-
  • Short wavelength radiation from the sun, like visible light, passes easily through the atmosphere and is absorbed by the land and sea. 
  • There it gets re-radiated back as longer wavelength infra-red (heat) radiation. 
  • This longer wavelength is blocked by certain gasses in the atmosphere, like carbon dioxide, and stays, which raises the world´s temperature.
  • If we didn´t have a greenhouse effect the world would be much, much colder, probably so cold the oceans would freeze over. 
  • So we need some level of greenhouse effect to keep the planet habitable.
Now, the "runaway" greenhouse effect is  when the effect starts to be self-feeding, and the rising temperatures add more gasses to the atmosphere, trapping more heat, raising the temperature and so on.

If this happened it might not end with all the oceans evaporating, since the increased clouds could reflect a lot of solar energy back into space. But it would likely mean very much higher temperatures than we have now.

Runaway Greenhouse effect faced by VENUS
  • Venus was originally cooler than what it is now and it had a greater abundance of water several billion years ago. 
  • Also, most of its carbon dioxide was locked up in the rocks. 
  • Through a process called as runaway greenhouse, Venus heated up to its present blistering hot level. 
  • Because Venus was slightly closer to the Sun than the Earth, its water never liquified and remained in the atmosphere to start the greenhouse heating. 
  • As Venus heated up, some of the carbon dioxide in the rocks was "baked out." 
  • The increase of atmospheric carbon dioxide enhanced the greenhouse heating. 
  • That baked more carbon dioxide out of the rocks (as well as any water) and a runaway positive feedback loop process occurred. 
  • This positive feedback loop occurred several hundred million to a few billion years ago so Venus has been very hot for that length of time.
runaway greenhouse positive feedback
  • The loss of water from the rocks means that Venus' rocks are harder than the rocks of Earth and its lithosphere is now probably too thick and hard and its aesthenosphere is too poorly lubricated for plate tectonics to occur. 
  • The water Venus originally had is now gone because of a process called dissociation.


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