>>6512463A lot.
I can see mainly two solutions
1. Spray reflective particles into the air, reducing the greenhouse effect. To do this, you would need quite a lot of projectors, either from planes or ships, to cover a large area. You would then need huge cannons capable of firing reflective particles (something like shredded aluminum foil).
2. Build large reflective surfaces on the ground.
Bu building huge mirrors, it would be possible to reduce once again the greenhouse effect. And, if you were to install these mirrors on hinges, you could effectively turn them on/off.
Both methods take lots of efforts. The 2nd would be a lot more expensive, but would have the main advantage of being fairly clean, while spraying the air with particles is probably a huge ecological disaster waiting to happen. Unless, of course, you manage to make it clean.
