| NSF Org: |
OPP Office of Polar Programs (OPP) |
| Recipient: |
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| Initial Amendment Date: | July 21, 2011 |
| Latest Amendment Date: | August 28, 2014 |
| Award Number: | 1111882 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Jennifer Mercer
jmercer@nsf.gov (703)292-7453 OPP Office of Polar Programs (OPP) GEO Directorate for Geosciences |
| Start Date: | September 1, 2011 |
| End Date: | February 28, 2017 (Estimated) |
| Total Intended Award Amount: | $1,096,395.00 |
| Total Awarded Amount to Date: | $1,096,395.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1960 KENNY RD COLUMBUS OH US 43210-1016 (614)688-8735 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1960 KENNY RD COLUMBUS OH US 43210-1016 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): |
ARCSS-Arctic System Science, AON-Arctic Observing Network |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.078 |
ABSTRACT
This project would continue operation of the Greenland GPS Network (GNET) for four years. GNET has proven highly successful in "weighing" the Greenland ice sheet by tracking the earth's elastic response to changing surface loads. The PIs propose to extend the life of GNET so that it can continue to sense space-time changes in ice mass, including spatial shifts in the position of the centers of ice loss, and accelerations in mass change rates. GNET is now resolving seasonal oscillations in the vertical position of the earth's crust as well as longer-term trends. These oscillations reflect seasonal changes in the loads placed on the solid earth by both the atmosphere and the ice sheet. The atmospheric pressure signal will be removed, using state-of-the-art weather models, in order to isolate the signal due to changes in the ice sheet. As part of the proposed pressure analysis, they would also produce a time series of integrated water vapor for each station in GNET, with benefits for weather and climate predictions. Broader impacts include the training of a graduate student and support of a postdoctoral researcher as co-investigator. Scientifically, GNET is addressing a problem of great interest and also considerable societal importance: the trajectory and magnitude of Greenland mass balance. With current and potential additional gaps in satellite geodesy coverage of Greenland, GNET is a cornerstone of our observations of the cryosphere. The data are openly accessible and are available in near real-time through UNAVCO.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The Greenland GPS Network (GNET) comprises about 50 autonomous, continuously operating GPS stations installed in bedrock near the margins of the Greenland Ice Sheet, the second largest ice sheet on Earth, and the ice sheet that has been losing the most ice since the turn of this century. GNET stations can be positioned every day with about 2 mm accuracy in the horizontal and 4-6 mm accuracy in the vertical. By modeling the coordinate time series since the GNET stations were installed (mostly in 2007-2009) we can observe the displacement of the earth, the vertical and horizontal velocity of the bedrock, and accelerations in these motions. Crustal displacement in Greenland is driven by the earths instantaneous elastic response to changes in the ice loads ('elastic rebound'), and by the delayed, viscoelastic response to past changes in ice loads, especially those that have occured since the end of the last ice age . The delayed response to past changes in ice mass is often referred to as glacial isostatic adjustment (GIA) or postglacial rebound (PGR).
GIA is important for many reasons, including the fact that the GRACE satellite mission, which measures mass changes from space, needs a "PGR correction" in order to distinguish between changes in ice mass and changes in rock mass. GNET has proven that existing GIA models are not very accurate, and that GRACE's mass solution are locally biased as a result. The GNET project has inferred that GIA models are incorrect because in some parts of Greenland they are not constrained by any observations, and because the GIA modellers made incorrect assumptions about the viscosity of the Earth's interior. Inaccurate viscosity models will also cause our GIA models to make inaccurate assesments about ice loss history. Disciplining numerical GIA models with the observations collected by GPS networks like GNET, in addition to geological observations, should help us improve our GIA, and increase our confidence in their predictions.
The other process driving crustal motion in Greenland is elastic rebound. The earth acts like a spring, and the ice sheet acts like a load sitting on top of that spring. The ice load compresses that spring. As the ice melts, the load decreases and the spring relaxes upwards. The ground rises as the ice melts, and if ice loss accelerates in some prt of Greenland so does the vertical (and horizontal) displacement of the nearby GNET stations. GNET stations uplift with time-averaged velocities that mostly fall in the range of 5 - 40 mm/year. This is about 10 times faster than the uplift rates observed over most of the USA, Europe and Australia, for example. Crustal motion tends to be dominated by elastic rebound in response to modern ice loss. That ice loss is driven by climate change - warming of the oceans and the atmosphere. Ocean warming enhances the calving rate of glaciers than flow into the sea causes them to discharge ice at a greater rate. Atmospheric warming incfreases ice loss too, mainly by driving surface mass balance more negative.
We have just completed a study in which we focus on the spatial and temporal patters of accelerations in ice loss recorded by GRACE and GNET. The greatest sustained acceleration in ice mass from the beginning of 2003 to early 2013 was focused in SW Greenland in an area almost devoid of marine-terminating glaciers. This means that this acceleration was driven by changes in SMB rather than by changes in glacial discharge. Remarkably, after a decde of accelerating ice loss an abrupt chnage occured near the beginning of the summer of 2013, and very little ice was loss in Greenland as a whole for the following 12-18 months. We have shown that this abrupt change in the rate of mass loss was also concentrated in SW Greenland. Again this implies that the major dirver was SMB. We argue that the sustained acceration and its abrupt reversal we tracking the phase of the North Atlantic Oscillation (NAO). This major implication for the future of ice loss in Greenland as global warming continues.
Last Modified: 08/01/2018
Modified by: Michael Bevis
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