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See detailGeodetic measurements reveal similarities between post–Last Glacial Maximum and present-day mass loss from the Greenland ice sheet
Knudsen, Shfaqat; Bamber, Ingo; Bevis, Michael et al

in Science Advances (2016), 2(9),

Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in ... [more ▼]

Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year. [less ▲]

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See detailVertical and horizontal surface displacements near Jakobshavn Isbræ driven by melt-induced and dynamic ice loss
Nielsen, Karina; Khan, Shfaqat A.; Spada, Giorgio et al

in Journal of Geophysical Research. Solid Earth (2013), 118(4), 1837--1844

We analyze Global Positioning System (GPS) time series of relative vertical and horizontal surface displacements from 2006 to 2012 at four GPS sites located between ∼5 and ∼150 km from the front of ... [more ▼]

We analyze Global Positioning System (GPS) time series of relative vertical and horizontal surface displacements from 2006 to 2012 at four GPS sites located between ∼5 and ∼150 km from the front of Jakobshavn Isbræ (JI) in west Greenland. Horizontal displacements during 2006–2010 at KAGA, ILUL, and QEQE, relative to the site AASI, are directed toward north-west, suggesting that the main mass loss signal is located near the frontal portion of JI. The directions of the observed displacements are supported by modeled displacements, derived from NASA's Airborne Topographic Mapper (ATM) surveys of surface elevations from 2006, 2009, and 2010. However, horizontal displacements during 2010–2012 at KAGA and ILUL are directed more towards the west suggesting a change in the spatial distribution of the ice mass loss. In addition, we observe an increase in the uplift rate during 2010–2012 as compared to 2006–2010. The sudden change in vertical and horizontal displacements is due to enhanced melt-induced ice loss in 2010 and 2012. [less ▲]

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See detailThe use of GPS horizontals for loading studies, with applications to northern California and southeast Greenland
Wahr, John; Khan, Shfaqat; van Dam, Tonie UL et al

in Journal of Geophysical Research. Solid Earth (2013), 118

We describe how GPS measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements, to improve and extend GPS studies of surface loading. We show that the ratio of ... [more ▼]

We describe how GPS measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements, to improve and extend GPS studies of surface loading. We show that the ratio of the vertical displacement to the horizontal displacement, combined with the direction of the horizontal motion, can help determine whether nearby loading is concentrated in a small region (for example, in a single lake or glacier), and where that region is. We illustrate this method by applying it to two specific cases: an analysis of GPS data from northern California to monitor the level of Lake Shasta, and the analysis of data from a single GPS site in southeast Greenland to determine mass variability of two large, nearby outlet glaciers: Helheim Glacier and Midgaard Glacier. The California example serves largely as a proof-of-concept, where the results can be assessed by comparing with independent observations (Lake Shasta tide gauge data, in this case). Our Greenland results show that both Helheim and Midgaard have experienced notable interannual variations in mass loss rate over the last decade. Helheim’s mass loss accelerated rapidly in mid-2003, decelerated in late 2005, and increased again in 2008–2009 before returning to about its pre-2003 rate in late 2010. Midgaard’s mass loss accelerated in mid-2004, and remained more-or-less constant before returning to its pre-2003 rate in late 2008. [less ▲]

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See detailBedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change
Bevis, Michael; Wahr, John; Khan, Shfaqat A. et al

in Proceedings of the National Academy of Sciences of the United States of America (2012), 109(30), 11944-11948

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in ... [more ▼]

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly. [less ▲]

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See detailElastic uplift in southeast Greenland due to rapid ice mass lost
Khan, Shfaqat A.; Wahr, John; Stearns, Leigh A. et al

in Geophysical Research Letters (2007), 34(L21701), 1-6

The rapid unloading of ice from the southeastern sector of the Greenland ice sheet between 2001 and 2006 caused an elastic uplift of ~35 mm at a GPS site in Kulusuk. Most of the uplift results from ice ... [more ▼]

