True Polar Wander and the Cooling of Earth

It is commonly assumed that Earth must have been fairly hot in its early history, though few observational constraints exist on the cooling rate of the bulk Earth. We demonstrate that maximum rates of true polar wander—wholesale rotation of mantle and crust around the core—have drastically decreased over the past one billion years. As true polar wander is rate-limited by the viscosity structure of the mantle, we interpret that the observed decay reflects the secular cooling of Earth. The magnitude of the decay indicates that the viscosity of the lower mantle has increased by an order of magnitude, and such a viscosity increase requires a cooling rate of greater than 100 K/Ga for the lower mantle. This rate is compatible with a recent petrological estimate on the cooling rate of the upper mantle, suggesting that the mantle as a whole has experienced rapid cooling since Neoproterozoic time.