It is staying about the same temperature. Geological estimates are that the Earth loses heat at a rate of about 50 terawatts.  That’s about 50,000 1 gigawatt power stations worth of heat loss, i.e. if power stations pump out power at a rate of 1 gigawatt then you’d need about 50,000 of them – that’s how fast the Earth is losing heat through the oceans, continental surfaces, volcanic eruptions and so on, and that means that the processes inside the Earth produced heat energy at a similar rate to balance things out because the Earth isn’t cooling down that much.
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