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We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods--especially in Greenland and West Antarctica--and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 ± 49, +14 ± 43, -65 ± 26, and -20 ± 14 gigatonnes year(-1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year(-1) to the rate of global sea-level rise.

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). Copyright © 2010 by the American Geophysical Union.

In this study we estimate a time series of regional groundwater anomalies by combining terrestrial water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) satellite mission with in situ soil moisture observations from the Oklahoma Mesonet. Using supplementary data from the Department of Energy's Atmospheric Radiation Measurement (DOE ARM) network, we develop an empirical scaling factor with which to relate the soil moisture variability in the top 75 cm sampled by the Mesonet sites to the total variability in the upper 4 m of the unsaturated zone. By subtracting this estimate of the full unsaturated zone soil moisture anomalies, we arrive at a time series of groundwater anomalies, spatially averaged over a region approximately 280,000 km2 in area. Results are compared to observed well level data from a larger surrounding region, and show consistent phase and relative inter-annual variability.

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 climate perturbations than previously thought, particularly near the coast. Here we use a satellite gravity survey by the Gravity Recovery and Climate Experiment (GRACE) conducted from April 2002 to April 2006 to provide an independent estimate of the contribution of Greenland ice mass loss to sea level change. We detect an ice mass loss of 248 +/- 36 km3 yr(-1), equivalent to a global sea level rise of 0.5 +/- 0.1 mm yr(-1). The rate of ice loss increased by 250 per cent between the periods April 2002 to April 2004 and May 2004 to April 2006, almost entirely due to accelerated rates of ice loss in southern Greenland; the rate of mass loss in north Greenland was almost constant. Continued monitoring will be needed to identify any future changes in the rate of ice loss in Greenland.

Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment satellites, we determined mass variations of the Antarctic ice sheet during 2002-2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 +/- 80 cubic kilometers of ice per year, which is equivalent to 0.4 +/- 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.

This study presents the first direct comparison of terrestrial water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to in situ hydrological observations. Monthly anomalies of total water storage derived from GRACE gravity fields are compared with combined soil moisture and groundwater measurements from a network of observing sites in Illinois. This comparison is achieved through the use of a recently developed filtering technique designed to selectively remove correlated errors in the GRACE spectral coefficients. Application of this filter significantly improves the spatial resolution of the GRACE water storage estimates, and produces a time series which agrees quite well (RMS difference = 20.3 mm) with the in situ measurements averaged over an area of ∼280,000 km2.

The GRACE satellite mission is mapping the Earth's gravity field at monthly intervals. The solutions can be used to determine monthly changes in the distribution of water on land and in the ocean. Most GRACE studies to-date have focussed on producing maps of mass variability, with little discussion of the errors in those maps. Error estimates, though, are necessary if GRACE is to be used as a diagnostic tool for assessing and improving hydrology and ocean models. Furthermore, only with error estimates can it be decided whether some feature of the data is real, and how accurately that feature is determined by GRACE. Here, we describe a method of constructing error estimates for GRACE mass values. The errors depend on latitude and smoothing radius. Once the errors are adjusted for these factors, we find they are normally-distributed. This allows us to assign confidence levels to GRACE mass estimates.

We use 22 monthly GRACE (Gravity Recovery and Climate Experiment) gravity fields to estimate the linear trend in Greenland ice mass during 2002-2004. We recover a decrease in total ice mass of 82 ± 28 km3 of ice per year, consistent with estimates from other techniques. Our uncertainty estimate is dominated by the effects of GRACE measurement errors and errors in our post glacial rebound (PG) correction. The main advantages of GRACE are that it is sensitive to the entire ice sheet, and that it provides mass estimates with only minimal use of supporting physical assumptions or ancillary data. Copyright 2005 by the American Geophysical Union.

Eleven monthly GRACE gravity field solutions are now available for analyses. We show those fields can be used to recover monthly changes in water storage, both on land and in the ocean, to accuracies of 1.5 cm of water thickness when smoothed over 1000 km. The amplitude of the annually varying signal can be determined to 1.0 cm. Results are 30% better for a 1500 km smoothing radius, and 40% worse for a 750 km radius. We estimate the annually varying component of water storage for three large drainage basins (the Mississippi, the Amazon, and a region draining into the Bay of Bengal), to accuracies of 1.0–1.5 cm.

The NASA/DLR satellite gravity mission GRACE, launched in March, 2002, will map the Earth's gravity field at scales of a few hundred km and greater, every 30 days for five years. These data can be used to solve for time-variations in the gravity field with unprecedented accuracy and resolution. One of the many scientific problems that can be addressed with these time-variable gravity estimates, is post glacial rebound (PGR): the viscous adjustment of the solid Earth in response to the deglaciation of the Earth's surface following the last ice age. In this paper we examine the expected sensitivity of the GRACE measurements to the PGR signal, and explore the accuracy with which the PGR signal can be separated from other secular gravity signals. We do this by constructing synthetic GRACE data that include contributions from a PGR model as well as from a number of other geophysical processes, and then looking to see how well the PGR model can be recovered from those synthetic data. We conclude that the availability of GRACE data should result in improved estimates of the Earth's viscosity profile.

