Perturbations of the Earth's rotation and their implications for the present-day mass balance of both polar ice caps

Abstract

The present-day perturbations of the Earth's rotation are sensitive to the glacial isostatic adjustment (GIA) arising from the Late Pleistocene glacial cycles and also to the recent mass balance of polar ice caps. In this study, we evaluate the polar wander and the change of degree-two harmonic of the Earth's geopotential , proportional to the rotation rate, for four Late Pleistocene ice models. We examine these perturbations as a function of lower- and upper-mantle viscosities and lithospheric thickness and rheology (elastic or viscoelastic), in which a compressible Earth model with elasticity and density given by the seismological model PREM is used. By considering the observations and predictions including the GIA process arising from the Late Pleistocene ice and recent mass balance of polar ice caps, we discuss the recent mass balance of the Antarctic and Greenland ice sheets. We also examine the effects of internal processes and the melting of mountain glaciers, although this work is only preliminary. The results shown below seem to be supported even if these effects are included. Two solutions are obtained for source areas of the recent Antarctic melting. We denote an equivalent sea level (ESL) rise (mm yr−1)) from the Greenland and Antarctic ice sheets as and , respectively, being positive for melting and negative for growth. One, solution s1, is a solution satisfying the relationship , and the other, solution s2, generally satisfies the relationship and . In most cases, the magnitude of for solution s2 is larger than that for solution s1. The melting area of the Antarctic ice sheet for solution s1 roughly corresponds to the Weddell Sea region, approximately located on the symmetric part of Greenland to the Equator. The area for solution s2 is located on the symmetric part of Greenland to the centre of the Earth. In both solutions, therefore, the polar wander direction caused by the mass imbalance of each ice sheet is in an opposite direction. The reason for this is that the observed polar wander direction is nearly identical to the prediction from the GIA process for the Late Pleistocene ice models. However, it is difficult to independently examine which solution is better. If we consider a recent ESL rise of ∼0.6 mm yr−1 from the Greenland ice sheet, then a similar ESL rise of 0.5–1.0 mm yr−1 is also suggested for the Antarctic ice sheet around the Weddell Sea region. This solution also suggests the lower-mantle viscosity to be ∼1022 Pa s.