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Will all of the drilling and digging to use the earth's natural heat as geothermal energy be affecting Earth's core, causing it to cool down?
If so, would it result in an ice age?
If not, how does it retain its heat?

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6 Answers 6

up vote 20 down vote accepted

Part 1, see Neos answer. Earth will lose its heat no matter what we do, and our extraction of geothermal energy is insignificant (Wikipedia quotes a BP figure of 11.4 GW electrical, 28 GW heating).

To answer part 2 of your question: if the Earth's core loses its heat, this will not have a major direct impact on climate. Internal heat generation is estimated by Davies and Davies (2010) to be roughly 47 TW. With a surface area of 5.1 × 1014 m2, this translates to roughly 0.1 W/m2. This can be compared to the other flows in the climate system, illustrated by Trenberth and Fasullo, 2012:

Global energy flows
—Trenberth, Kevin E., and John T. Fasullo. "Tracking Earth’s energy: From El Niño to global warming." Surveys in geophysics 33, no. 3-4 (2012): 413-426. Weblink

So, from a climate perspective, internal heat generation is not important. See also this post on skepticalscience.

However, we might lose our atmosphere, which would have inconvenient consequences. An ice age would be the least of our worries. A subsequent question would be: (How long) would Earth's atmosphere last without a global magnetic field? That is a different question and I'm not sure if we really know the answer.

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You could still add the comparison to the total extracted geothermal energy, to make this a complete answer. Wikipedia cites a report that estimates 10715 MW globally (0,010715 TW). Which doesn't really contribute to the Earth cooling down. –  Spießbürger Apr 22 at 17:52
    
@Spießbürger Wikipedia cites two reports, one by BP, another by an IPCC subgroup. The second gives me an error 404. I'm hesitant to rely on BP for objective energy information. I lack the domain expertise to find other reliable figures. I'm sure that the conclusion that it is insignificant holds, though. –  gerrit Apr 22 at 17:57
    
@Gerrit Would the solar wind remove the whole atmosphere? And here was me thinking this question could be simply answered with something like there's enough radioactive decay heating the inside of the earth, together with tidal heating effects for us never to really worry about cooling the core. –  Siv Apr 22 at 18:15
    
@Siv I don't know, I have asked. –  gerrit Apr 22 at 18:16
    
hydrogen stripping I could go for. –  Siv Apr 22 at 18:17

This question is relevant, Why is the inside of the Earth so hot?

The short answer is the core is losing heat no matter what we do. You see, heat is transported from the core to the surface, but its important to think of heat in terms of energy. Since there is a finite amount of energy within the earth, we are actually transferring energy from the inside to the out. It works similar to a internal combustion engine found inside your car. You are converting a potential difference (High and Low temperature) into mechanical energy. In Earth's case, this mechanical energy is represented as as convection cells, which drive plate tectonics. Eventually, an engine runs out of gas, or in Earth's case, energy, and will begin to cool off.

When the core cools down, I don't think an ice age would be the right think to say. Mars would be a good example of what might happen to Earth when it loses most of its heat. There will no longer be tectonic events like volcanism, Earth will be a cold ball of mass. There will be a lot of ice, but eventually cosmic radiation and solar winds will destroy the atmosphere without the protection of the Earth's magnetic field leaving a barren planet with a largely homogenous surface. So sure, it will cause an ice age, but the ultimate destiny of the planet is barren, with a solid non-convecting mantle and core.

Edit:

I want to add this this is largely speculation, that we really don't know what will happen. I just assume that Earth will share a similar fate to Mars. Mars once had a magnetic field protecting its atmosphere, but as the planet cooled off the field disappeared. Mars's historic magnetic field is an area of contentious research.

As gerrit pointed out, Venus has an atmosphere without a magnetic field, so this is clearly postulation. Perhaps an expert will shed light on this question (How long) would Earth's atmosphere last without a global magnetic field?

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Internal heat provides only on the order of 0.1 W/m² to the surface. From a climate perspective, that is a minor source of energy. Losing this heat source will not cause an ice age. –  gerrit Apr 22 at 17:25
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@Neo - I think the answer could be improved by citing a few numbers. –  Spießbürger Apr 22 at 17:36
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@gerrit however, if the core solidifies we lose the magnetic field, after which our atmosphere is left largely unprotected and be blown away, decreasing the earth's surface temperature drastically. So in terms of heat the molten core is not leading to an ice age, although without atmosphere most water will probably evaporate eventually as well so perhaps you are right, cold but no ICE age. –  hugovdberg Apr 22 at 17:39
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@hugovdberg That may be true, but the answer makes no mention of any of that. But what will the activity of the Sun be by the time the Earth's core has cooled down, and how long will it take for the atmosphere to be blown away? –  gerrit Apr 22 at 17:43
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@Neo But Venus doesn't have much of a magnetic field either. Maybe an expert can share xyr knowledge answering this question. –  gerrit Apr 22 at 18:10

Put a frying pan on a stove burner and make the pan hot. Measure its temperature every minute over half an hour or so to get an idea of how rapidly it naturally cools.

Then start the experiment over again. This time, take a needle and touch and hold its tip to the frying pan so that it acts as a heat sink. The relative sizes of frying pan and needle will approximate "Earth and current methods of geothermal energy transfer". Measure the cooling over the next half hour.

If you run the two experiments a few times and compare results, you should find that it's practically impossible to detect any difference. The results can be extrapolated to estimate effects on the Earth, and a plausible conclusion is that it won't make a meaningful difference.

The simple reason is that our current geothermal efforts (as well as any currently projected future efforts) are so vanishingly small when compared to the size of the Earth that it has less effect than we can measure.

