... i.e. a movement of the axis of rotation with respect to an earth-fixed reference system (polar motion) and a change in its rotational speed (length of day). Mass redistributions within the Earth and on its surface change the moment of inertia and cause the polar motion. The length of day depends on the main moment of inertia in the direction of the Earth's axis, the so-called J2 term. The polar motion is caused by the ice mass redistribution and leads to a further deformation, which again influences the polar motion. This feedback leads to an average velocity of the pole of a few centimeters per year, which, seen from today's North Pole, points in the direction of eastern Canada. The viscoelastic deformations caused by the polar motion must be taken into account accordingly in the geodetic observables as well as in the sea level. The movement is influenced also by other processes such as mantle convection, present-day mass redistribution at the surface and the pressure field at the core-mantle boundary (e.g. Mitrovica & Vermeersen, 2002).
References:
Martinec, Z., Hagedoorn, J. (2014): The rotational feedback on linear-momentum balance in glacial isostatic adjustment. - Geophysical Journal International, 199, 1823-1846. doi:10.1093/gji/ggu369
Martinec, Z., Hagedoorn, J. (2005): Time-domain approach to linearized rotational response of a three-dimensional viscoelastic earth model induced by glacial-isostatic adjustment: I. Inertia-tensor perturbations. - Geophysical Journal International, 163, 443-462. doi:10.1111/j.1365-246X.2005.02758.x