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See detailUsing GPS and absolute gravity observations to separate the effects of present-day and Pleistocene ice-mass changes in South East Greenland
van Dam, Tonie UL; Francis, Olivier UL; Wahr, J. et al

in Earth and Planetary Science Letters (2017), 459

Measurements of vertical crustal uplift from bedrock sites around the edge of the Greenland ice sheet (GrIS) can be used to constrain present day mass loss. Interpreting any observed crustal displacement ... [more ▼]

Measurements of vertical crustal uplift from bedrock sites around the edge of the Greenland ice sheet (GrIS) can be used to constrain present day mass loss. Interpreting any observed crustal displacement around the GrIS in terms of present day changes in ice is complicated, however, by the glacial isostatic adjustment (GIA) signal. With GPS observations alone, it is impossible to separate the uplift driven by present day mass changes from that due to ice mass changes in the past. Wahr et al. (1995) demonstrated that viscoelastic surface displacements were related to the viscoelastic gravity changes through a proportionality constant that is nearly independent of the choice of Earth viscosity or ice history model. Thus, by making measurements of both gravity and surface motion at a bedrock site, the viscoelastic effects could be removed from the observations and we would be able to constrain present day ice mass changes. Alternatively, we could use the same observations of surface displacements and gravity to determine the GIA signal. In this paper, we extend the theory of Wahr et al. (1995) by introducing a constant, Z, that represents the ratio between the elastic changes in gravity and elastic uplift at a particular site due to present day mass changes. Further, we combine 20 yrs of GPS observations of uplift with eight absolute gravity observations over the same period to determine the GIA signal near Kulusuk, a site on the southeastern side of the GrIS, to experimentally demonstrate the theory. We estimate that the GIA signal in the region is 4.49 ± 1.44 mm/yr and is inconsistent with most previously reported model predictions that demonstrate that the GIA signal here is negative. However, as there is very little in situ data to constrain the GIA rate in this part of Greenland, the Earth model or the ice history reconstructions could be inaccurate (Khan et al., 2016). Improving the estimate of GIA in this region of Greenland will allow us to better determine the present day changes in ice mass in the region, e.g. from GRACE. [less ▲]

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See detailGPS Measurements of Crustal Uplift near Jakobshavn Isbrae due to Glacial Ice Mass Loss
Khan, S. A.; Liu, L.; Wahr, J. et al

in Journal of Geophysical Research (2010), 115

We analyze 2006–2009 data from four continuous Global Positioning System (GPS) receivers located between 5 and 150 km from the glacier Jakobshavn Isbræ, West Greenland. The GPS stations were established ... [more ▼]

We analyze 2006–2009 data from four continuous Global Positioning System (GPS) receivers located between 5 and 150 km from the glacier Jakobshavn Isbræ, West Greenland. The GPS stations were established on bedrock to determine the vertical crustal motion due to the unloading of ice from Jakobshavn Isbræ. All stations experienced uplift, but the uplift rate at Kangia North, only 5 km from the glacier front, was about 10 mm yr−1 larger than the rate at Ilulissat, located only "45 km further away. This suggests that most of the uplift is due to the unloading of the Earth’s surface as Jakobshavn thins and loses mass. Our estimate of Jakobshavn’s contribution to uplift rates at Kangia North and Ilulissat are 14.6 ± 1.7 mm yr−1 and 4.9 ± 1.1 mm yr−1, respectively. The observed rates are consistent with a glacier thinning model based on repeat altimeter surveys from NASA’s Airborne Topographic Mapper (ATM), which shows that Jakobshavn lost mass at an average rate of 22 ± 2 km3 yr−1 between 2006 and 2009. At Kangia North and Ilulissat, the predicted uplift rates computed using thinning estimates from the ATM laser altimetry are 12.1 ± 0.9 mm yr−1 and 3.2 ± 0.3 mm yr−1, respectively. The observed rates are slightly larger than the predicted rates. The fact that the GPS uplift rates are much larger closer to Jakobshavn than further away, and are consistent with rates inferred using the ATM!based glacier thinning model, shows that GPS measurements of crustal motion are a potentially useful method for assessing ice!mass change models. [less ▲]

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See detailGeodetic measurements of postglacial adjustments in Greenland
Khan, S. A.; Wahr, J.; Leuliette, E. et al

in Journal of Geophysical Research (2008)