Now, that doesn't mean that some radical future change in technology won't change things. But no one can answer on those terms except to assert that we could make a significant difference if we could advance technology far enough.

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are we even using geothermal energy that wouldn't otherwise be expended? –  Neo Apr 23 at 0:05
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@Neo : No, it'll all be 'expended' no matter what we do. The only element of concern here is how rapidly any heat transfer happens between the interior and an area where radiation out to open space happens. For the relatively tiny amount involved, the transfer happens more quickly. It's just so small in relation to the scales involved that it doesn't matter. –  user2338816 Apr 23 at 0:31
    
Oh I see, that makes sense. I was a bit confused by the metaphor. –  Neo Apr 23 at 0:33
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Incidently this metaphor is very appropriate as far of the history of geology go since it s by warming a canon ball and looking at how long it took to cool down that Buffon made one of the first approximation of the age of earth (he was way off but at least closer in term of order of magnitude). –  plannapus Apr 23 at 8:03

Could all the drilling and digging to use the earth's natural heat as geothermal energy be affecting Earth's core, causing it to cool down?

Yes. But by how much? Let's do some rough math. We'll just be concerned with orders of magnitude here.

Suppose we have a uniform sphere the size of the Earth. Call it 1021 cubic meters.

Suppose this sphere is made of rock that is four times more dense than water. Water weighs 1000 kg per cubic meter.

Of course the Earth is not uniform; it is made up of rocks that are less dense and metals which are more dense. We're doing some rough math here.

And let's suppose that the interior of our planet is of uniform temperature, say, 5000 Kelvin.

Again, of course the Earth is not uniformly hot throughout. Again, we're doing rough math here, just to get an idea of the order of magnitude involved.

Let's suppose that our ball of rock is not producing new heat. Of course the Earth is producing new heat inside it, for instance, from radioactive elements in the core. But let's suppose that it is not.

And let's suppose that our ball of rock has a specific heat capacity of 0.8 joules per kilogram * kelvin. The specific heat capacity roughly speaking tells us how much energy is in some amount of a substance at a particular temperature. So multiply that out.

(1021 cubic meters) x

(4000 kg / cubic meter) x

(5000 kelvin) x

(0.8 Joules per kilogram * kelvin) =

1.6 x 1028 joules

We're just looking for an order of magnitude here. Our ball of rock has roughly 1028 joules of thermal energy.

Now let's suppose that we extract some amount of those joules. Total energy consumption of humanity from all sources -- nuclear, gas, etc -- is about 1018 joules per year. If we got 100% of that from our ball of hot rock, it would cool it off by one-ten-billionth of its total heat every year.

That's making the worst possible assumptions; of course we do not get anywhere even close to all our power from geothermal, the energy we do get was just going to be wasted into the atmosphere eventually anyway, the earth does make its own heat, and so on. We could get our total power needs met by geothermal energy for trillions of years without worrying about cooling the core.

how does it retain its heat?

The same way anything other ball of rock retains its heat. Heat, like all forms of energy, is retained indefinitely until something acts to remove it. I'm not clear on what question you're actually asking here.

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... causing it to cool down?

This answer to the question 'Why has Earth's core not become solid?' over on Physics seems to claim the answer is no.

The core is heated by radioactive decays of Uranium-238, Uranium-235, Thorium-232, and Potassium-40, all of which have half-lives of greater than 700 million years (up to about 14 billion years for Thorium).

The core isn't hot just because of remnant heat left over from formation, the heat energy in the core is continually renewed by radioactive processes.

If so, would it result in an ice age?

This energy from the core must already be continually dissipated up through the mantel, through the crust, into the atmosphere and eventually into space (or else the planet would be heating up).

All we could possibly do is speed the dissipation of this energy through the crust, any energy we extract would get to the surface anyway. As others have pointed out geothermal energy is a tiny fraction of what heats our atmosphere, most of that comes from the sun. Even if this were not the case, for us to cause an ice age, would require us to have near complete control over geothermal release through artificial means. We would have to extract enough energy over a long enough period from deep enough in the earth that there was no longer significant natural heat dispersion through the crust. Then we would have to stop and bottle up our manual extraction so that the heat had no other means to escape but rising through the crust in the natural way. The heat present in the atmosphere would dissipate into space far quicker than new heat would rise through the crust.

I imagine both the process of our intervening to the point of control, and our sudden relinquishing control would both have significant effects aside from climate change: earthquakes, volcanic eruptions, disrupting continental drift...

If not, how does it retain its heat?

I hope it is clear that it doesn't.

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its losing heat slowly, because of its Thermal Diffusivity is very high. Thermal Diffusivity is the relative measure to how much energy a substance can store (per volume) to how much it can output (conductivity). –  Neo Apr 23 at 18:57

You have to start with the cause of the heat: radioactive isotopes are distributed throughout the earth and, since thermal radiation occurs at the surface, the deeper you go the hotter it gets. Radioactive isotopes decay at a fixed rate and some have very long half lives so this heat is released over the lifespan of the earth. The earth's heat is not (mostly?) due to kinetic energy left over from its formation or tidal squeezing by the moon. Because of the scale nothing we do will make much difference to this situation.

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groundwateruk.org/downloads/4%20Busby.pdf states "Much of the Earth's heat, between 45-85% is derived from the decay of the radioactive isotopes of Uranium Thorium and Potassium concentrated in the crust and mantle. The other main source is the primordial energy of planetary accretion. The Earth is cooling very slowly, the temperature of the mantle has decreased no more than 300-350 degrees C in 3 billion years, remaining at about 4000 degrees C at its base". –  Siv Apr 23 at 18:02

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