The project


At present, global sea level is rising at around 3 mm/yr, as measured by satellite altimetry and a discrete set of tide gauges (e.g. Church et al. 2011). The increases in sea surface heights are caused by a number of factors as thermal expansion of the oceans, salinity changes and the transfer of water from the continents to the oceans, consequence of melting of glaciers and ice caps.

To enable accurate predictions of what future sea levels the world may experience, we have to quantify the rate of present day melting of the Antarctic ice sheet, representing the largest volume of ice that could participate to sea level rise. To do so, scientists use space geodetic observations from satellite altimetry (e.g. Envisat, ICESat, Cryosat-2) and space gravity (e.g. the Gravity Recovery and Climate Experiment mission, known as 'GRACE'), respectively. The altimetric approach enable to measure the ice thickness and topography (More information here), whereas the latter is used to 'weight' Antarctica and derive the mass balance changes (and here). Both of these observations types sense not only present-day changes in the ice sheet but also the ongoing response of the Antarctic continent to the melting of the ice sheet that occurred thousands of years ago (known as Glacial Isostatic Adjustment or 'GIA'). Thus, in order to use these space geodetic observations to estimate present-day mass balance, it is necessary to first separate the GIA signals from the mass balance change signals.

Geodetic estimation of Glacial Isostatic adjustment


One particular region in Antarctica with positive anomaly rates detected by GRACE lies in Enderby Land. Chen et al. (2009) suggested "that this feature is either from unquantified snow accumulation in this region or more likely due to unmodelled postglacial rebound" (in other words, uplift of the Antarctic continent).



Long-term mass change rates (in units of cm/yr of equivalent water height) observed by GRACE. From Chen et al (2009). Enderby Land is located at the cross 'D'.

There are three possible causes of the observed signal:

    Unmodelled Glacial Isostatic Adjustment. The melting of the Antarctic Ice Sheet since the Last Glacial Maximum (~20,000 years ago) has removed mass from the continent, which then rebounds. Imagine that there is cubic metre of ice sitting on the bow of a boat. The 1 tonne mass would make the boat sink lower in the water. When the ice melts, the boat will rise. The continents behave in a similar manner to the boat, with the mantle playing the role of the water.We would expect the new GPS installation to show a positive uplift rate, where the rate of rise of the continent provides information on the amount of ice that has melted.
    Increased recent mass accumulation. GRACE senses changes in mass. If over the past 4 years there has been a progressive increase in snow/ice accumulation in the region then this would explain the signal detected by GRACE. In this case, the additional mass would cause subsidence of the continent, thereby producing a negative vertical velocity at the GPS site.
    Errors in the GRACE analysis. One should never discount the possibility that the analysis of the GRACE data is in error and that the reported signal is an artifact of incorrect processing. If this is the case then there should be no (or very little) vertical movement detected at the GPS site.
The combination of GRACE and GPS has been used with great success in detecting changes due to continental hydrology (Tregoning et al. 2009) but also glacial changes in Greenland (e.g. Khan et al., 2010). Thus, the ability to discriminate between these different processes using these two techniques is well established. 

The project is held by Paul Tregoning and Simon McClusky from the Research School of Earth Sciences - Australian National University.

> Know more about the studying polar caps with satellites

In the field

The first step will be to reach Mawson station, the Western part of Australian Antarctica Territory. The field work will then be held approximately 500 km away from the station. Hence, a couple of Twin-Otters and helicopters will be responsible for the cargo and pax transit.

We will work at replacing the Richardson Lake Station, that has been installed in January 2007.

 Location of the Richardson Lake GPS station

The second step will be to install a second station on a chosen rocky site. With no close access to any Antarctic Station, we will have to install a camp site during the time of the operations.

> Know more about the previous field work in Enderby Land


References

Chen, J.L., C.R. Wilson, D. Blankenship and B.D. Tapley (2009). Accelerated Antarctic ice loss from satellite gravity measurements, Nature Geoscience.

Church, J.A. et al. (2011). Revisiting the Earth's sea-level and energy budgets from 1961 to 2008. Geophysical Research Letters. 

Tregoning, P., G. Ramillien, H. McQueen and D. Zwartz (2009). Glacial isostatic adjustment and non-stationary signals observed by GRACE, J. Geophysical Research.

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