Well, the Earth's paying for it, and it'll turn into solid rock slightly earlier than it otherwise would have due to modern Icelandic district heating practices.
As the Earth is mostly an approximately Earth-sized ball of molten rock it's generally agreed that this will take "a while".
We've also decided that it's such a large value of "a while" that we're going to split the difference and proclaim that this non-renewable geothermal energy extraction practice is actually renewable.
Nevermind that we're talking about time scales where we might expect all the non-renewable fossil fuel we're extracting to have been renewed several times over by natural processes.
I think geothermal energy might get away with it on a technicality: The Earth won't actually turn solid. Before it can do that it's expected to be swallowed up by the Sun turning into a Red Giant.
By that time geothermal heat might have become too much of a good thing.
As long as the heat is not turned into other forms of energy (electricity, potential energy, chemical energy, etc.), it is actually not being "used". Just moved to a slightly different place. Such usage actually wouldn't speed up cooling off the Earth even by a miniscule amount since the heat is at the surface already.
While in Iceland good sources of heat may be close to the surface, so exploiting them may not change a lot the cooling speed of the Earth, in most places in order to obtain enough geothermal energy the heat must be brought from high depth and its use is equivalent with significantly reducing the thermal resistance of the upper layer of Earth's crust at that point, increasing the speed at which the Earth is cooling.
The deepest borehole ever drilled is just over 12km, it's 6371km to the Earth's core. If the Earth was the size of a basketball that would get you around 1/8th of a millimeter into the surface.
The reason geothermal's easier in Iceland is that the heat's close to the surface, so you can get away with shallower boreholes.
I'd think the overall effect would be trivial either way, but I don't see how the thickness of the crust would make a difference.
As the Earth is mostly an approximately Earth-sized ball of molten rock it's generally agreed that this will take "a while".
We've also decided that it's such a large value of "a while" that we're going to split the difference and proclaim that this non-renewable geothermal energy extraction practice is actually renewable.
Nevermind that we're talking about time scales where we might expect all the non-renewable fossil fuel we're extracting to have been renewed several times over by natural processes.
I think geothermal energy might get away with it on a technicality: The Earth won't actually turn solid. Before it can do that it's expected to be swallowed up by the Sun turning into a Red Giant.
By that time geothermal heat might have become too much of a good thing.