EFFECT OF MELTING GLACIERS ON THE EARTHS ROTATION AND GRAVITATIONAL-FIELD - 1965-1984

Yearly mass balance data for 85 glaciers in 13 of the 31 mountain glacier systems designated in Meier's (1984) paper are averaged for the years 1965-1984. Average rates of volume change for the 13 regions are compared to Meier's (1984) estimates of glacial melting. These data cover regions that account for about 74 per cent of the volume change estimated to have occurred in all the 31 mountain glacier systems from 1900 to 1984. The volume changes in the 13 regions tend to be less than those calculated by using changes in the volume of glaciers between 1900 and 1961 (Meier 1984). Although the reason for this difference is not well known, it could result from melting, during the first half of the 20th century, of ice accumulated during earlier periods, or a decrease in the rise in atmospheric temperatures from 1945 to 1980. Spatial and temporal empirical orthogonal functional analysis is used to bring out the similarities and differences among the 13 regions and identify those regions contributing most to the variance of the data set. These regions are not necessarily those with the greatest average rates of volume change. Enough spatial variability is present to support the hypothesis that satellites may eventually be able to discern changes in the Earth's glaciers through the gravitational field even though the signal from the mass change is small and changes in the gravitational field of the Earth are contaminated with gravity signals from other mass changes on the Earth's surface and interior. Zonal coefficients of the spherical harmonic expansion of the gravitational potential are calculated for yearly mass changes for these 13 regions up to l = 8. There is little similarity, at decadal periods, between the polar motion excitation from mountain glacier systems and the observed excitation. The majority of observed polar motion excitation could be due to other surface mass changes including continental water storage; see, for example, Chao (1988). The amplitude of the interannual excitation due to the glacial signal computed here is less than 10 per cent of the observed excitation and the secular trend of the excitation from melting glaciers in the 13 regions is about one quarter of that of the observed excitation. Yearly changes in the length of day (LOD) are calculated from the glacial signals and compared to the observed yearly DELTA-LOD (excess LOD) signal. The contribution of glaciers to the total excess LOD signal is less than one part in 150 of the observed signal, as DELTA-LOD is caused mainly by core-mantle coupling at decadal periods, and atmospheric coupling at shorter periods. The contributions to displacement of the centre of mass of the solid Earth as seen by a satellite tracked from the Earth's surface are calculated for the mountain glaciers and an 80 year record of global sea-level. For the glaciers, the average displacements are as much as 1 mm over 20 years and for sea-level, the Z-coordinate of the average displacement is 5 mm over its 80 year record. Interannual signals from tide gauge data are so noisy, we judge the interannual changes in sea-level to be unfit for use in calculating rotational or gravitational effects.