A body emits thermal radiation at a rate proportional to its surface area and absolute temperature.

If we do anything on the earth to generate heat at a rate greater than the earth emits the heat the earth gets hotter, and so reaches equilibrium emitting more heat at the higher temperature.

The only way to reach this equilibrium without the earth getting hotter is to either increase the surface area of the planet or reduce the amount of heat in the planet.

Since any form of energy that involves stuff up will generate heat we must reduce the heat coming from the other source.

See black body radiation.

Mines the very very black one with a parasol.