[Will]

More 'slight' problems:

3) Good God, do you realize how much time, energy, and money it would take to build such a thing? Even beyond the huge engineering problems of preventing the thing from falling apart from constant meteor and space-junk impacts, we still have barely progressed beyond Apollo-era space technology. And some would say we have regressed to pre-Apollo technology, by swamping everything in more complexity. And you're proposing to create something that massive?

4) As a corollary, the time, energy, and money that would need to be poured into such an endeavor would have a much bigger impact if put into, say, infrastructure to reduce current emissions of carbon dioxide, and/or methods of recapturing carbon, research into sustainable energy like fusion, and growing enough food to feed every person on Earth.

Also, I have to say that Jack's first objection isn't that big a deal if this is something that only partially blocks the sun's energy, either by turning "on and off", or by only blocking out a percentage. In either case, plants get the energy during daylight hours and store it in ATP, and then constantly use that stored ATP to convert carbon dioxide and water into sugars and oxygen, whether there is light or not. Reduce incoming energy enough to lower the global average temperature by a few degrees centigrade will not have a significant impact on the total energy the plants recieve to keep the rest of us breathin'.