[jbert]

Is the "but at that kind of energy output we'll boil the planet" limiting case really a hard limit though?

If we're at the stage of having super-dense energy sources and super-tech, couldn't we we use those to cool the planet effectively?

Use a heat pump to move that heat off-planet? Drop some icy asteroids in the seas? Take compressed hot atmosphere off-planet in super-blimps, run it through a heat exchanger on Titan and then bring it back?

[Symmetry]

In the end the resources available to a civilization can only expand as the third power of time due to the speed of light limit. That means that the laws of physics do prohibit an indefinite exponential increase in energy use, baring new discoveries that look pretty unlikely.