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Where does the Energy that reaches the Earth Go?

  1. Nov 28, 2012 #1

    I've been thinking about something that recently has puzzled me. There is a net increase in the energy on the Earth from the sun. Where does this energy go? I assume it either goes to bonds, such as to produce biomass, or heat. However, does that mean long-term, the Earth's fate is to accumulate oil everywhere or get very hot?

    Factoring in blackbody effects, does this mean that Earth's temperature is steady-state, and a certain percentage of the energy coming in is turning to biomass? If so, what are the effects of humans burning such biomass, and converting it to heat? Does this simply shift Earth's black body steady state temperature to a higher value?

    I'm sorry there are a lot of questions. I'm leaving the Earth's core out of this because there's no need to consider such crazy timescales (at least, I hope not). And I'm assuming it doesn't contribute significantly to the energy balance. Please let me know if I am mistaken.
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  3. Nov 28, 2012 #2


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    Most of the energy the Earth receives from the sun gets radiated back into space. The Earth is approximately a blackbody at about 300K, so it radiates power at a rate of
    [tex] 4 \pi r^2 \sigma T^4[/tex]
    In fact, if you calculate the energy the Earth receives from the sun and set it equal to the above and use this to solve for the surface temperature of the Earth, you come reasonably close to the actual temperature (within 50 degrees or so).
  4. Nov 28, 2012 #3


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    The amount of heat that the Earth absorbs from the Sun, in addition to any heat generated internally by radioactive decay, is equal to the energy radiated away by blackbody radiation. Any energy that is used to grow plants, break bonds, etc is not converted to heat.
  5. Nov 28, 2012 #4
    I mean we've done the balance for our engineering class, and I understand that most of the energy transfer is dominated by blackbody effects.

    I think this is a better question: Does the energy absorbed by plants on Earth get accounted for in the blackbody balances?

    The source of this question is a thought experiment - "What if humans did not exist?". Plants would keep growing, dinosaurs would keep eating them, and dying, and making oil. Would the whole world eventually be filled with oil eventually?
  6. Nov 28, 2012 #5


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    In the grand scheme of things, the total amount of energy stored on Earth as fossil fuels and the biosphere corresponds to a few days of the Sun's input over billions of years. It can't be significant to the energy balance equation you are describing. Animals and bacteria are converting the energy that has been stored by photosynthesis. There will be fluctuations in the balance of producers and consumers in the biosphere and these will affect the balance but to a very small degree. Humans are making a bit of a difference at the moment but that will pass, eventually. Fossil fuels are being laid down continually but will come to the surface, eventually and broken down by organisms (they will evolve to use an energy resource). But it is such a tiny fraction of the whole process.
  7. Nov 28, 2012 #6


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  8. Nov 28, 2012 #7


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    What does one thing have to do with the other? Regardless of what you actually are driving at, the influence of humans is lost in the error margins of such a question. We just haven't been around long enough to have a major impact on the issue.

    For example, the dinosaurs have been gone for 65,000,000 years, while humans have been around for about 20,000.... roughly 0.03% of that time.
  9. Nov 28, 2012 #8
    thanks for the great answers. I think I was getting confused from my thermo class, which we talked about the Second Law of Thermodynamics. Sometimes it just gets jumbled up what can be converted, or accumulated, or generated...:)
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