[ben_w]

Tidal energy comes from the moon (and the sun) pulling the oceans as the Earth spins. Extracting energy from this effectively increases the drag between the ocean and the land, which slows down the rotation of the Earth. This effect even occurs without extracting tidal energy, and some of the dinosaurs (IIRC) had 19 hour days.

The amount of energy in the rotation of the Earth is so huge that this basically doesn't matter, 10 TW for 660 million years, which is into the realm of "will too much carbon be subducted for photosynthesis to continue?" timescales.

[Y-bar]

I propose two possible solutions:

A. Build some huge-ass rockets on the moon to accelerate it to offset any slowdown from changes in tidal energy.

B. Divert a few billion tonnes of asteroids to crash into the moon's aft, adding both to the moon's velocity and mass.

I believe that if we begin working on any of these solutions within a hundred thousand years or so from now we should be able to offset most of the projected long-term lengthening of the day.

[ben_w]

You'd have to put the rockets/target the asteroid impacts on the Earth, the effect on the Moon from this interaction is to push it into a higher orbit to conserve angular momentum of the combined Earth-Moon system.

[Mistletoe]

> "will too much carbon be subducted for photosynthesis to continue?"

Great, you gave me a new fear. :)