Is the core temperature of the Earth changing?

Is the core temperature of the Earth getting higher, getting lower, or stable? And what would be the long term implications of this?

Question posed by Rob

The deepest mine in the world is a South African gold mine called TauTona and is located in the West Witts gold field, west of Johannesburgh. It is just under 4km deep, and represents the deepest physical exploration of our planet by mankind. Compare this with the radius of the Earth at about 6,378 km, and you'll realise that we've barely scratched the surface of our planet: we've explored further upwards into space than we have inside our own planet.

Having said that, there are methods we can use to make inferences about how Earth works without actually needing to go there. We can study the planet's magnetic field, for example, or the way in which seismic waves travel through the Earth at different depths after an Earthquake.

How hot is the Earth's core?
The Earth's core is made up mostly of iron and nickel, with around 10% of it being some other substance (most likely sulphur or oxygen (the current favourite).Estimates of its pressure and composition allow us to estimate its temperature: The outer core is molten and thought to range from about 4,400°C (at the surface) to 6,100°C where it meets the (mostly) solid inner core, which has a temperature of around 5,430°C.

Is the Earth's core temperature changing?
The Earth (including its core) is cooling over time, for similar reasons to those which govern the cooling of a cup of tea: The surroundings are generally cooler, and heat is lost to them through a variety of mechanisms (mostly radiation in the case of the Earth). This cooling process is, however, very slow: The Earth's temperature is reducing by something like 1°C every ten million years*, so there's no need to start panicking just yet.

What are the implications of this change?

As the internal temperature of the Earth decreases, this will have a small effect on the atmosphere and hence things like weather and climate. There are, however, more important long-term considerations:

• The outer core will solidify

Our planet's liquid outer core is responsible for its magnetic field, which in turn keeps us safe from harmful radiation and charged particles blasted at us by the solar wind. Without a liquid core there is nothing to generate a powerful enough magnetic field and that protection will be removed.

• The mantle will solidify

The Earth's mantle is the layer of viscous rock upon which (to grossly oversimplify) the crust 'floats'. Bits of the crust move around on top of the mantle and we call this plate tectonics. This is where Earthquakes and volcanoes come from. When the mantle solidifies, plate tectonics will cease. The Earth will be smoothed out by erosion as there will be no processes by which mountain ranges are renewed. Tectonic processes also recycle carbon dioxide. Without them, all of the planet's CO₂ will be locked up in the rocks with no process to release it, cooling the planet's atmosphere and making it less hospitable to life as we know it.

• The Earth will shrink

As the cooling process occurs, much of the interior of the Earth will become more dense as temperatures no longer supply the energy required to keep the surrounding pressures at bay. This means that the same amount of stuff will take up less space, collapsing in on itself slightly. As a result, the crust will have less support and will also start to collapse inwards, buckling as it does (much as an overripe piece of fruit ends up with a wrinkly skin). This will probably result in a period of Earthquake and maybe even volcanic activity before settling down.

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* It's worth noting here that this cooling can be removed entirely from any discussion about 'global warming' or climate change. It's just not relevant to include it, and if you come across anyone who does use this fact as an argument against climate change you can be fairly sure that they don't know what they're talking about.