Sea-level rise is caused by adding mass (water) and heat to the ocean. The Antarctic ice sheet represents a potentially large source of water. Knowing the speed and direction (velocity) at which the Antarctic ice sheet is flowing to the ocean is important for assessing current and future sea-level rise. Scientists from the University of California, Irvine, and the NASA Jet Propulsion Laboratory have published the most precise map ever of Antarctic ice velocity in the journal Geophysical Research Letters (July 2019). The new map will be useful for mass-balance and ice-thickness studies of the Antarctic ice sheet as well as for modeling climate and sea-level rise. Compared to earlier maps, the new map is 10 times more accurate in terms of ice-flow direction and speed and shows ice movement over 80 percent of Antarctica versus 20 percent shown in past maps. See also: Antarctica; Climate modeling; Glaciology; Global climate change; Sea-level rise

Mapping of ice motion in Antarctica using synthetic-aperture radar interferometric phase data. Upper panels show the distribution of data tracks used from the Canadian RADARSAT-2, Japanese ALOS PALSAR, European ERS and Canadian RADARSAT-1, and European Envisat ASAR. Lower main panel shows a complete map of ice motion in Antarctica combining phase data in the interior and speckle tracking in the fast-moving sectors, with speed colored on a logarithmic scale from brown (less than 1 m/year) to red (more than 3 km/year) and flow direction indicated by colored lines and showing the flow pattern of ice across the continent. (Credit: J. Mouginot et al., Geophys. Res. Lett., 2019)

Scientists measure the speed of ice moving across the Earth's surface by analyzing sequences of images using either synthetic aperture radar (SAR) speckle or phase interferometry tracking techniques. Speckle tracking techniques monitor the movement of two-dimensional parcels of ice over time, which works well for measuring fast-flowing ice. However, about 60 percent of Antarctica—particularly the interior—is flowing at slow speeds of less than 10 meters (33 feet) per year. Alternatively, phase interferometry measures the phase difference of two or more SAR signals acquired from slightly different orbit positions and at different times. As a result, a much smaller area can be measured but more data are required than for speckle techniques because a three-dimensional parcel of flowing ice is being measured. See also: Interferometry; Radar; Synthetic aperture radar (SAR)

To overcome the data acquisition requirements, the researchers combined 25 years of phase data obtained from six satellites: the Canadian Space Agency’s Radarsat-1 and Radarsat-2; the European Space Agency’s Earth remote sensing satellites 1 and 2 and Envisat ASAR; and the Japan Aerospace Exploration Agency’s ALOS PALSAR-1. As a result, the researchers were able to produce a map showing ice movement at a resolution (accuracy) of 20 centimeters (about 8 inches) per year in speed and 5 degrees per year in flow direction. In contrast, speckle tracking of fast‐flow areas has an accuracy of only 2 to 5 meters (6.6 to 16.4 feet) per year. See also: Remote sensing