IntroductionOver the last decade, the Greenland and Antarctic ice sheets contribute about 0.8 mm per year to sea level rise (Hanna et al., 2013), which is almost one third of the annual global signal. Projections of mass loss for the next 100 to 200 years point to a significantly more dominant contribution of both ice sheets in the global sea-level budget. However, despite increasing technical advances in estimating the present-day budget of ice sheets (and the Antarctic ice sheet in particular), it remains difficult to reconcile global mass balance estimates obtained from different satellite-based methods, such as interferometry, altimetry and gravimetry. A typical approach is to balance the mass input from atmospheric modelling with the outgoing mass flux at the ice-sheet boundary (Shepherd et al., 2012). The flux calculations at the boundary rely on satellite-derived surface velocities, which currently are only available as snapshots in time, and which need ground truth for validation. Here, we propose continuous, year-round measurements that aim at improving the input-output method in several aspects.
AimThe project aims at continuously monitoring the motion (horizontal and vertical) of a well-studied ice shelf in Dronning Maud Land, Antarctica over the course of one year using both GPS and phase-sensitive radar as part of a larger network, already installed in the region (Pattyn et al., 2012; Drews et al., submitted; Callens et al., 2014; submitted). The innovative part is that the measurements will be year-round and that the the information is broadcasted to the office in Brussels (ULB) on a daily basis, so that real-time positioning and change can be monitored remotely and followed by a larger community of interest. This will be our tweeting ice shelf (@TweetingIceShelf).
Science: the mass balance of ice shelves
Ice shelves play a crucial role in Antarctic ice dynamics. Ice shelves form when the ice reaches the ocean and starts to float according to the Archimedes principle. They spread under their own weight and typically extend hundreds of kilometers sewards before the ice eventually calves. Most ice shelves are fringed at their sides by ice islands and embayments and as long as this fringing remains, they slow down the inland ice flow. However, ocean and surface melting may destabilize ice shelves, hence leading to an upstream ice speed-up (such as they way you remove the cork on a bottle so that the liquid may pour out). This speed-up invokes an increased ice discharge into the ocean, hence sea level rise.
To determine the mass budget of ice shelves, one needs to carefully map its flow speed, which is generally done using satellite interferometry, calibrated with known velocities (for instance via GPS measurements, Berger (2013)). It is essential to know the mean flow speed, but also its variation over the course of a year to detect seasonal variability and the influences of tides. Ice flux is then determined as the product of ice thickness (known from radar surveys) and flow-speed (to be measured by GPS) . Surface input (accumulation through snowfall) is determined from a combination of mass balance models and direct measurements in the field, through firn coring. Both ice core and radar data are already available in our area of interest from previous surveys. The remaining unknown is the basal mass balance: at the ice-shelf bottom the ice can either melt or refreeze in form of marine ice. Due to the high salinity content of marine ice, radar measurements cannot detect the thickness of this marine ice layer . However, novel techniques, such as a phase sensitive radar (pRES) are capable of detecting the rate of change of basal layers in the bottom part of the ice-shelf ice, which enable to determine both accretion and melt rates.
During the 2014-15 BELARE (Belgian Antarctic Research Project), a pRES will be installed in the studied ice shelf to monitor this change. For the purpose of our tweeting ice-shelf project, we propose to attach two low-cost, low-power consumption GPSses to the system to monitor surface flow speed and vertical change on a hourly basis at differnent sites on the ice shelf. At the same time, the pRES will retrieve data on the internal layering, enabling to monitor at high precision the rate of change within the ice column, this on top of its position.
Next to the GPSses, an Iridium data link will send the essential data on a daily basis to our office. Assemblage of the GPS-Iridium units will be done by the British Antarctic Survey. In the framework of this project, the GPSses and the Iridium modem (+ transmission cost) will be acquired. Installation of the equipment will be done by the IceCon project during BELARE 2014. The scientific value of the real-time data transmission is twofold: (i) it allows to reconfigure the system remotely (if necessary), and (ii) the known results will impact the mission design of the following field campaign in 2015/16.