Has climate changed the Earth's tilt?

Abstract

Have the ice ages secularly altered the obliquity (axial tilt) of the Earth over geologic time? The waxing and waning of ice caps in response to obliquity oscillations plus mantle adjustment to the weight of the caps alter the Earth's dynamical flattening. This affects the lunar and solar torques on the Earth so as to secularly change the Earth's axial tilt. This mechanism is dubbed “climate friction,” since analogous to tidal friction, it arises from lags in the Earth's response to cyclic external forcing. But the existence of two processes, ice cap waxing and waning and mantle viscous flow, can lead to either an increase or decrease in the obliquity. Evidence indicates that the growth and decay of the ice caps greatly lag the orbital forcing; this causes the axial tilt to increase with time. But the ice cap effects are partly canceled by viscous compensation in the mantle. Low mantle viscosities (about 1021 Pa·s) lead to rapid compensation and have only a slight effect on obliquity. High viscosities (about 1022 Pa·s) slow the compensation enough so that there could be significant secular change in the obliquity over geologic time, perhaps explaining all of the present tilt of 23.5°. However, in addition to the current uncertainty as to the effective viscosity of the mantle, knowledge of past ice ages is incomplete, so that the amount of obliquity increase presently remains unknown.