We aren't talking about a system with an evenly distributed energy. It wouldn't be life on Earth if the global temperature average was between what is now the temperature of the atmosphere, and all the ones below surface. As long as the thermal isolation remains in place it shouldn't be a problem and the biggest foe should be the unbalance between heat in and heat out from sun/space, but what if we do a big scale and sustained thermal transfer between the core and the surface?

I know, I'm oversimplifying a lot, and the orders of what humans can do may be dwarfed by any volcanic eruption, but it should be a safe margin at some point at which the scale and time of heat transfer with underground may start to affect global average temperature.