This is the perfect question. The answer isn't easily computed. Even though we know it is just stored heat energy, not somehow from within Earth, we would still require a knowledge of the heat gradient, all the way to the center of Earth, to know the total stored energy, thus compute the percentage change. Otherwise it is guesswork, which of course is always taken advantage of by whoever funds the research, seeking returns.
Logically it shouldn't be messed with, as we are seeing, there is always an environmental cost for using the stored energies of Earth, regardless of what form, it could even be that there are costs associated with using wind and wave power that we have yet to acknowledge.
Solar is the only actual source of energy, so that is what we need to plug into. Everything else is just a waste of time and precious stored energy, it seems to me. I think we should define all business which can't exist solely on solar power as bullshit industry. That will reduce our power requirements by a very large fraction, I believe.