This is more of a theoretical thought-experiment question.
Basically, how much geothermal energy can we extract before the loss of the magnetic field makes it a terribly bad idea?
Will the geothermal energy inside the earth now, which is lost through "natural processes", last until the end of the Sun (say, another 5billion years?) And if so, how much excess is leftover?
a) do we know how much thermal energy there is? Call this: A
b) do we know at what point the core would stop producing a magnetic field? (ie: is it when the iron in the core is no longer molten?) Call this: B
c) do we know the rate that heat is radiated out naturally? Call the annual rate: C
So I guess: A-B - (5.0 * 10^9 * C) is the amount of heat for "safe" geothermal extraction?
also: Considering such extreme timescales -- are there any other factors adding heat into the Earth? Maybe tidal interactions with Sun/Moon/etc add anything over billions of years?