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Heat from the Earth’s interior does not control climate

Posted on 17 September 2011 by Andy Skuce

This blog post is the intermediate-level rebuttal to the climate myth “Underground temperatures control climate”. 

The myth:

"There are other possible causes for climate change which could be associated with solar activity or related to variations in the temperature of the liquid core of the Earth, which is about 5,400 degrees Celsius.  We don't need a high heat flow - just a high temperature for the core to affect the surface climate.  There is massive heat inside the Earth." Link. See here, also.

Consider:

  • The center of the Earth is at a temperature of over 6000°C, hotter than the surface of the Sun.
  • We have all seen pictures of rivers of red-hot magma pouring out of volcanoes.
  • Many of us have bathed in natural hot springs.
  • There are plans to exploit geothermal energy as a renewable resource.

Common sense might suggest that all that heat must have a big effect on climate. But the science says no: the amount of heat energy coming out of the Earth is actually very small and the rate of flow of that heat is very steady over long time periods. The effect on the climate is in fact too small to be worth considering.

The Earth’s heat flow

Where does the heat come from?

  • There are radioactive elements in the Earth, mainly potassium, uranium, and thorium, that have long half-lives. When their nuclei decay, they give off heat, as in a nuclear reactor.  Some researchers say that  "the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes", while other scientists claim that "heat from radioactive decay contributes about half of Earth’s total heat flux". More here.
  • The Earth is still hot from the time the planet formed from the agglomeration of smaller bits and pieces.  Even more heat was gained as the high-density materials, such as iron and nickel, subsequently separated out and formed the core of the Earth.

The mostly solid, rocky outer layers of the Earth, the crust and mantle, have low thermal conductivity, acting as a thermal blanket slowing down the passage of heat to the surface.  In the very early stages of the Earth’s history, internal temperatures and heat flows were probably much higher than they are today, partly because the planet had only just started to cool, and partly because the energy flow from radioactive decay was much larger then.

How does the heat get to the surface?

According to Stein and Stein (10MByte download) most of the heat energy (about 70%) that makes its way to the surface is transported by the convection of the mantle.  This is the process that drives plate tectonics. Most of the rest of the heat flow, 25%, is by conduction.  The small remainder is transported by mantle plumes, hot spots associated with certain volcanoes.

Figure 1: Showing mantle convection cells, which are responsible for transporting most of the Earth’s heat from the interior to the surface. Wikipedia

Mantle convection cells are the super tankers of global tectonics, transporting vast quantities of hot rock but changing speed and direction only gradually. Conduction of heat through the rocks of the Earth’s continental crust is also an unhurried and stable process; with the supply of heat metered by atomic clockwork. There are a few well-known hot spots around the world, where magma and hot water quickly bring heat to the surface but the energy released at these places does not add up to much in the global scheme of things. The rate of heat escape from the Earth is slow and very steady.

Figure 2: Red indicates the oceanic ridges where mantle convection comes to the surface and where new ocean crust is formed. The colors indicate the age of the oceanic crust, with the purple being the oldest. Source.

How do we measure heat flow?

The temperature gradient in the upper part of the crust is determined by directly measuring temperatures at different elevations in boreholes. On land, temperature measurements are usually made at depths greater than 100 metres to avoid any effect of variable surface temperatures. In the oceans, water temperatures at the sea bed are generally steady; measurements are  made in the uppermost layer of sediments and yield reliable results.  Once the thermal conductivity is known (it can be measured in a laboratory) the heat flow can be calculated using Fourier’s equation:

q = -ku

Where q is the heat flow, k is the thermal conductivity, and u is the temperature gradient.

Figure 3: Heat flow at the surface of the earth, from Davies and Davies (2010).  Heat flow units are in mWm-2. Note how the areas of highest heat flow follow the mid-ocean ridges. The largest areas of measurement uncertainty are along the very crests of the ridges and under the Greenland and Antarctic ice caps. The total heat flow for the planet is 47 TW +/- 2TW, which is equivalent to 0.09Wm-2   (90mWm-2).

Typically, the rate at which temperature increases with depth (the geothermal gradient) is in the range of 25-30°C per kilometer, with higher values at volcanoes, ocean ridges and rifts, and lower values in places that have recently received thick blankets of sediments.  The top several hundred metres of boreholes often show changes in the geothermal gradient that are caused by changes in the surface temperature that have modified the temperature of the rocks at these shallow depths.  These observations can be inverted to reveal paleoclimate information over the past few hundred years; see Huang et al (2000) and Beltrami et al (2011).

