We live on maybe ±1km of the surface or so; we don't dig much deeper than that except perhaps when exploring very deep caves or so. The radius of the Earth is approximately 6300 km. One volume is (4 pi/3) R^3 and the other is 4 pi R^2 dR, so the one volume is larger by a factor of 6300/3; we can only hope to visit one two-thousandth of the Earth's total volume with holes 1km deep. Up that to 10km deep and you'll still only get one two-hundredth of the Earth. Oh, and we've only got about a third of that because oceans.

Oddly enough, constantly growing usage can cut into that time-to-depletion estimate fairly quickly. Present estimates are that US coal reserves are ~200 - 1,000 years, depending on the recoverable yield.

Or you can fear monger and oppose change and prefer that we burn fossil fuels indefinitely, I guess.

Given the large amount of statistics and facts that you bring to the question it is clearly evident that you know more about this problem than I do. I merely wish to point out that a simple linear scaling of current consumption levels is not a convincing model for adoption of any new technology / paradigm. You had me on the facts, but lost me on the projection.

I am aware that constant usage is not generally a valid assumption. I made a simplifying assumption that I believe reasonable given the sheer size of the energy store. It doesn't matter much if our usage is constant or increases a thousandfold. There's so much energy in the crust that we essentially could not use it all, and I doubt that we would use enough to cause global issues (vs local issues caused by overuse in certain areas which seems more feasible).

P.S. To be clear, I don't know much about this problem at all. My only data is from Wikipedia and its references.

One of the points that TeMPOraL made above is "and again, trees were a renewable resource too, before the industrial revolution.". The point being that resources that are seemingly infinite when first used can end up being not so unlimited.

So for example, the person downthread who uses energy trends estimates a lower bound of about 1000 years (if current energy consumption trends continue, we immediately switched to only geothermal, and we used the entire heat of the earth edit: Earth's crust, not the whole earth). So now we have a range of 1,000 to 800,000 years (based on varying assumptions).

800,000 years to take up 0.001% of the total heat would be fine. 1,000 years to take up 100% of the heat would be really bad.

The 1,000 year quote is based on historical data, and 800,000 years is based on flat/zero growth. So there appears to be room for discussion, not all of which is 'fear monger'ing.

It's legitimate to ask what our energy usage might grow to. It's not legitimate to dismiss potential energy sources because we cannot predict the future with perfect accuracy. We will never predict the future perfectly, or even close, and if we have a choice between killing the planet in 1000 years or killing it in 100, we'd still be better off taking the first option.

The "1000 years" comment also assumes we'll double our energy usage every 25 years. If we accept that as true then our only viable strategy is to get off this planet because it assumes that in 1000 years we will literally have extracted all energy from the core. We're doomed if this is our future.

Also, please stop mentioning the trees. This has been addressed multiple times now. The industrial revolution didn't result in trees being cut down. Quite the opposite. The large-scale adoption of fossil fuels resulted in forests rebounding worldwide as we stopped relying on timber as a fuel.

> *The 1,000 year quote is based on historical data, and 800,000 years is based on flat/zero growth. So there appears to be room for discussion, not all of which is 'fear monger'ing.

It's rather generous to say it was based on historical data. It was based on taking the log2() of our current energy usage and eyeballing an energy usage chart to produce a guess at a 25-year doubling rate. Our energy growth is no more a constant than our usage, though, so extrapolating this way is as unrealistic as assuming we'll stay at present-day usage forever.

There is certainly room for discussion. Again, though, I'll point out that a discussion should be about options and not just the worst case for geothermal exhaustion. If we found that we could replace all of our fossil fuel use with geothermal but we'd only get to extract energy for 1000 years safely, that would be a great deal, because the obvious alternative is that we literally burn every bit of fossil fuel we can extract from the Earth.

The argument sometimes gets confused, since conservationists are concerned about the loss of old-growth forests, and animal habitats vs. the quantity and area of forests.

Fine, let's quadruple consumption – we're good for 20 million years. Ok, okay, let's go to 100x consumption – we're good for 800,000 years. I think we're ok.

It's obvious that the answer is less than 100%, since a completely solid core would provide no magnetic field. Is there some way to tell whether it's closer to 0.1%, 1% or 10%?

It's not really possible to use enough geothermal for it to have a noticeable effect on the lifespan of the magnetic field. Even completely removing the solid crust of the earth would not really register.

So we could take out some percentage of the heat of the crust, thus lowing the total temperature by some amount, but then that loss would be replenished from further down in the core.

It looks like from the calculations elsewhere in the thread that it probably wouldn't even be physically possible to extract that much energy from the crust. We'd just run out since there would be some lag time in replenishing the heat from the core through to the areas of the crust.

But also let's hope it's not 0.001% because I'm pretty sure the core will cool more than that on its own over a shorter timespan than 19 million years.

19 million years is 1.9% of the billion year rate. So it suggests the core should lose 1.9C in 19 million years. That's about 0.035% of the total temp, using Wikipedia's 5700C as the temp estimate.

That's a great initial guess, given how much uncertainty is (i've recently heard core temp could be 4x the wiki number, and some people estimate the earth's core only has lost a few degrees since it formed, rather than nearly 500 degrees). Neat stuff, thanks for providing the opportunity to look in to this.

No idea how good the source is, however it looks like we have a nice exponential there.

Lets see what the envelope says for an exponent of 2:

Using numbers from parent commentors, log2(6.6 x 10^13) is about 45.9, so we are in our 46th doubling period. A guess on the graph says its 25 years to double.

> The total heat content of the Earth is of the order of 12.6 x 10^24 MJ, and that of the crust the order of 5.4 x 10^21 MJ (Dickson and Fanelli, 2004).

so that's end of crust energy usefulness at the start of the 72nd period and end of Earth energy less than halfway through the 84th period.

So that gives us 500-1000 years of energy from now if we solely use the planets heat. Whether we soley use it or not becomes largely irrelevant due to the exponent, we can stretch it as a major source a few hundred more years or so.

Of course, if in 1000 years we aren't only using this planet we should be good. How many years to the next system again?

We are willing to concede that usage patters from 100 years ago are unrealistic current realities.

However we would simultaneously like to use the current reality to project future realities that are much farther away (in some cases 19 million years away).

Yeah, our dependence on fossil fuels is a huge problem, and not just because of climate change (which might also kill us).

Mobile phones didn't exist when I was young, and there are certainly a lot of them now even if they are low power devices, so it would be interesting to see electrical usage patterns pre-mobile phones vs. today.

The same goes for cars. Cars were using leaded gasoline in the US and getting 8 to 12 mpg, and now 30 to 35 mpg or greater on the highway, but do people drive more frequently, and with single riders?

Another question is would any cooling of the total Earth's heat energy be offset by global warming? The thermal differential would be less thereby slowing the migration of heat to the surface?

Or maybe we want to terraform our own planet to counter climate change. These things could use amounts of energy orders of magnitude higher than we use today.

I'm sure 100 years ago didn't expect we would launch 100,000 airplane flights every day [0] :) Nor did they think we would send spaceships to Mars.

[0] http://www.garfors.com/2014/06/100000-flights-day.html

It seems somewhat unreasonable to assume that our energy usage is going to exponentially increase due to as-yet-unforeseeable future needs and at the same time assume that we'll completely fail to find additional energy sources in the same timeframe.

Earth has a fixed surface area. And the only way to get rid of waste heat from energy production is to radiate it into space. Finding this equilibrium for given quantities of energy generation into waste heat is a pretty straightforward calculation.