Sunshade
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Variations on an umbrella have been used for thousands of years to shade people from the Sun. Megastructure sunshades in orbit have been proposed by science fiction writers and more recently are becoming serious engineering proposals.
In theory a sunshade with a diameter of 130 miles can be orbited at an altitude of 10,000 to 15,000 miles which is a solar synchronous orbit. The sunshade would produce a continuous solar eclipse which would circle Earth's tropic zone continuously. This sunshade would reduce the solar input to Earth by an amount sufficient to reverse the recent global warming trends and cancel the effect of green house gases put in Earth's atmosphere by humans.
Carbon nanotubes may soon reduce the mass of a viable sunshade considerably and make possible a space elevator to move the sunshade materials to orbit.