How does heat flow from the interior of the Earth compare with other inputs of energy into the climate system?

 

Figure 4: The volumes of the cubes are proportional to the magnitude of the energy flow from various sources. The solar irradiance is the incident energy, averaged over the area of the Earth (divided by four); irradiance varies over 11 year cycles and, at the top of recent cycles, can reach 341.7 Wm-2. The increase in anthropogenic forcing since pre-industrial times comes from the IPCC. The heat flow from the Earth’s interior is the 47 TW figure (see Figure 3 caption) averaged over the surface area.   The energy flow from the human energy production is based on Flanner (2009).  Tidal energy is the total energy input from the gravitational interaction between the Earth, Moon and Sun; a small part of this energy is included in the energy flow from the Earth’s interior (see below for further discussion).

The net increase in the amount of planetary energy flow arising from human activities (mainly the greenhouse effects from emissions of carbon dioxide) since the industrial revolution is more than twenty times the steady-state heat flow from the Earth’s interior.  Any small changes in the Earth’s heat flow over that time period—and there is no evidence for any change at all—would plainly be inconsequential.

Tidal Energy

From the Skeptical Science comments:

"Over the last two weeks I have been doing calculations on borehole data and this very convincingly supports the theory. We see different underground temperatures which are related to latitude, thus confirming that frictional heat (due to the moon) is being generated in the core, more at the equator than at the poles."

The spinning of the Earth, as well as the rotation of the Moon around the Earth and the orbit of both bodies around the Sun, do indeed have an impact on the energy of the Earth, through tidal friction. The ultimate source of this energy is the Earth’s rotation, to which the Moon and the Sun provide a gentle brake, resulting the generation of frictional heat and the slowing down of the Earth’s rotation (days were two hours shorter 600 million years ago).  The Moon gains some energy from this interaction, being gradually boosted into a higher orbit above the Earth. The total Earth energy flow from tidal effects is about 3.7 TW (0.007 Wm-2 ), of which 95% goes into the familiar ocean tides and some 5% (0.2 TW or 0.0004 Wm-2) goes into Earth tides, which are small deformations of up to a few centimetres that occur on twice-daily or longer timescales.  Earth tides contribute approximately 0.5% to the heat flow of the Earth.

Figure 5: From Munk and Wunsch (1998) showing an “impressionistic” (their word) budget of tidal energy fluxes.

The energy from tides in the oceans is dissipated as heat in marginal areas (shallow waters) and around ocean ridges and seamounts (the “stir sticks” of the oceans). All of this energy is therefore added immediately to the ocean-atmosphere system. As for the Earth tides, the slight flexing of the crust and mantle is dissipated as heat  there. This is a very small amount relative to the heat coming from radioactive decay and from the heat associated with the formation and differentiation of the Earth.

The amount of Earth tide energy flow, 200 gigawatts is miniscule by any planetary standard, it hardly varies at all over periods of millions of years and has no significant effect, globally or regionally, on the energy balance of the climate system.

Science isn’t always common sense

Diagrams such as the one below and its accompanying article make no mention of geothermal heat, tidal energy or “waste” heat from human fossil or nuclear energy use. Is this because its author, Kevin Trenberth, is negligent and unaware how big these sources of energy are?  No, it’s actually because he knows how inconsequential they are.

Figure 6. The global annual mean Earth’s energy budget for 2000 to 2005 (W m–2). The widths of the columns are proportional to the sizes of the energy flows.  From Trenberth et al (2009).

For example, on this figure, a line representing geothermal energy flow would have a thickness of 6 microns, the thickness of a strand of spider-web silk; ocean tidal energy, one-tenth of that; Earth tidal energy less than one-tenth even of that. Our intuitions tell us that earthquakes, volcanoes, geysers and tides are mighty forces of nature and, in relation to a human individual, they are. But compared to the transfers of energy within the climate system, they are too puny to merit consideration.

[Thanks to jg for drafting Figure 4 and to Tom Curtis for helpful comments.]

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Comments

Comments 1 to 26:

  1. One quibble: In the Tidal Energy section, if rotation is being slowed, then day length would have been shorter (not longer) 600 million years ago.
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    Moderator Response: [AS] Indeed! Thanks, I'll correct that.
  2. “Underground temperatures control climate”.

