... and Arthur C. Clarke rolls in his grave
Isn't it obvious that the sunshade should be kept *outside* the atmosphere? I'm no physicist, but wouldn't some of the diffuse light from such a shade be deflected away from earth due to angle of incidence anyway (reducing the initial shade requirement)?
Also, can't help thinking that it'd be easier to spiral a non-atmospheric shade off into the sun (or burn it up on reentry) than it would be to depollute the atmosphere (or oceans) of aerosols... I mean global warming is an issue, but it's just one class of the more general problem of *pollution*. (Eg. a food supply full of mercury will leave us with significant problems even if we don't cover the planet in hot water.) So polluting the atmosphere to address warming seems to ignore the one thing we've learnt from all the Great Ideas of the past, which is that their most remarkable consequences were rarely intentional. (Cane toads anyone?)