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Question - could one theoretically build a big enough array of this to counter-act globally warming? Any back of the envelope math for what it would take would

Question - could one theoretically build a big enough array of this to counter-act globally warming? Any back of the envelope math for what it would take would be very interesting.

hwillis 9y ago
Radiative forcing from global warming will be somewhere between 2.6 and 6.0 Watts/m^2 by the end of the century[1]. Currently it is around 1.75 W/m^2[2], which translates to a total increase in absorption of 223 terawatts. That's about 3x the total energy produced by all photosynthesis[3][citation needed], and over 12x the energy in all forms consumed by humanity. So, to a first estimate, no. It's absolutely beyond what we're capable of. To a second estimate: these panels reject 70 W/m^2, so to counteract current warming we would need to cover .625% of the earth's surface in them, which is 3.2 million square kilometers, which is over 6x the area you'd need to replace all of humanity's energy use with solar power. All in all, it doesn't make much sense. [1]: https://en.wikipedia.org/wiki/Representative_Concentration_Pathways https://en.wikipedia.org/wiki/Representative_Concentration_P... [2]: https://en.wikipedia.org/wiki/Radiative_forcing#/media/File:Radiative-forcings.svg https://en.wikipedia.org/wiki/Radiative_forcing#/media/File:... [3]: https://en.wikipedia.org/wiki/Orders_of_magnitude_(power)#Terawatt_.281012_watts.29 https://en.wikipedia.org/wiki/Orders_of_magnitude_(power)#Te...
- swsieber 9y ago
  Cool. I thought it wouldn't be feasible, but those numbers really help cement that idea. Thank you.
  - lucb1e 9y ago
    And thank you for asking :). I had the same question and the answer is fantastic. I really wish HN had some mechanism of extra upvotes, e.g. by being allowed to hand out one extra upvote per day or week, or by paying 5 cents (eur/usd) to buy an extra upvote (with a maximum of X per post or Y per day, or both, or whatever)... but alas.
    - nkurz 9y ago
      A possible mechanism for this is the "favorite" link that you can get to by clicking on the timestamp of a comment. Currently, I don't think much is being done with it other than adding it to a private per-user list, but it could also be used to highlight or boost the chosen comments? A quick fix might be to just add another item to the "Lists" link at the bottom that shows the top "Favorited" comments, based on how many people have it as their most recent (or 10 most recent) favorite.
- logfromblammo 9y ago
  It seems like solar panels work best when in sunlight, and infrared radiators work best when shaded. If you have a square 1 m^2 solar panel, and a square 1 m^2 area to put it in, and you live at 35 degrees north latitude, you angle your panel up 35 degrees facing south. That gives you .428 m^2 of area, angled at 55 degrees, facing north, which will always be in the shadow of the panel. So stick a rectangular infrared radiator behind the solar panel. So the area needed would be less than that needed to replace all of humanity's energy use with solar power, but the exact amount less depends on the latitude at which you place the solar panels.
- spenrose 9y ago
  Agreed on geoengineering. But given that AC is a non-trivial fraction of current energy use [1] and that there are ~2B low-energy consumers in the tropics who would enormously benefit from AC, the in-principle impact on GHG output here is substantial. Install to cool the earth: no. Install to cool human-occupied spaces: yes. [1] https://energy.gov/energysaver/air-conditioning https://energy.gov/energysaver/air-conditioning
- PhantomGremlin 9y ago
  these panels reject 70 W/m^2 What about plain mirrors? How efficiently do they reflect photons back into space? Presumably they're highly efficient for visible light; how easy is it to reflect infrared passively?
  - hwillis 9y ago
    A mirror rejects close to 0 heat compared to a matte surface, it just causes it to be absorbed in the air instead of the ground.
  - sjruckle 9y ago
    The problem with using mirrors as radiators is that they're terrible radiators. They might be great at reflecting the sky-photons away, but they won't launch their own photons away either. Shiny things are great at reflecting incident radiation (which is why they're good mirrors) but bad at radiating their own heat. Dull black things are bad at reflecting but good at radiating. The technical terms are reflectance, absorptance, and emissivity. http://www.optotherm.com/emiss-physics.htm http://www.optotherm.com/emiss-physics.htm