The Gravity Recovery and Climate Experiment (GRACE) satellite mission was launched in March 2002 and has an expected 5-year lifetime. One potential application of GRACE measurements of time-variable gravity will be to isolate the postglacial rebound signal, which can then be used to estimate the Earth's viscosity structure. In this paper we present a sensitivity analysis of simulated GRACE data, designed to assess the accuracy with which those data can be used to recover a simple model of Earth viscosity. We find that without combining with any other data type, but ignoring complications caused by uncertainties in the global ice loading history, GRACE data alone would allow us to determine the viscosity of a uniform lower mantle layer and an upper mantle/transition zone layer to within ±30–40% and to estimate lithospheric thickness to within ±15–20%. GRACE will have a harder time differentiating between the separate viscosities of the transition zone and upper mantle, but accuracies of within a factor of 2 might still be achievable for those parameters. Errors in the ice loading history could significantly degrade these viscosity estimates, particularly for the transition zone and upper mantle. The accuracy of recovery of the true Earth viscosity will depend in part on how well the model parameterization used for the grid search can represent the true Earth structure. However, combining GRACE data with data from other more traditional measurements of postglacial rebound has the potential of dramatically improving viscosity estimates throughout the Earth, particularly in the lower mantle.

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 Gravity Recovery and Climate Experiment (GRACE) satellite mission will resolve temporal variations in gravity orders of magnitude more accurately and with considerably higher resolution than any existing satellite. Effects of atmospheric mass over land will be removed prior to estimating the gravitational field, using surface pressure fields generated by global weather forecast centers. To recover the continental hydrological signal with an accuracy of 1 cm of equivalent water thickness down to scales of a few hundred kilometers, atmospheric pressure must be known to an accuracy of 1 mbar or better. We estimate errors in analyzed pressure fields and the impact of those errors on GRACE surface mass estimates by comparing analyzed fields with barometric surface pressure measurements in the United States and North Africa/Arabian peninsula. We consider (1) the error in 30-day averages of the pressure field, significant because the final GRACE product will average measurements collected over 30-day intervals, and (2) the short-period error in the pressure fields which would be aliased by GRACE orbital passes. Because the GRACE results will average surface mass over scales of several hundred kilometers, we assess the pressure field accuracy averaged over those same spatial scales. The atmospheric error over the 30-day averaging period, which will map directly into GRACE data, is generally < 0.5 mbar. Consequently, analyzed pressure fields will be adequate to remove the atmospheric contribution from GRACE hydrological estimates to subcentimeter levels. However, the short-period error in the pressure field, which would alias into GRACE data, could potentially contribute errors equivalent to 1 cm of water thickness. We also show that given sufficiently dense barometric coverage, an adequate surface pressure field can be constructed from surface pressure measurements alone. Copyright 2001 by the American Geophysical Union.

Glaciers and ice caps (GICs) are important contributors to present-day global mean sea level rise (1-4). Most previous global mass balance estimates for GICs rely on extrapolation of sparse mass balance measurements (1,2,4) representing only a small fraction of the GIC area, leaving their overall contribution to sea level rise unclear. Here we show that GICs, excluding the Greenland and Antarctic peripheral GICs, lost mass at a rate of 148 ± 30 Gt [yr.sup.-1] from January 2003 to December 2010, contributing 0.41 ± 0.08 mm [yr.sup.-1] to sea level rise. Our results are based on a global, simultaneous inversion of monthly GRACE-derived satellite gravity fields, from which we calculate the mass change over all ice-covered regions greater in area than 100 [km.sup.2]. The GIC rate for 2003-2010 is about 30 per cent smaller than the previous mass balance estimate that most closely matches our study period (2). The high mountains of Asia, in particular, show a mass loss of only 4 ± 20 Gt [yr.sup.-1] for 2003-2010, compared with 47-55 Gt [yr.sup.-1] in previously published estimates (2,5). For completeness, we also estimate that the Greenland and Antarctic ice sheets, including their peripheral GICs, contributed 1.06 ± 0.19 mm [yr.sup.-1] to sea level rise over the same time period. The total contribution to sea level rise from all ice-covered regions is thus 1.48 ± 0.26 mm [yr.sup.-1], which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources (6)., Interpolation of sparse mass balance measurements on selected glaciers is usually used to estimate global GIC mass balance (1,2,4). Models are also used (3,7), but these depend on the quality [...]