I thought I read some time ago about some ultra-thin Mylar self-unfurling technology that could potentially be used to advertise from orbit. Mylar reflects light damned well, seems like we could just stick one or three of these in a decent orbit for much less than trillions of dollars. Plus how did these people calculate the effects of a sun shade? It doesn't have to cover a given spot on the planet, or be stationary relative to the sun. Seems to me like a rather straight forward way to cool the planet off.
That is assuming that there actually is any global warming, and that if there is we should do anything about it at all. Anything we orbit to block sun will have to be removable in case it gets too cold out there.
And yes, beaming down power would be a bad thing. Everyone talks about heat, thinking that lasers or electricity aren't related to heat. We need to worry about the total energy balance, not just thermal energy, since eventually that laser power will get run through someone's ice box and thus be converted to heat. The whole idea of a sun shade is to prevent energy from getting to the atmosphere at all.
And we should never get an accurate climate model, since that Chaos theory someone mentioned basically says that there is a limit to the ability to predict anything in a chaotic system, even if you know the governing equations really well. Which we probably never will.