ALBERT

Notes: [Auszug] In 2001 the Intergovernmental Panel on Climate Change projected the contribution to sea level rise from the Greenland ice sheet to be between -0.02 and +0.09 m from 1990 to 2100 (ref. 1). However, recent work has suggested that the ice sheet responds more quickly to ...

Notes: 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 ΔLOD (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 ΔLOD 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

Notes: We discuss the isostatic adjustment of the earth in response to Pleistocene deglaciation, and derive constraints on the earth's viscosity profile. We model the earth as viscoelastic, self-gravitating, realistically stratified and spherically symmetric. It is shown that the earth's relaxation eigenspectrum, besides possessing a small set of well-known discrete modes, also includes infinitely dense sets of modes (a continuous spectrum) which separate into classes that depend on the principal restoring force of the mode. Methods for estimating the continuous spectrum are discussed, and the effects on the predicted uplift are assessed. We find that the effects of the continuous spectrum are far less important than those of the discrete modes, though the presence of the continuous spectrum can make the discrete modes more difficult to identify. The continuous spectrum does become important when computing the rebound due to more recent loading, such as might be associated with present-day changes in polar ice.We use our model to refine predictions of the earth's response to Pleistocene glacial loading, with particular emphasis on implications for mantle viscosity. We have used time histories that are consistent with the recently recalibrated 14C time-scale. A special effort is made to match the geologic records of sea-level at sites in the vicinity of Hudson Bay. We find, consistent with the results of other authors, that the curvatures of the records are sensitive indicators of lower-mantle viscosity, but that there are inconsistencies between the records at different sites. However, we find we are also able to change the predicted curvatures by modifying the temporal and spatial distribution of the ice loads. If we assume a factor of 50 increase in viscosity between the upper and lower mantles, and if we assume that the ice on the eastern side of Hudson Bay melted a few thousand years later than the ice on the western side (a result consistent with recent geologic evidence), we are able to match the observed sea-level data about as well as we are able to, if we use, instead, a uniform mantle viscosity profile. A model with a factor of 50 jump in viscosity predicts a large free-air gravity anomaly over northern Canada that is reasonably consistent with what is observed, whereas a uniform viscosity model does not (though some authors have argued that the observed North American gravity anomaly may be mostly a consequence of mantle convection, and largely unrelated to postglacial rebound). Our objective here is not to present a preferred ice model or viscosity profile. Rather, we conclude only that the interpretation of the data is ambiguous, and that the mantle viscosity profile can not yet be uniquely inferred from them.

Notes: Synthetic body-wave traveltime inversion experiments were performed to evaluate the effect of unmodelled 660km discontinuity topography on the inference of aspherical, volumetric mantle structure. Synthetic residuals were computed for S waves refracted through a 660 km discontinuity with topography. We employed a spherical harmonic parametrization for lateral variations of the topography of the 660 km discontinuity and the mantle velocity structure. the radial dependence of the velocity structure in the mantle was parametrized in two ways: smooth Chebyshev polynomial functions and uniform shells. the results of both para-metrizations show that significant smearing of the input topographic signal appears in the models of volumetric mantle structure inferred from the synthetic data. Solving for higher order radial structure with the smooth Chebyshev functions reduces the smearing, but resolution is limited to the half-wavelength of the radial basis functions. More of the input synthetic residual variance is absorbed into the solution for volumetric structure by solving for higher order radial structure with smooth polynomials or with thinner shells directly below 660km depth. However, fundamental differences between the kernels for volumetric and topographic structure restrict recovery of the input signal to approximately 80 per cent. This work points to the possible value of relating volumetric and topographic structure when inverting seismic observations. This may be addressed by incorporating geodynamic modelling and mineral physics results into the modelling of seismic observations.

Notes: We invert ISC PcP and PKP absolute and differential traveltimes in an attempt to infer the long-wavelength topography of the core-mantle boundary (CMB). The data selection and processing methods are described and evaluated. These travel-time data are very noisy and the geographic distribution of the data is highly non-uniform, inhibiting reliable inference of CMB topography. Spatial averaging enhances the coherent component of the residual variance (related to heterogeneity), however, the random component of the variance is much larger than the coherent component. We show that for PcP data the coherent signal due to mantle heterogeneity overshadows that arising from the CMB, and that the effects of mantle heterogeneity are mapped into our inferred CMB solutions. The PcP data are not correlated across the spatial averaging bins and seem to have a strong bias due to small-scale structure and/or noise. The non-uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the CMB. Spatial patterns of CMB models inferred from different phases do not agree. Amplitudes of seismically inferred CMB undulations vary greatly. The sensitivity of inferred CMB models to the processing, spatial averaging procedure, and inversion techniques are investigated. Topographic amplitudes increase strongly with increasing input residual variance. The power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher. Based on this work, we conclude that reliable inference of long-wavelength CMB topography is not likely with the current ISC data set or with a spherical harmonic expansion truncated to degree and order 6.

Notes: Wahr (1987) used satellite gravity solutions for C21 and S21 to place constraints on certain properties of the core and core/mantle boundary. It is shown, here, that Wahr's constraints on the products of inertia of the core (that Wahr used to place bounds on the l= 2, m= 1 coefficients on the boundary topography and on the angular orientation of the inner core) are not justified, although his conclusions about the mean rotational flow in the core are correct, and his constraints on the time-averaged torque between the core and mantle and on the long-wavelength fluid pressure at the top of the core can be strengthened. C21 and S21 results from a recent satellite gravity field model are used to update and improve Wahr's core constraints. The new results are:(1) the time-averaged rotation axes of the core and mantle are tipped relative to each other by less than 3 X 10-8 radians;(2) the time-averaged torque on the core from the mantle in the equatorial plane is less than 2 X 1028 dyne cm;(3) the l= 2, m= 1 spherical harmonic components of dynamically supported pressure at the top of the core are less than 2 X 104 dyne cm-2.

Notes: Abstract Changes in polar ice could cause vertical crustal motion of up to several mm yr-1 along the edge of the Greenland and Antarctic ice caps. Measurements of the uplift could help constrain the changing ice volumes. The problem is complicated by the Earth's visco-elastic response to past loading, including the Late Pleistocene deglaciation. A method is described for removing these visco-elastic effects, by using simultaneous measurements of vertical motion and surface gravity. A linear combination of these two measurement types can be formed which is relatively independent of visco-elastic effects, and which can be interpreted in terms of present-day fluctuations in ice.