Some thoughts on Benford's proposal to mitigate global warming by putting a giant concave fresnel lens at the earth-sun L1 point:
1) The exact optical properties don't matter - just scattering sunlight will be good enough. A fine mist of volatiles (e.g., water ice) will do the job nicely, and is much easier to generate in space than a 1000km diameter lens.
2) The L1 point is unstable at a time scale of about 3 weeks. It would be hard to maintain position for the lens, and impossible for a cloud of volatiles. So: let the volatiles diffuse out all along a solar orbit, forming a miniature planetary nebula. This would require a vast amount (technically: a shitload) of volatiles.
3) Where does a vast cloud of boiled-off volatiles make a spectacular show of scattering sunlight? Cometary tails. So: steer a comet to earth-sun L1, then use (much smaller) smoke^H^H^H^H^Hlenses & mirrors to selectively control the rate and direction (so as to maintain orbit) in which it boils off its volatiles.
Questions: how long would the average comet last if we want to reduce sunlight on earth by, say, 1%? How many comets would it take to fill the entire solar orbit passing through earth-sun L1 with sufficient volatiles to achieve this reduction?
4) Far more parsimonious to put that comet into a terrestrial rather than solar orbit. Dispersing material in LEO or GEO is a bad idea though - we don't want to create a navigation hazard for current or future spacecraft. So let's place a comet or two at the Moon's trojan points wrt. earth (L4 and L5, stable).
5) The solar wind may kill this plan by sweeping the volatiles off their intended orbit. The thus-formed cometary tail may constitute a spacecraft navigation hazard, especially if/when it periodically crosses earth's path. Bummer.