Geodetic measurements reveal similarities between post–Last Glacial Maximum and present-day mass loss from the Greenland ice sheet

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 ▲]

The use of GPS horizontals for loading studies, with applications to northern California and southeast Greenland

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 ▲]

Elastic uplift in southeast Greenland due to rapid ice mass lost

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 ▲]

GPS measurements of vertical crustal motion in Greenland

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 ▲]

Global Positioning System and Gravity Used to Study Greenland Ice

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

Modeling environmental loading effects, Invited, Proceedings EGGS, Ed. H.-P. Plag and S. Zerbini

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 ▲]

Displacements of the Earth's surface due to atmospheric loading: Effects on Gravity and Baseline Measurements

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 ▲]