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What part of the Earth should be "shaded"; the poles or the equator?

Since the size of the sunbrella would depend on its ability to reduce certain wavelengths of radiation; what material or technology would best provide the shade, and when parked at L1 what size would it need to be?

Once in place, could the amount of shade provided be regulated from earth?

In the event of an approaching ice-age, could the sunbrella be adapted to intensify rather than decrease heat generation?

If it could be built, would our scientists know how to properly employ it?

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    $\begingroup$ This question might be better suited to SE Space Exploration. But, lose the bit about the lawyers - on either site that could result in the question being closed because it could be deemed as being either too broad or subject to too much personal opinion. $\endgroup$ – Fred May 21 '17 at 12:03
  • $\begingroup$ The answer to the question in the title is presumably "yes"? More interesting questions might be how big it would need to be, or whether current technology could provide it... $\endgroup$ – Semidiurnal Simon Jun 22 '18 at 8:27
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There are several journal articles on this topic, for example Optimal Sunshade Configurations for Space-Based Geoengineering near the Sun-Earth L1 Point.

Being truly at the L1 point is unsuitable because the radiation pressure of the photons acting on the shade (like solar sail) would disrupt the equilibrium, so the equilibrium point is shifted toward the sun from the L1 point and depends on the degree to which the shade reflects versus absorbs.

The shade should be made of black material to minimize the effect of radiation pressure on the equilibrium point, thereby permitting it to be as close as possible to the L1 point, and closer to the Earth, where it can more effectively shade.

The size required is roughly a 1,500 kilometer diameter structure.

All parts of the Earth would be shaded to some degree. From Earth, the sun would appear as it does during a transit of Venus or Mercury, but the position on the sun's disc would vary more with ones location on Earth since the shade would be much closer to Earth than Venus is.

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    $\begingroup$ I'm just going to mention the usual caveats: Lack of launch and management capability to actually do this; environmental impacts from construction and launch of so much material; a shade only offsets the effect of increased CO2, so it would have to be maintained for centuries or millennia; doesn't address ocean acidification, may make it worse by permitting further CO2 emissions; unknown ecosystem effects from abrupt insolation change. Plus the creation of a global authority to manage the thing... $\endgroup$ – jeffronicus May 23 '17 at 14:56
  • $\begingroup$ Something to bear in mind, in common with other forms of "reduce the sunlight" geoengineering, is that since it does nothing to deal with the underlying problem, while CO2 in the atmosphere keeps increasing the sunshade will need to keep expanding to keep pace! $\endgroup$ – Semidiurnal Simon Jun 22 '18 at 11:49

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