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Or would the heat dissipate back into space? For my world building purposes, I need it to cause global warming, I just don't really know if it would work. Maybe the cities would need to be bigger then they are know?

EDIT: I meant mirrors pointed up like giant reflective parasols. And now I see why that would decrease the temperature, not increase it, so... silly me! I'll fix it somehow, find something more compelling to produce global warming. That story with the town in Norway was pretty cool! Thanks for all the feedback, my question has been answered! :)

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    $\begingroup$ Giant mirrors above cities could reduce temperature locally. Effectively by reflecting excessive sunlight over the cities. This might have little impact on the rest of the planet's environment. Global warming could continue unabated. $\endgroup$ – a4android Feb 14 '18 at 8:28
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    $\begingroup$ You mean like this one? bbc.com/future/story/… $\endgroup$ – SZCZERZO KŁY Feb 14 '18 at 8:45
  • $\begingroup$ I've voted to close as it's pretty unclear what you're asking. Do you mean mirrors pointed up like giant reflective parasols? Mirrors in orbit aiming downwards? Random mirrors so people can look up and see a map of the city?? $\endgroup$ – Joe Bloggs Feb 14 '18 at 9:52
  • $\begingroup$ Don't use mirrors, use dull black surfaces. That way, mostly IR and microwaves will be radiated, which can't escape the atmosphere as easily. $\endgroup$ – nzaman Feb 14 '18 at 13:15
  • $\begingroup$ Yeah... I meant mirrors pointed up like giant reflective parasols. And now I see why that would decrease the temperature, not increase it, so... silly me! I'll fix it somehow, find something more compelling to produce global warming. That story with the town in Norway was pretty cool! Thanks for all the feedback, my question has been answered! :) $\endgroup$ – roxana_z Feb 14 '18 at 19:22
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If you want to cause global warming, you need to keep in mind the total heat balance, considering the Energy Delivered (by the Sun in your case), the Energy Stored in the system and the Energy Re-emitted by the system:

$$E_d + E_s + E_r =0$$

You cannot tweak the Sun, therefore we assume the delivered energy stays constant. The only way to increase the value of the stored energy, which is reflected into the raise of temperature, is to lower the amount of the re-emitted energy.

Since the re-emission of energy mostly happens in the infrared spectrum, your mirrors have to stop such radiation from escaping your system. Instead of mirrors you can use "greenhouse gases", such as water vapor, CO2 and methane.

Coming to your question

If there were giant irregular mirrors above cities that would reflect sunlight, would that heat the atmosphere and cause global warming?

Such mirrors would lower the amount of delivered energy and thus negatively contribute to your energy balance, making your target harder to achieve.

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  • $\begingroup$ You're right, I thought that might happen, the equation explains it clearly. I need something to trap the heat, not reflect it. Thanks so much for replying! $\endgroup$ – roxana_z Feb 14 '18 at 19:11
  • $\begingroup$ @user307328, greenhouse gases it's what you are looking for $\endgroup$ – L.Dutch - Reinstate Monica♦ Feb 14 '18 at 19:24

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