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.
Abstract
Investigating changes in terrestrial water storage (TWS) is important for understanding response of the hydrological cycle to recent climate variability worldwide. This is particularly ...critical in India where the current economic development and food security greatly depend on its water resources. We use 129 monthly gravity solutions from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites for the period of January 2003 to May 2014 to characterize spatiotemporal variations of TWS and groundwater storage (GWS). The spatiotemporal evolution of GRACE data reflects consistent patterns with that of several hydroclimatic variables and also shows that most of the water loss has occurred in the northern parts of India. Substantial GWS depletion at the rate of 1.25 and 2.1 cm yr
−1
has taken place, respectively in the Ganges Basin and Punjab state, which are known as the India's grain bowl. Of particular concern is the Ganges Basin's storage loss in drought years, primarily due to anthropogenic groundwater withdrawals that sustain rice and wheat cultivation. We estimate these losses to be approximately 41, 44, and 42 km
3
in 2004, 2009, and 2012, respectively. The GWS depletions that constitute about 90% of the observed TWS loss are also influenced by a marked rise in temperatures since 2008. A high degree of correspondence between GRACE‐derived GWS and in situ groundwater levels from observation well validates the results. This validation increases confidence level in the application of GRACE observations in monitoring large‐scale storage changes in intensely irrigated areas in India and other regions around the world.
Key Points:
Spatiotemporal changes in water storage of India are characterized
GRACE records are validated using in situ groundwater levels from observation wells
Significant water storage loss from the indirect effect of climate variability
Greenland's main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade. Recent work has shown that Greenland's mass loss is still increasing. Here we ...show that the ice loss, which has been well‐documented over southern portions of Greenland, is now spreading up along the northwest coast, with this acceleration likely starting in late 2005. We support this with two lines of evidence. One is based on measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission, launched in March 2002. The other comes from continuous Global Positioning System (GPS) measurements from three long‐term sites on bedrock adjacent to the ice sheet. The GRACE results provide a direct measure of mass loss averaged over scales of a few hundred km. The GPS data are used to monitor crustal uplift caused by ice mass loss close to the sites. The GRACE results can be used to predict crustal uplift, which can be compared with the GPS data. In addition to showing that the northwest ice sheet margin is now losing mass, the uplift results from both the GPS measurements and the GRACE predictions show rapid acceleration in southeast Greenland in late 2003, followed by a moderate deceleration in 2006. Because that latter deceleration is weak, southeast Greenland still appears to be losing ice mass at a much higher rate than it was prior to fall 2003. In a more general sense, the analysis described here demonstrates that GPS uplift measurements can be used in combination with GRACE mass estimates to provide a better understanding of ongoing Greenland mass loss; an analysis approach that will become increasingly useful as long time spans of data accumulate from the 51 permanent GPS stations recently deployed around the edge of the ice sheet as part of the Greenland GPS Network (GNET).
Sea Level is Rising: Do We Know Why? Meier, Mark F.; Wahr, John M.
Proceedings of the National Academy of Sciences - PNAS,
05/2002, Volume:
99, Issue:
10
Journal Article
Peer reviewed
Open access
The gradual rise of sea level is one of the most troubling aspects of global change, especially because it is likely to accelerate in the future as global warming progresses. Meier and Wahr discuss ...some of the possible causes of this change and its implications.
The Gravity Recovery and Climate Experiment (GRACE), along with other relevant field and remote sensing datasets, was used to assess the performance of two land surface models (LSMs: CLM4.5-SP and ...GLDAS-Noah) over the African continent and improve the outputs of the CLM4.5-SP model. Spatial and temporal analysis of monthly (January 2003–December 2010) Terrestrial Water Storage (TWS) estimates extracted from GRACE (TWS
GRACE
), CLM4.5-SP (TWS
CLM4.5
), and GLDAS-Noah (TWS
GLDAS
) indicates the following: (1) compared to GRACE, LSMs overestimate TWS in winter months and underestimate them in summer months; (2) the amplitude of annual cycle (AAC) of TWS
GRACE
is higher than that of TWS
LSM
(AAC: TWS
GRACE
> TWS
GLDAS
> TWS
CLM4.5
); (3) higher, and statistically significant correlations were observed between TWS
GRACE
and TWS
GLDAS
compared to those between TWS
GRACE
and TWS
CLM4.5
; (4) differences in forcing precipitation and temperature datasets for GLDAS-Noah and CLM4.5-SP models are unlikely to be the main cause for the observed discrepancies between TWS
GRACE
and TWS
LSM
; and (5) the CLM4.5-SP model overestimates evapotranspiration (ET) values in summer months and underestimates them in winter months compared to ET estimates extracted from field-based (FLUXNET-MTE) and satellite-based (MOD16 and GLEAM) ET measurements. A first-order correction was developed and applied to correct the CLM4.5-derived ET, soil moisture, groundwater, and TWS. The corrections improved the correspondence (i.e., higher correlation and comparable AAC) between TWS
CLM4.5
and TWS
GRACE
over various climatic settings. Our findings suggest that similar straightforward correction approaches could potentially be developed and used to assess and improve the performance of a wide range of LSMs.
Measurements of ice elevation from the Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud, and Land Elevation Satellite can be combined with time‐variable geoid measurements from the ...Gravity Recovery and Climate Experiment (GRACE) satellite mission to learn about ongoing changes in polar ice mass and viscoelastic rebound of the lithosphere under the ice sheet. We estimate the accuracy in recovering the spatially varying ice mass trend and postglacial rebound signals for Antarctica, from combining 5 years of simulated GRACE and GLAS data. We obtain root‐mean square accuracies of 5.3 and 19.9 mm yr−1 for postglacial rebound and ice mass trend, respectively, when smoothed over 250 km scales. The largest source of error in the combined signals is the effect of the unknown time‐variable accumulation on the density of the ice column. To estimate this contribution and so obtain better estimates of ice mass trend and postglacial rebound, we add Global Positioning System (GPS) measurements of vertical velocities as additional constraints. Using an empirical relation between the errors in postglacial rebound and ice mass trend that result from the unknown density variation within the ice column, we are able to solve for all three unknowns in the problem: ice mass trend, postglacial rebound, and the snow compaction trend. The addition of a plausible distribution of GPS measurements reduces the errors in estimates of postglacial rebound and ice mass trend to 3.4 and 15.9 mm yr−1, respectively.
The GRACE satellite mission, scheduled for launch in 2001, is designed to map out the Earth's gravity field to high accuracy every 2–4 weeks over a nominal lifetime of 5 years. Changes in the gravity ...field are caused by the redistribution of mass within the Earth and on or above its surface. GRACE will thus be able to constrain processes that involve mass redistribution. In this paper we use output from hydrological, oceanographic, and atmospheric models to estimate the variability in the gravity field (i.e., in the geoid) due to those sources. We develop a method for constructing surface mass estimates from the GRACE gravity coefficients. We show the results of simulations, where we use synthetic GRACE gravity data, constructed by combining estimated geophysical signals and simulated GRACE measurement errors, to attempt to recover hydrological and oceanographic signals. We show that GRACE may be able to recover changes in continental water storage and in seafloor pressure, at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer, with accuracies approaching 2 mm in water thickness over land, and 0.1 mbar or better in seafloor pressure.
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