    Next myth:

    Lunar rotation causes tides therefore, it stands to reason, it pulls up on the Earth's crust. Thus the friction caused by the crust being raised by tidal forces causes temperatures to rise.

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  3. villabolo: "tidal forces causes temperatures to rise."

    Since the moon is gradually receding from the earth, tidal forces are growing weaker. That means that the heat from tidal friction is diminishing, leading to an inevitable cooling. With that mechanism, a glacial advance cannot be far off. Global warming? Problem solved!
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  4. The rounded figures are much more comforting. 0.9 of a watt per square meter imbalance does not sound much, yet that tiny imbalance is going to give us so much grief.
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  5. Very nice post, and hope you put Figure 4 in your figures area. One other interesting point you might mention is that about 50% of the total heat flow from the earth is 'primordial', i.e., remaining from the heat generated by gravitational collapse of the proto-planet. See all the articles on geoneutrinos.
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  6. I like the figure.

    A better representation would be 240 W m-2 for the solar figure though, since ~30% is reflected without doing anything. And we're on course to get more like 7 W m-2 from CO2 alone by 2100...
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  7. Where is the cosmic ray cube?
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  8. mlyle@5: I looked at a few references to see what the proportions of heat come from radioactivity compared to the three other proposed sources of heat (initial formstion, gravitational fractionation and latent heat coming from ongoing growth of the solid inner core). The precise numbers seemed to vary considerably, which is why I settled on the imprecise "Most comes from radioactivity". Wikipedia cites 45-90% coming from radioactivity but there's no reference given for the low end of the range. If you, or anyone else, could point me to a definitive reference on geoneutrinos, or a recent review paper on sources of internal heat, I would be most grateful.

    MarkR@6: You're correct about the reflected energy and I thought about including that that in Fig 4. The reflected energy is shown clearly in Figure 6. With all these figures there's a trade-off between getting the basic message across and rigorously including all the factors.

    And yes, we do plan to put Figure 4 in our figures area.
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  9. John Hartz#7: Are you suggesting that cosmic rays are a meaningful source of heat? On the scale of the diagram (fig 4), how big would that cube be?
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  10. Good debunking of yet another climate myth!

    MarkR in #6: You were faster than me with your comment about the 30% of reflected solar energy, the 102 Watts shown to the left in figure 6!

    Something that should put geothermal heat in the correct climate perspective:
    A couple of years ago I debated this issue with a denier on a Norwegian science website. My two central arguments were exactly what is mentioned here: The energy flow from the Earth’s interior is several thousand times smaller than that from the sun, and there are no reasons to believe is has changed much for millions of years. He then claimed that undersea volcanoes are a major heat source and that they can explain the recent warming. After some research on the heat capacity of water and the energy output from volcanoes measured from satellites, I came up with an interesting calculation:

    If 1000 undersea volcanoes, each with the same energy output as Etna in 1992 (12 gigawatts), had continuous eruptions, it would take them about 15,000 years to heat the oceans by 1 K if all that heat stayed in the oceans and wasn’t lost to the atmosphere and space. The heat added to each cubic metre of water every year would equal the body heat from a mouse swimming in that water for a few minutes!
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  11. "The heat added to each cubic metre of water every year would equal the body heat from a mouse swimming in that water for a few minutes! "

    Hmmm...I don't know about you, but I've noticed a substantial increase in mice rummaging through my garden recently.

    Next climate myth, coming right up! ;-)
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  12. @ muoncounter #9:

    About the size of a pinhead I would imagine.

    My question about cosmic rays was tongue-in-cheek.

    PS -- I'm suffering from Pielke-fatigue syndrom so please forgive me.
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  13. JohnH#12:

    I was hoping you were kidding; it's just so easy to put the blame for all the world's ills on the poor cosmic rays.

    Let's see: LBL's muon page gives an average surface flux of 167 muons per second per sq meter, with an average of 4 GeV each; convert eV per sec to watts; that's a delightfully toasty 10^-7 W/sq meter.
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  14. It looks like a really nice article presentation. However I have to protest the promotion of the idea that radioactive material heats the interior of the Earth. I mean, how do you go about testing this hypothesis? The answer is you cannot. No one knows what is in the center of the Earth. Maybe some people think that this is great since without a means of testing a hypothesis then no one can prove it wrong. But actually this is not good since if a hypothesis cannot be tested then it does not qualify as being a scientific hypothesis. To clarify why this is wrong let me give an example of another untestable hypothesis such as ‘all good people go to heaven after they die’. Untestable hypotheses are not actually science.

