[thedoc (OP)]

Keith Reeves  asked the Naked Scientists:
   
Dear Chris
 
We are aware that the earth has a molten core, but surely over time this core will cool down; if so, I imagine it will be in millions of years' time?

My question is what would the effect of this occurrence be on life on earth? [Assuming that other un-related factors do not wipe out all life].
 
Regards
   
Keith Reeves
 
What do you think?

[damocles]

(1) Only the outer part of the Earth's core is liquid; the earth also has a solid "inner core"

(2) There are three main reasons why the Inner part of the Earth is hot:
• Residual heat from gravitational collapse when earth first formed
• Heat from the radioactive decay of thorium and uranium mostly, and other elements with long -lived isotopes.
• Heat from friction associated with tidal flexion of the solid Earth.

We do not have a really solid handle on the relative importance of these three, but most authorities say that the first is fairly minor, and that radioactive decay is a greater source of heat than tidal flexion.
Of these factors, radioactive decay will slowly become a lesser source of heat, but it will take billions of years to do so. Uranium decay energy will halve over 4 billion years. Thorium decay -- 3 times more abundant source, but only about 1/5 of the energy release for an overall factor around 60% that of uranium -- will take 14 billion years. I think the star physicists tell us that we only have about 5 billion years before the Earth is swallowed up by the sun, so we can look forward to little reduction in geothermal energy from radioactive decay. Tidal friction will produce less geothermal heating as the moon moves away from the Earth, and lengths of both the day and the month become greater. But this is happening at a very slow rate as well. We can look forward to a decline in geothermal energy over the next 4 billion years, but the decay will be so slow that it should still be about half the present level in 4 billion years time.

And geothermal energy is a tiny proportion of the heat reaching the Earth's surface: somewhere between 1/1,000 and 1/10,000 of the ground heating by solar energy.

I would suggest that geothermal heat has only a minute global effect on climate (it can have large effects locally, in the microclimates near hot springs or volcanoes, or temporary global effects, as with large volcanic eruptions throwing dust up into the stratosphere, which blocks out sunlight reaching the surface).

It is much more important to be worrying about possibly erratic behaviour by the sun than about the extremely slow cooling of the Earth's interior when thinking about future climates. But more important still is the introduction of large changes in the chemical composition of the atmosphere brought about by human activities.

[CliffordK]

One should add primordial heat to the heat budget of the Earth.

Perhaps one should think of Uranium as that we now have about half as much uranium on Earth as when Earth was created.  And, in 4 billion years, we will halve it again, or about 1/4 as much as when Earth was formed.  Along with the primordial heat, the radioactive decay portion of Earth's heat will reduce over time.

Over a period of billions of years, the moon will continue to recede from Earth, and thus cause weaker tides. 

Effects?
I agree that it will likely be minimal.  Fewer volcanoes may in fact be good

Antarctica melts from the bottom up from geothermal heat.  If the amount of heat being released from geothermal sources was halved, then it is possible that the antarctic lakes could freeze, and there could be greater ice buildup.

Of course, some of these effects could be offset if the sun slowly increases in thermal output as is predicted.  And, it all depends on stuff such as carbon dioxide and greenhouse gases.

[CliffordK]

As far as the effect on life.

Volcanism may be important for recycling minerals on the planet.  And Tectonic activity may counteract erosion. 

Without this activity, it is quite possible that much of the continents would slowly erode into the sea.  And, while there is a cycle for minerals, it is possible that slowly many of the nutrients would also leach into the sea. 

Thus, life on the continents might struggle as land mass and nutrients might decrease.  The oceans may also depend on the turnover of nutrients, so if less soil was being turned over on the continents, the oceans could also suffer.

Antarctica might freeze more completely, which could cause a greater buildup of ice at the poles.... Which would cause?

If there is a gradual warming of the sun, then a gradual cooling of the core may counteract the temperature changes somewhat, and help maintain a stable temperature, which would be good.

[Ophiolite]

ClifordK, I am not sure how - if at all - your primordial heat differs from Damocles' "Residual heat from gravitational collapse when earth first formed."

Both of you have also missed certain  pressure related phase changes which are exothermic.