Question - could one theoretically build a big enough array of this to counter-act globally warming? Any back of the envelope math for what it would take would be very interesting.

hwillis 9y ago

Radiative forcing from global warming will be somewhere between 2.6 and 6.0 Watts/m^2 by the end of the century[1]. Currently it is around 1.75 W/m^2[2], which translates to a total increase in absorption of 223 terawatts. That's about 3x the total energy produced by all photosynthesis[3][citation needed], and over 12x the energy in all forms consumed by humanity. So, to a first estimate, no. It's absolutely beyond what we're capable of. To a second estimate: these panels reject 70 W/m^2, so to counteract current warming we would need to cover .625% of the earth's surface in them, which is 3.2 million square kilometers, which is over 6x the area you'd need to replace all of humanity's energy use with solar power. All in all, it doesn't make much sense. [1]: https://en.wikipedia.org/wiki/Representative_Concentration_Pathways https://en.wikipedia.org/wiki/Representative_Concentration_P... [2]: https://en.wikipedia.org/wiki/Radiative_forcing#/media/File:Radiative-forcings.svg https://en.wikipedia.org/wiki/Radiative_forcing#/media/File:... [3]: https://en.wikipedia.org/wiki/Orders_of_magnitude_(power)#Terawatt_.281012_watts.29 https://en.wikipedia.org/wiki/Orders_of_magnitude_(power)#Te...

swsieber 9y ago
Cool. I thought it wouldn't be feasible, but those numbers really help cement that idea. Thank you.
- lucb1e 9y ago
  And thank you for asking :). I had the same question and the answer is fantastic. I really wish HN had some mechanism of extra upvotes, e.g. by being allowed to hand out one extra upvote per day or week, or by paying 5 cents (eur/usd) to buy an extra upvote (with a maximum of X per post or Y per day, or both, or whatever)... but alas.
  - nkurz 9y ago
    A possible mechanism for this is the "favorite" link that you can get to by clicking on the timestamp of a comment. Currently, I don't think much is being done with it other than adding it to a private per-user list, but it could also be used to highlight or boost the chosen comments? A quick fix might be to just add another item to the "Lists" link at the bottom that shows the top "Favorited" comments, based on how many people have it as their most recent (or 10 most recent) favorite.
logfromblammo 9y ago
It seems like solar panels work best when in sunlight, and infrared radiators work best when shaded. If you have a square 1 m^2 solar panel, and a square 1 m^2 area to put it in, and you live at 35 degrees north latitude, you angle your panel up 35 degrees facing south. That gives you .428 m^2 of area, angled at 55 degrees, facing north, which will always be in the shadow of the panel. So stick a rectangular infrared radiator behind the solar panel. So the area needed would be less than that needed to replace all of humanity's energy use with solar power, but the exact amount less depends on the latitude at which you place the solar panels.
spenrose 9y ago
Agreed on geoengineering. But given that AC is a non-trivial fraction of current energy use [1] and that there are ~2B low-energy consumers in the tropics who would enormously benefit from AC, the in-principle impact on GHG output here is substantial. Install to cool the earth: no. Install to cool human-occupied spaces: yes. [1] https://energy.gov/energysaver/air-conditioning https://energy.gov/energysaver/air-conditioning
PhantomGremlin 9y ago
these panels reject 70 W/m^2 What about plain mirrors? How efficiently do they reflect photons back into space? Presumably they're highly efficient for visible light; how easy is it to reflect infrared passively?
- hwillis 9y ago
  A mirror rejects close to 0 heat compared to a matte surface, it just causes it to be absorbed in the air instead of the ground.
- sjruckle 9y ago
  The problem with using mirrors as radiators is that they're terrible radiators. They might be great at reflecting the sky-photons away, but they won't launch their own photons away either. Shiny things are great at reflecting incident radiation (which is why they're good mirrors) but bad at radiating their own heat. Dull black things are bad at reflecting but good at radiating. The technical terms are reflectance, absorptance, and emissivity. http://www.optotherm.com/emiss-physics.htm http://www.optotherm.com/emiss-physics.htm