21 - The global energy budget

Summary

Preamble

If the Earth's internal heat were not maintained by radioactivity, the present rate of heat loss, 44.2 × 10¹² W, would cause cooling at an average rate of about 120 K per billion years. Even over 4.5 billion years, cooling at this rate would have had only a moderate effect on the temperature of the deep interior. Although radioactivity is the dominant continuing source of internal energy, it is only topping up the primordial heat generated by the Earth's accretion. It is slowing the cooling rate but is not the reason why the interior is hot. But radioactivity is decreasing slowly with time and does not balance the heat loss. The Earth is cooling at a rate determined by the difference between the surface heat flux and the radiogenic heat. Mantle convection and tectonics are slowing down as the Earth cools and the rate of change is determined by this unbalance.

A substantial source of internal energy is necessary also to maintain core convection, which drives the geomagnetic dynamo. The Earth has had a magnetic field for at least 3.5 billion years and probably for its entire life. The power requirement for dynamo action is probably less than the drain on core energy by thermal conduction, and if the core has some radioactivity then its effect is to compensate for the conductive heat loss. The necessity for this depends on the core conductivity, which is still poorly determined.