    Even without direct evidence, there is still plenty of indirect evidence that strongly weighs in against radioactive heating being correct. For example, if the entire interior of the 4.6 billion year old Earth was filled with radioactive potassium this still would not produce enough heat to account for the heat coming from the Earth’s interior. So there you have it, the radioactive heating claim is preposterous. It is only because so few people are capable of doing the math and / or willing to take the time to do it that the weakness of the radioactive heating claim is not apparent to everyone.

    No doubt it makes us feel good to have answers to life’s questions, but quickly filling every mystery with the first answer that ‘sounds right’ is counterproductive since it brings an end to science inquire. If either one of these propose answers to what heats the Earth’s interior were correct, then they should bring us greater insight into understand our Earth and our solar system. But of course they do not; their purpose is to just to explain away the paradox without concern if the answer is right or wrong. Obviously they are wrong since astronomers have rejected them in their efforts to explain Jupiter’s moon Io is being heated from within. In all likelihood, an answer that explains the heating of one heavenly body should be general such that it works for all the planets and moons of our solar system.

    Astronomers say that tidal heating is what heats Io’s interior. And here is the best part. It can be shown that tidal heating alone can explain the interior heating of all the planets and moons throughout our solar system. The theory correctly predicts which planets should be hot and which ones should not. As an extra bonus, the theory correctly predicts the density of these objects. For example, the Earth has the highest density of all the planets in our solar system due to close proximity of the Moon producing a disproportionally large amount of tidal heating within the Earth.

    One of the biggest problems with science today is that so many people blindly follow science dogma rather than thinking about the new progressive ideas that allow us to understand our reality.

    Read more at http://www.dinosaurtheory.com/hell.html.

    Galileo
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    Response:

    [DB] But then again, one of the biggest problems with science today is that so many people easily reject established science in favor of just about any other alternative that comes along, no matter how tenuous the evidenciary chain that supports it.

  15. From Galileo's website:

    "When scaling, the ratio between the area and volume changes: the area of my kite scaled by a factor of three squared while the volume scaled by a factor of three cubed. I was astonished that the Square-Cube Law was not covered in my grade school science class."

    Someone was asleep in math class ...
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  16. Bizarre claim on Doug Cotton's website:

    "It is the heat flow from the core of the Earth that maintains a "base" temperature simply because the underground conduction rate is far slower than the rate of convection for warm air rising in the atmosphere. Life as we know it would not exist on this planet if the temperature gradient caused by the heat flowing from the core were such that the mean "break out" temperature at the surface were not something like 9 deg.C give or take a few degrees."

    I would put it Doug Cotton that he should consider what the surface temperature of the Earth would be if it had the same solar insolation and same atmosphere, but didn't have a hot centre.

    More to the point, he should calculate what the surface temperature would be if you were able to turn off the sun and let the temperature equilibriate...
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  17. 14, Galileo,

    You make some interesting assertions, but the actual science weighs against you. You say:
    So there you have it, the radioactive heating claim is preposterous.


    The following paper pertains to actual measurements detecting such a source of heat:

    Earth's heat budget: Clairvoyant geoneutrinos (Jun Korenaga, 2011)
    The quantity of heat generated by radioactive decay in Earth’s interior is controversial. Measurements of geoneutrinos emitted from the mantle during this decay indicate that this source contributes only about half of Earth's total outgoing heat flux.


    You said:
    ...this is not good since if a hypothesis cannot be tested then it does not qualify as being a scientific hypothesis.


    And yet it has been tested.

    I'm not going to argue all of the details of a fairly theoretical aspect of science that doesn't entirely interest me, but it is one in which real scientists are now developing ways to actually observe and measure.

    I'm afraid your rather lordly and holy dismissal of the science was a bit premature.
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  18. Stu#16: "Bizarre claim ... Doug Cotton"

    I can't imagine how you could pick out just one bizarre claim, like identifying a single tree in that very large forest.

    For example, his geothermal gradient of 27 deg C/km puts the temperature at the outer core boundary at a mere 79700 C; why worry about an 1800% error?

    But sadly, that's not the most bizarre claim out we're hearing these days.
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  19. Some bizarre claims there #14 Galileo. You claim the radioactive heating hypothesis is preposterous, yet don't tell us why?

