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I recently came across this article outlining the use of our advancements in oil and gas drilling to implement "district heating" (or geothermal heating for our buildings/cities as a whole) by pumping water underground to be heated by the mantle, then pumped back up to the surface. I like the premise and it sounds great, but what are the effects? This seems like alcohol expanding our capillaries, we feel warmer but as a whole are getting colder as heat is lost more rapidly on the surface.

If accomplished globally would there be a significant decrease in the temperature of the mantle?

Would areas of high use cool the mantle there, forcing a deeper crust at those locations? Or is the amount of energy used so minuscule in comparison there'd be no discernible difference? What about after a millennia of use?

Hopefully helpful diagram for reference? Maybe redundant in this SE, I just joined so lmk

crust-mantle-core-diagram

(image credit)

I know I asked a few questions, but those all stem from the title "How would district heating affect the temperature of the earth's mantle/core?"

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    $\begingroup$ This is done within the crust, it never reaches the mantle (the deepest borehole ever reached 12 km, while the continental crust is 30 km thick on average). So it would not change the mantle's temperature. Note that the mantle already loses heat all the time (about 90 mW/m$^2$) by conduction through the continental crust, so it would just mean to actually gather it and use it instead of just losing it to space. en.wikipedia.org/wiki/Earth%27s_internal_heat_budget $\endgroup$ – Jean-Marie Prival Nov 19 '20 at 8:30
  • $\begingroup$ @Jean-MariePrival But wouldn't it lose more heat at those specific points as cool water is pumped down? That cools the crust which in turn draws more heat from the mantle. i.e. a baked potato fresh out of the oven with internal temp of 400F with 5 layers of paper towels around it, one side of the paper towels has straws with cold water pumping through them, wouldn't that side of the potato cool faster? I can see how we might write it off as negligible, but from a physics perspective I don't see how it wouldn't change the temperature at all - either way, thanks for the link. Still reading. $\endgroup$ – TCooper Nov 19 '20 at 17:15
  • $\begingroup$ Good point, but I'm not sure it would matter. In the mantle heat is transferred by convection, which is highly efficient. So, if you lower the temperature at a point in the mantle by pumping too much heat from the crust above, it would go back to equilibrium temperature by heat transfer from neighboring mantle. At least that's how I picture it, but I'd love an input from someone else! $\endgroup$ – Jean-Marie Prival Nov 20 '20 at 9:25
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    $\begingroup$ Actually, there is a place on Earth where what you describe is kind of happening: the mid-ocean ridges. Look at the first map in the Wikipedia article: you'll see that mid-ocean ridges are the places where the heat flux is the highest, and it's under water. If the underlying mantle was significantly cooled, the magmatic output would stop at some point. But these ridges can keep spreading for 100–200 millions years. $\endgroup$ – Jean-Marie Prival Nov 20 '20 at 9:31

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