The rapid unloading of ice from the southeastern sector of the Greenland ice sheet between 2001 and 2006 caused an elastic uplift of ~35 mm at a GPS site in Kulusuk. Most of the uplift results from ice dynamic-induced volume losses on two nearby outlet glaciers. Volume loss from Helheim Glacier, calculated from sequential digital elevation models, contributes about ~16 mm of the observed uplift, with an additional ~5 mm from volume loss of Kangerdlugssuaq Glacier. The remaining uplift signal is attributed to significant melt-induced ice volume loss from the ice sheet margin along the southeast coast between 62°N and 66°N. [less ▲]

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See detailThe New IERS Special Bureau for Loading (SBL)
van Dam, Tonie UL; Plag, Hans-Peter; Blewitt, Geoffrey et al

in International VLBI Service for Geodesy and Astrometry: General Meeting Proceedings (2002)

Currently, the establishment of the International Earth Rotation Service (IERS) Special Bureau for Loading (SBL) is in progress as part of the IERS Global Geophysical Fluids Center (GGFC). The main ... [more ▼]

Currently, the establishment of the International Earth Rotation Service (IERS) Special Bureau for Loading (SBL) is in progress as part of the IERS Global Geophysical Fluids Center (GGFC). The main purpose of the SBL is to provide reliable, consistent model predictions of loading signals that have been thoroughly tested and validated. The products will describe at least the surface deformation, gravity signal and geo-center variations due to the various surface loading processes in reference frames relevant for direct comparison with existing geodetic observing techniques. To achieve these goals, major scientific advances are required with respect to the Earth model, the theory and algorithms used to model deformations of the Earth as well as improvements in the observational data related to surface loading. [less ▲]

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See detailGPS measurements of vertical crustal motion in Greenland
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(D24), 33755-33759

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component ... [more ▼]

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component gives a negative secular uplift rate of −5.8±1.0 mm/yr. A negative rate (i.e., a subsidence) is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr. However, it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice prior to 4 ka years ago, which predict that Kellyville should be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy is too large to be the result of loading from present-day changes in nearby ice. We show, instead, that it is consistent with independent suggestions that the western ice sheet margin in this region of Greenland may have advanced by ≈50 km during the past 3–4 kyr. [less ▲]

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See detailGlobal Positioning System and Gravity Used to Study Greenland Ice
van Dam, Tonie UL; Larson, Kristine; Wahr, John et al

in Earth in Space (2001), 13(5), 1-16

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See detailGeodetic measurements in Greenland and their implications
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(B8), 16567-16581

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two ... [more ▼]

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two locations along the edge of the ice sheet: Kellyville, located about one third of the way up the western ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crustal uplift rates of -5.8±1.0 mm/yr at Kellyville and -2.1±1.5 mm/yr at Kulusuk. There have not yet been enough absolute gravity occupations to permit useful secular gravity solutions at either location. The negative uplift rate at Kellyville is consistent with independent archeological and historical evidence that the southwestern edge of the continent has been subsiding over the last 3000 years, but it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice during the early Holocene, which predict that Kellyville is likely to be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting -7.8±3.6 mm/yr discrepancy between the observed and predicted uplift rates is too large to be caused by loading from present-day changes in nearby ice. However, it is consistent with independent suggestions that the western ice sheet margin in this region may have advanced by ≈50 km during the past 3000-4000 years. If this advance did occur and if the crustal subsidence it induces is not removed from altimeter measurements of Greenland ice sheet elevations, then the altimeter solutions could underestimate the true snow/ice thickness change by 5-10 mm/yr along portions of the western margin of the ice sheet. [less ▲]

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See detailUsing GPS and Gravity to Infer Ice Mass Changes in Greenland
van Dam, Tonie UL; Larson, Kristine; Wahr, John et al

in EOS : Transactions, American Geophysical Union (2000), 81(37), 421-427

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See detailModeling environmental loading effects, Invited, Proceedings EGGS, Ed. H.-P. Plag and S. Zerbini
van Dam, Tonie UL; Wahr, John

in Physics and chemistry of the earth (1998), 23

Temporal variations in the geographic distribution of atmospheric, hydrologic and oceanic mass load and deform the surface of the Earth. In many instances, the deformation is large enough to be detected ... [more ▼]

