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We already know how to heat things up, but what about cooling them? How do we make Earth's climate like that of Mars? Or go further: can we, by any hypothetical ways, make Earth somewhat like Pluto?

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Reflect sunlight

  • Release particles into the atmosphere that reflect more light than they trap (think nuclear winter)
  • Deploy a massive sunshade at the Earth-sun L1 point
  • Increase albedo by painting the surface white, or growing white GMO crops, etc

Reduce the greenhouse effect

  • Knock off some of Earth's atmosphere with a giant impactor
  • Shut off plate tectonics - which should slow the mantle convection that produces Earth's magnetic field, allowing the atmosphere to erode due to solar wind
  • Pull greenhouse gases out of the atmosphere

You're limited in part by the mass of the Earth. Without drastically shrinking the planet or reducing its density, its gravity will be the same, meaning it still holds on to its atmosphere relatively well. enter image description here

Compare the locations of Earth and Pluto on this graph. Without moving Earth so far away from the Sun that it's as dimly lit (and therefore cold) as Pluto, or reducing it in mass to bring it to such a low escape velocity as Pluto, Earth will always be able to hold a much thicker atmosphere. That insulating air should always retain a little more heat.

Luckily, once the planet cools a certain amount through some of the aforementioned methods, you might get a runaway refrigerator. Astronomynotes describes how this contributed to the cooling of Mars:

Since Mars was slightly further from Sun than the Earth, Mars' initial temperature was lower. This meant that the water vapor condensed to form a liquid water layer on the surface. Gaseous carbon dioxide dissolves in liquid water and can then be chemically combined with rocks. This would have happened on Mars long ago. The removal of some of the carbon dioxide caused a temperature drop from the reduced greenhouse effect. This caused more water vapor to condense, leading to more removal of atmospheric carbon dioxide and more cooling, etc. This positive feedback process is called a runaway refrigerator.

In theory, once your sunshade and impacts cause enough cooling, it might progress into a positive feedback loop that finishes the job.

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  • $\begingroup$ The giant impact probably does not help. The cooling period will likely be short compared to the overall warming and subsequent rebalancing. If you create a large enough impact to remove a non-trivial proportion of the atmosphere, the resulting event might not cause any measurable cooling at all. +1 for the sunshade though, which is probably the only practical solution at this point. $\endgroup$ – Starfish Prime Jan 19 at 10:47
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A volcanic eruption dispersing aerosol and ashes in the high atmosphere can already cool down the planet, since those substances reflect back the solar radiation which is our main heath source. Famous example is the year without a summer following the eruption of Krakatoa.

Therefore I would say that a substantial amount of similar substances in the high atmosphere would be an effective way to lower the surface temperature.

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