Re: Saving the world
A very large solar sail at the L2 Earth-Sun Lagrange Point would block sunlight. If it were constructed like a windmill, then feathering its "blades" would change its projected surface area, and hence the amount of light transmitted, while simultaneously controlling its position wrt L2.
It may be several hundred million years before this is an issue. By that time we could easily have a fully developed spacefaring infrastructure. In fact we could do it in a few hundred with the right technologies. After a million years, I don't expect us to even resemble "humans" anymore.