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare ... [more ▼]

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare our results with predictions based on the ICE-5G deglaciation model of Peltier (2004). Results from the GPS receiver at Kellyville (-1.2 ± 1.1 mm/a) and from the tide gauge at Nuuk (-2.2 ± 1.3 mm/a), indicate that ICE-5G overestimates the subsidence rates at those locations by 2.1 and 1.1 mm/a, respectively. Kellyville and Nuuk are located along the southwestern margin of the Greenland ice sheet, and the observed negative uplift rates are consistent with independent evidence that the ice margin along the southwestern edge readvanced during the last ~8 ka to its current position. The ICE-5G glaciation-deglaciation history includes a readvance between the latitudes of 62°N and 72°N. The GPS measurements suggest the ICE-5G readvance may be too large or mistimed. Our GPS results at Qaqortoq, located at the southern tip of Greenland, suggest a secular subsidence rate of 􏰀0.3 ± 1.1 mm/a, while ICE-5G predicts an uplift rate of 1.0 mm/a. ICE-5G assumes no ice sheet readvance in south Greenland, including no readvance of the Qassimiut lobe. The difference of 1.3 ± 1.1 mm/a can tentatively be explained as due to a ~33 km readvance of the Qassimiut lobe during the last ~3 ka. For the other GPS sites, the observed/predicted uplift rates are 3.6 ± 1.1/-0.1 mm/a at Thule, 0.0 ± 1.1/2.0 mm/a at Scoresbysund, and -0.4 ± 1.1/-1.7 mm/a at Kulusuk. For Thule, Kulusuk, and Scoresbysund the differences between the observed and predicted rates are on the order of 1.3 – 3.7 mm/a, though with opposite signs, and indicate that ICE-5G does not exactly reproduce the correct rebound signal at those locations. [less ▲]

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See detailA comparison of annual vertical crustal displacements from GPS and GRACE
van Dam, Tonie UL; Wahr, J.; Lavallée, David

in Journal of Geophysical Research (2006), 111

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity ... [more ▼]

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity Recovery and Climate Experiment (GRACE) gravity fields for stations in Europe. An annual signal fit to the residual monthly heights, corrected for atmospheric pressure and barotropic ocean loading effects, should primarily represent surface displacements due to long-wavelength variations in water storage. A comparison of the annual height signal from GPS and GRACE over Europe indicates that at most sites, the annual signals do not agree in amplitude or phase. We find that unlike the annual signal predicted from GRACE, the annual signal in the GPS heights is not coherent over the region, displaying significant variability from site to site. Confidence in the GRACE data and the unlikely possibility of large-amplitude small- scale features in the load field not captured by the GRACE data leads us to conclude that some of the discrepancy between the GPS and GRACE observations is due to technique errors in the GPS data processing. This is evidenced by the fact that the disagreement between GPS and GRACE is largest at coastal sites, where mismodeling of the semidiurnal ocean tidal loading signal can result in spurious annual signals. [less ▲]

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See detailCrustal displacements due to continental water loading
van Dam, Tonie UL; Wahr, J.; Milly, P. C. D. et al

in Geophysical Research Letters (2001), 28(4), 651-654

The effects of long-wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root-mean-square (RMS ... [more ▼]

The effects of long-wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root-mean-square (RMS) values for 1994–1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for horizontal deformation. We compare ΔrM with observed Global Positioning System (GPS) heights (ΔrO) (which include adjustments to remove estimated effects of atmospheric pressure and annual tidal and non-tidal ocean loading) for 147 globally distributed sites. When the ΔrO time series are adjusted by ΔrM, their variances are reduced, on average, by an amount equal to the variance of the ΔrM. Of the ΔrO time series exhibiting a strong annual signal, more than half are found to have an annual harmonic that is in phase and of comparable amplitude with the annual harmonic in the ΔrM. The ΔrM time series exhibit long-period variations that could be mistaken for secular tectonic trends or postglacial rebound when observed over a time span of a few years. [less ▲]

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See detailPredictions of crustal deformation and of geoid and sea level variability caused by oceanic at atmospheric loading
van Dam, Tonie UL; Wahr, J.; Chao, Y. et al

in Geophysical Journal International (1997), 129(3), 507-517

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