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?