I would strenghten the comments about tectonics, eorions, recylced minerals etc. The Earth is as it is on the surface because of plate tectonics. Without them carbon dioxide would not be recycled, the carbon reservoir for organisms would be massively reduced and temperature would fall as CO2 atmospheric content fell. The mountains would all be eroded in the sea, leaving a water planet. On the plus side, the sun will have burnt us to a crisp before those changes can happen.

[damocles]

ClifordK, I am not sure how - if at all - your primordial heat differs from Damocles' "Residual heat from gravitational collapse when earth first formed."

Both of you have also missed certain  pressure related phase changes which are exothermic.

I would strenghten the comments about tectonics, eorions, recylced minerals etc. The Earth is as it is on the surface because of plate tectonics. Without them carbon dioxide would not be recycled, the carbon reservoir for organisms would be massively reduced and temperature would fall as CO2 atmospheric content fell. The mountains would all be eroded in the sea, leaving a water planet. On the plus side, the sun will have burnt us to a crisp before those changes can happen.

I am a little mystified Ophiolite! Perhaps you can help me. Firstly, I also did not see either how primordial heat differed from my "gravitational collapse" (it did not worry me greatly though). But I realise that I should have said that there was a large initial heating which has slowly been conducted to the surface, and a smaller term associated with continuing gravitational collapse.

But my biggest problem is these "certain phase change" exotherms. I was trained as a physical chemist, and this training taught me to look at initial and final state, and not to worry too much about the middle man. Phase changes are surely not a source of energy in their own right? Something, whether it be gravitational differentiation, tidal flexion, or chemical reaction, must be driving the phase change, and so any resulting exotherm should be passed back up the line to one of these factors? A phase change can only occur because a system is not in equilibrium, and that means either that it was primordially not in equilibrium, or that some other factor has taken it out of equilibrium meanwhile. Are there any factors operating other than the three I have mentioned? Hydrodynamic effects from the liquid core perhaps? (And what is the driving force for them?)

Secondly, why is the geophysical carbon cycle important? Carbon is both being removed and being added to the surface environment by plate tectonics. Is the system not in a steady state? Why would the supply of biological carbon decline: it surely relies on the balance between the respiration of plants, aerobes, and animals, and the photosynthetic activities of plants?
Are you considering the main geophysical problem to be balance between formation and weathering of carbonate minerals? Or deposition of reduced carbon in the deep ocean and failure to return it to the surface environment? These are both very slow processes with a time scale of the order of 10 kyr, as opposed to around 8 yr for atmospheric recycling of carbon, and perhaps a matter of decades for oceanic recycling in the shallows.

[Mazurka]

OK wikipedia is frowned on, but in this case it accords with my understanding and a quick quote may be illuminating...

In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of Earth at between 20 million and 400 million years. He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface to cool to its present temperature. His calculations did not account for heat produced via radioactive decay (a process then unknown to science) or convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface

To answer the question...
As the earth cools certain types of volcanism become less frequent (there have been no recent kimberlite diatremes for example and large volume of basalt eruptions (flood basalt) decline through geological time).  It is reasonable to suggest that seafloor spreading (that in the simplest terms is the volcanic activity that drives plate tectonics) will also slow in due course.  This could result in a significant imbalence between erosion of continental land mass and subduction related volcanism or tectonic uplift resulting in a shallow sea across the globe. 

[Ophiolite]

Firstly, I also did not see either how primordial heat differed from my "gravitational collapse" (it did not worry me greatly though).

I'm sorry I was not clear. Your statement was wholly correct, but Cliford K said we needed to add primordial heat to your list. In my book, and it seems in yours, primordial heat arises from gravitational collapse, so your list had that covered. I was correcting ClifordK, not you.

Your point on the phase changes is a delight to me. I've been stumbling across passing references to this for years and could never be bothered to look into it more closely. I hesitated to put it into my post, but reasoned I might be challenegd which would force me to do a proper literature/text book search into the matter. I'm now starting the process. Be warned, I have been known to take four years to arrive at a uesful conclusion, but I promise progress reports.

My point on the carbon cycle is that the long term cycle depends upon carbon being recylced from carbonates that are carried into the mantle by subducting plates. If plate tectonics halts, then the bulk of that carbon remains sequestered in the mantle. Meanwhile carbon is continually taken out of circulation by deposition as limestone, or as burial of organic matter in anoxic conditions. With plate tectonics halted and therefore none of these deposits lifed and recycled through weathering and erosion the available carbon reservoir must fall.