Polar Ice Caps
Altimetry is a powerful tool for measuring both the dynamics and mass balance of ice sheets. Continental ice has an impact on sea level: if both of the major Greenland and Antarctica ice sheets were to melt, the sea level would rise by about 80 m.
fig 1. Rate of elevation change of the Antarctic ice sheet between 2010 and 2013 determined from CryoSat-2 repeat altimetry and smoothed with a 25 by 25 km median filter. Solid grey and white (inset) lines show the boundaries of 27 ice sheet drainage basins [Zwally et al., 2012]. The CryoSat measurements reach to within 215 km of the South Pole, as compared to 930 and 430 km for the ERS/Envisat and ICESat altimeters, respectively. Also shown (inset) are the numbers used to identify ice sheet drainage basins, with East Antarctica and the Antarctic Peninsula defined as basins 2 to 17 and 24 to 27, respectively, and West Antarctica defined as the remaining basins and the mask developed to identify elevation changes occurring at the density of ice (inset, black regions). Elsewhere, it is assumed that elevation changes are caused by fluctuations in surface mass balance alone, and we, therefore, applied a density of snow to these signals (inset, grey regions). Ice dynamical imbalance (IDI) is evident as thickening of the Kamb Ice Stream (basin 18) and as widespread thinning across the Amundsen Sea sector (basins 21 and 22), with the latter signal affecting a considerably larger area than at any time in the past two decades [Shepherd et al., 2002, 2004; Wingham et al., 2009]. (Credits CPOM)
Topography is one of the parameters relevant to the processes acting on ice sheets. It contains the signature of the main physical processes (climate and dynamic), that act on an ice sheet on both a small and a large scale, and important information about local anomalies or general trend behaviour. Nowadays topography is also an initial condition for studying future evolution. Large-scale topography controls flow direction and its mapping enables the balance velocity to be derived. Moreover, the deformation and sliding velocities depend on the basal shear stress and thus on surface slopes. Accurate information about topography is, therefore, crucial to predicting future evolution and understanding ice dynamics, either by providing an empirical parameterisation of the flow laws or by pointing out unknown physical processes.
Moreover, altimeters also provide other parameters such as backscatter coefficient and waveform shape that give information on surface roughness and snow pack characteristics, from the global to the kilometre scale. Since ERS-1 was launched, with an orbit as high as 82°N and S, our vision of the ice sheets has been radically transformed. This long series, with ERS-2 and Envisat following ERS-1, has made it possible to discern changes in the shape and volume of both ice sheets, Greenland and Antarctica, which are related to climate change. Moreover, from April 1994 to March 1995, ERS-1 was placed on a geodetic orbit (two shifted cycles with a 168-day repeat) so that the topography of both ice sheets could be mapped with a resolution of 2 km. This precise topography has led to the detection of subglacial lakes, subglacial hydrological networks, outlet glacier anomalies, etc.
The 25 years of data from ERS-1, ERS-2, Envisat and Cryosat-2 have also made it possible to map the ice mass balance. Measurements from CryoSat mission, launched in 2010, have been used to map the height of the huge ice sheets that blanket Greenland and Antarctica and show how they are changing. It carries an interferometer altimeter, called SIRAL, the first of its kind to overcome the difficulties intrinsic to measuring icy surfaces. The instrument allows scientists to determine the thickness of ice floating in the oceans and to monitor changes in the vast ice sheets on land, particularly around the edges where icebergs are calved. Ice sheets gain mass through snowfall and lose it through melting and by glaciers that carry ice from the interior to the ocean.
Examining the ice sheet regions individually we show that the Greenland, West Antarctic and Antarctic ice sheets have all lost mass over the past two decades, whilst the East Antarctic ice sheet has undergone a slight snowfall-driven growth. The Greenland ice sheet has lost the largest mass and accounts for about two-thirds of the combined ice sheet loss over the study period. In Antarctica, the largest mass losses have occurred in the West Antarctic Ice Sheet. However, despite occupying just 4% of the total ice sheet area, the Antarctic Peninsula has accounted for around 25% of the Antarctic mass losses.
|fig 2. Time series of cumulative mass change and sea level contributions from the East, West and Antarctic Peninsula ice sheets (left), and the Antarctic, Greenland, and combined Antarctic and Greenland ice sheets (right). (Credits IMBIE)|
Satellite altimetry has been used extensively in the past few decades to observe changes affecting large and remote regions covered by land ice such as the Greenland and Antarctic ice sheets. Glaciers and ice caps have been studied less extensively due to the limitation of altimetry over complex topography. However, their role in current sea-level budgets is significant and is expected to impact over the next century and beyond (Gardner et al., 2011), particularly in the Arctic where mean annual surface temperatures have recently been increasing twice as fast as the global average.
Radar altimetry is well suited to monitor elevation changes over land ice due to its all-weather year-round capability of observing ice surfaces. Since 2010, the Synthetic Interferometric Radar Altimeter (SIRAL) on board the European Space Agency (ESA) radar altimetry CryoSat-2 (CS-2) mission has been collecting ice elevation measurements over glaciers and ice caps. Its Synthetic Aperture Radar Interferometric (SARIn) processing mode reduces the size of the footprint along-track and locates the across-track origin of a surface reflector in the presence of a slope. This offers new perspectives for the measurement of regions marked by complex topography.
More recently, data from the CS-SARIn mode have been used to infer elevation beyond the point of closest approach (POCA) with a novel approach known as “swath processing” (Hawley et al., 2009; Gray et al., 2013; Foresta et al., 2016). Together with a denser ground track interspacing of the CS mission, the swath processing technique provides unprecedented spatial coverage and resolution for space borne altimetry, enabling the study of key processes that underlie current changes of ice caps and glaciers.
|fig 3. Comparison between swath-processed (Swath) and conventional (POCA) surface elevation change rates over the six largest ice caps in Iceland, representing 90% of the glaciated area. V (Vatnajökull), L (Langjökull),H(Hofsjökull),M(Mýrdalsjökull), D (Drangajökull), and E (Eyjafjallajökull). The inset shows the location of individual elevation measurements by using Swath and POCA approaches over Langjökull. [Credit: Luca Foresta et al. (2016).]|
- Rémy, F., The new vision of the cryosphere thanks to 15 years of altimetry, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
- Zwally, H.J. and A.C. Brenner, Ice sheet dynamics and mass balance, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001
- McMillan, Malcolm, et al. “Increased ice losses from Antarctica detected by CryoSat‐” Geophysical Research Letters 41.11 (2014): 3899-3905.
- Shepherd et al., 2012 A reconciled estimate of ice sheet mass balance.
- Tepes, Paul, et al. “CryoSat swath altimetry to measure ice cap and glacier surface elevation change.” AGU Fall Meeting Abstracts. 2016.
- Gardner, A.S., Moholdt, G., Cogley, J.G., Wouters, B., Arendt, A.A., Wahr, J., Berthier, E., Hock, R., Pfeffer, 10 W.T., Kaser, G. and Ligtenberg, S.R., 2013. A reconciled estimate of glacier contributions to sea level rise: 2003 to 2009. Science, 340(6134), pp.852-857
- Foresta, L., et al. “Surface elevation change and mass balance of Icelandic ice caps derived from swath mode CryoSat‐2 altimetry.” Geophysical Research Letters 43.23 (2016).