    Apart from Sphaerica's excellent point, I might ask you a more philosophical question. If lightning strikes can start forest fires, does that mean that arsonists are incapable of starting forest fires? This is akin to your suggestion that the same single explanation should fully describe the interior states of all the planets in the Solar System.

    You compare Earth to Io, yet one is a planet orbited by a large moon, and the other is a moon orbiting closely to the most massive object apart from the Sun in the Solar System. Io's mass is 1.5% of the Earth's. The objects driving the tides are vastly different - Jupiter's mass is something like 26,000 times that of the Moon, and that mass is driving tides on Io at a similar distance to the lunar tides. Why would the internal processes necessarily have the same origin? Sadly, it's another bizarre claim to add to the increasing list.
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  20. Maybe it's just me, but I felt that Galileo's comment was satire....

    As for Cotton's stuff, apparently the Earth, like Venus (which is heating due to an internal source too, dontchaknow) has a blue supergiant star instead of a ball of solid nickel-iron for a core. Don't ask why we don't weigh a lot more, why the Sun doesn't orbit around the Earth, how the Earth's crust is solid, how life evolved in a barrage of radiation that doesn't exist, etc.

    I know - Romulan force field, augmented with Borg technology. Oh, wait, Romulans use captured miniature black holes for their warp drives, not blue supergiant stars. OH! Maybe that's it, then!

    Oh, Doug, I've got a new idea for you....
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  21. Angliss @20:

    I sincerely doubt that Galileo #14 was merely satire. Only he can confirm or deny your interpretation.
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  22. #14: "there is still plenty of indirect evidence ... if the entire interior of the 4.6 billion year old Earth was filled with radioactive potassium this still would not produce enough heat"

    I'm not sure how this unsubstantiated hypothetical qualifies as 'evidence.'

    However, here is some actual evidence: Radioactive potassium may be major heat source in Earth's core

    "Our new findings indicate that the core may contain as much as 1,200 parts per million potassium - just over one tenth of one percent," Lee said. "This amount may seem small, and is comparable to the concentration of radioactive potassium naturally present in bananas. Combined over the entire mass of the Earth's core, however, it can be enough to provide one-fifth of the heat given off by the Earth."

    "the Earth has the highest density of all the planets in our solar system due to close proximity of the Moon producing a disproportionally large amount of tidal heating"

    The moon's proximity is the reason earth's density (5.52) is as high as it is? Then please explain the densities of Venus (5.2) and Mercury (5.6), all relative to water=1. Where are their close proximity moons? Why would more tidal heating result in higher density? And isn't 5.6 > 5.52?

    Funny thing about the real Galileo; he was right. I fear what we have here is an example of it's turtles all the way down.
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  23. I have found a few references on the radioactive contribution to heat flow. This one, which describes how geoneutrinos are measured and how they may provide constraints on the quantity of radioactive heating in the solid Earth. That reference led to this recent paper on the same subject, which reports that "heat from radioactive decay contributes about half of Earth’s total heat flux". The original statement in the blog post "Most of the heat that flows to the surface comes from this source" therefore needs to be amended, which I will do.

    Lord Kelvin did some calculations in 1864 on the thermal age of the Earth that neglected entirely any contribution from radioactive decay, which had not been discovered at that time. The text of some of the paper and be seen here and images of the original article can be viewed here. Kelvin came up with an age of the Earth on the range of 20-400 million years, but he later leaned more to the low side of this range. These figures were disputed by contemporary geologists and by supporters of Charles Darwin, who believed that much more time was required to account for geological and biological evolution. Of course, the natural philosophers eventually won that particular dispute with the physicists.
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  24. It's just comical (albeit predictable) to see skeptics even tried this one. I think the next one has to be:

    "Heat Release From Bioluminescent Sea Animals The Real Cause For Ocean Heating."
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  25. I'm a bit late, but I'd like to take issue with the statement about heat from the Earth's core: "The effect on the climate is in fact too small to be worth considering."

    Net solar heat flow is 341.3 - 101.9 - 238.5 = 0.9 W/m2
    Heat flow from the core is 0.09 W/m2. That's 10% of the net heat flow. That's significant.

    Net gain from solar and core together is 0.99 W/m2.
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  26. Martin @25:

    It is wrong to call all that "solar", the 238.5 number you provide must be IR losses to space, not solar.

    When it comes to climate change, you need to focus on radiation change. The net IR change related to doubling CO2 is about 4 W/m^2. How much do you think the 0.09 W/m^2 geothermal heat flux has changed? How important is that compared to 4?
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