Through nanotech. J. Storrs Hall (aka JoSH) has some ideas. I have a problem with this one, though:
So you have this balloon and it floats up there twenty miles. They all have a little GPS and receiver so they can turn themselves. That’s all there is to it. What can you do with a machine like this? The machine is essentially a programmable greenhouse gas. If you set the mirrors facing the sun, it reflects all the sunlight back. If you set them sideways, it allows the sunlight to come through, and similarly for the longwave radiation coming from the back side of the earth at night.
He seems to be implying that GPS can be used to determine attitude. It can’t. It only provides velocity and position. Now it may want to know that information for other purposes, but there will have to be some other means of attitude knowledge. It seems to me the simplest way would be to just measure the sunlight hitting the mirror (which would actually be a solar cell). As you adjust your attitude, the power available will grow or shrink, and it can use the rate to control the angle. As for how to physically control the attitude, I would guess that little reaction wheels would do the job.
Of course, many of the warm mongers hate technical solutions like this, because they don’t require us to piously tighten up our hair shirts, and they don’t allow for sufficient control of the global economy.