"Current understanding is that fluctuations in the temperature boundary conditions at the surface of Earth do not affect the internal dynamics below about 1 meter in depth."

That jives with something I saw on a cable science program. I think it was Discovery's Miracle Planet mini-series. There was some excellent footage of a simulated large asteroid impact wherein the entire Earth's surface was effective vaporized. They then discussed the possibility of life surviving such a catastrophe and showed how it was possible for microbial life to survive underground. Even though the surface temperature was hundreds of degree's Celsius, IIRC it would take hundreds of years for the heat to reach even one kilometer under ground.

Brian