What will happen -- Mirrors on the Moon to illunimate the Earth....

[sophiecentaur]

It wouldn't increase the greenhouse gas effect as such

Why not? It could, if engineered perfectly, send all the sunlight arriving at the Moon, onto the Earth's surface. That could increase the energy arriving around the full Moon condition by about 1/16. The mirrors would need to focus and to track, perfectly. But that amount of extra energy would be more than any other source that has added to the Sun's energy input to the Earth.
But one needs to ask why we'd do that. It could well be more in our interest to use mirrors on the Earth's surface to get rid of a lot of the Sun's battering.

 

[Keith_McClary]

Rjukan sun: the Norwegian town that does it with mirrors

Last week for the first time ever, the winter sun shone on the Norwegian town of Rjukan. ... High on the mountain opposite, 450 metres above the town, three large, solar-powered, computer-controlled mirrors steadily track the movement of the sun across the sky, reflecting its rays down on to the square and bathing it in bright sunlight.​