I was thinking again that the easiest / cheapest way to cool the planet is probably just to block some amount of the sunlight. freebeard has discussed the idea of a lens at lagrange L1 in space between the sun and earth to shift the light a couple degrees so that it misses the earth.
Lagrange points are the neutral gravity points between 2 large objects, like planets, moons, and the sun. An object placed at any of these 5 points tends to maintain their relative distance between the 2 large objects because the gravity pulling on one side is cancelled by the gravity pulling on the other side.
L1 is the point between the 2 objects, in this case the sun and earth. Perhaps solar panels (films) could be deployed at this point about a million miles away at L1, generate electricity 24/7, and beam it to earth while blocking some of the sunlight that warms the planet.
Solar pressure would tend to push the solar array away, but perhaps that could be compensated by moving a little closer to the sun at a new compensated L1 point. Another idea is ion engines to compensate.
Anyhow, I found a fun website that discusses some of these ideas in more depth, and suggests it would be possible to shoot the payloads at escape velocity using rail guns.
https://www.pnas.org/content/103/46/17184
I haven't read all the way through yet, but I wonder how many Gs of acceleration over what distance would be required to achieve the necessary 28,600 MPH?