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1 Adam: The sub's ready!
1 Jamie: Please tell me it doesn't burn coal for fuel.
2 Adam: Of course not; that's environmentally irresponsible. It uses totally carbon-neutral renewable energy!
3 Jamie: Well okay then! Let's get going.
4 Adam: We just need to find a handy source of geothermal heat adjacent to Loch Ness.
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Geothermal energy is considered a renewable energy source, despite technically being limited to the sum total of energy produced by the decay of radioactive isotopes within the Earth's body. This total heat flow from the interior of the Earth to the surface is estimated at 47 terawatts (47×1012 watts).
The total world energy consumption is estimated to be of the order of 1021 joules per year, which is about 30 terawatts. So currently, in theory, geothermal power could supply all of the world's energy (assuming it's all captured and used with high efficiency).
However, since geothermal energy comes from radioactive decay of a combination of long-lived isotopes, it is decaying with a half-life of approximately 2 billion years. Our civilisation's energy consumption will likely increase much faster than that, and within a few decades geothermal power alone won't be enough, even theoretically.
Eventually, after many billions of years, the radioactive decays producing energy will die away to a point where the available energy to be harnessed from geothermal power will be too small to be worthwhile. In this sense, geothermal energy is, strictly speaking, non-renewable. However, before we reach that point our sun will run out of readily available nuclear fuel in its core in just 4 billion years, and expand into a red giant star, which will almost certainly destroy any civilisation left on Earth.
 I had no idea before I looked up these numbers and did the calculation how close these numbers would turn out to be. I guessed geothermal power would theoretically be thousands of times more power than humanity uses. But no! Given these numbers, geothermal power can essentially never supply all of our modern civilisation's energy requirements. We better get to work on those solar thingies.
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