Temporal variations in the geographic distribution of atmospheric, hydrologic and oceanic mass load and deform the surface of the Earth. In many instances, the deformation is large enough to be detected with space based geodetic techniques as well as with terrestrial gravity observations. For example, atmospheric loading induced crustal deformations on the order of 20 mm are possible at high latitudes with accompanying changes in gravity of 6 μGals. Non-tidal ocean loading effects can typically cause 5 mm (2 mm root-mean-square, RMS) in vertical positioning at coastal geodetic sites with displacements of up to 10 mm possible. Deformation associated gravity changes are usually on the order of 2-3 μGals, however peak-to-peak changes of 5 μGals are also predicted. The effects of regional ground water variations on geodetic measurements are less well known. Model results indicate that annual changes in gravity and vertical positioning can be as large as 2 μGals and 5 mm for sites where there is significant annual snowfall. We present a review of work done to date to address these issues. [less ▲]

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See detailAtmospheric load response of the oceans determined using Geosat data
van Dam, Tonie UL; Wahr, John

in Geophysical Journal International (1993), 113(1), 1-16

Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure ... [more ▼]

Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure is fit to the sea-surface height along individual altimeter tracks, the response is found to be only 60–70 per cent of the theoretical response or approximately -0.6 to -0.7 cm mb-1. Fits at fixed geographic locations show a clear dependence on latitude. There is a steady decrease in the absolute value o the regression coefficient between 70° and 20°, and then an abrupt increase again closer to the equator. A simple error analysis demonstrates that errors in the pressure data would reduce the along-track regression values, as is observed, and could produce a similar latitude dependence. But, the errors are unlikely to be large enough to explain the entire departure from inverted barometer. We estimate that pressure errors are apt to perturb the along-track track results by no more than about 0.1-0.2 cm mb-1. The possibility that the remaining disagreement is due to a global coherence between wind- and pressure-driven sea-surface height variability is considered. Winds driven by the pressure gradients of synoptic storms induce a sea-surface height response that is opposite in direction to that caused by the pressure cell. the wind-driven response is estimated for a stationary storm over a homogeneous barotropic ocean and for a moving storm over a two-layer baroclinic ocean by modeling the pressure cell as an idealized Gaussian distribution. the model results indicate that the wind-induced sea-surface height depends on both the radius and the translational velocity of the pressure cell. But, the winds associated with storms moving at average speeds of 10 ms-1 are apt to lower the theoretical pressure response in the model by only approximately 0.1 cm mb-1. the surface stress associated with those winds has the same latitudinal trend between 70° and 20° as the regression coefficients. But, the response of the ocean to that stress does not appear to exhibit the same trend. Nevertheless, the abrupt change in the regression coefficients near the equator suggests the apparent non-inverted barometer response may reflect a real change in sea-surface height related to atmospheric forcing (though the results near the equator are not as well defined as those at higher latitudes). [less ▲]

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See detailDisplacements of the Earth's surface due to atmospheric loading: Effects on Gravity and Baseline Measurements
van Dam, Tonie UL; Wahr, John

in Journal of Geophysical Research (1987), 92

Atmospheric mass loads and deforms the Earth's crust. By performing a convolution sum between daily global barometric pressure data and mass loading Green's functions, we estimate the time dependent ... [more ▼]

Atmospheric mass loads and deforms the Earth's crust. By performing a convolution sum between daily global barometric pressure data and mass loading Green's functions, we estimate the time dependent effects of atmospheric loading, including those associated with short-term synoptic storms, on surface point positioning measurements and surface gravity observations. We calculate the response of both an oceanless Earth and an Earth with an inverted barometer ocean. Peak to peak vertical displacements are frequently 15-20 mm with accompanying gravity perturbations of 3-6 microgal. Baseline changes can be as large as 20 mm or more. The perturbations are largest at higher latitudes and during winter months. These amplitudes are consistent with the results f Rabbel and Zschau (1985) who modeled synoptic pressure disturbances as Gaussian functions of radius about a central point. Deformation can be adequately computes using real pressure data from points within about 1000 km of the station. Knowledge of local pressure alone, is not sufficient. Rabbel and Zschau's hypothesized corrections for these displacements, which use local pressure and regionally averaged pressure, prove accurate at points well inland but are, in general, inadequate within a few hundred kilometers of the coast. [less ▲]

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