The early 1980s saw the very first attempt to measure ice sheets using satellite radar altimetry, with observations only of limited parts of Greenland and Antarctica [Remy and Parouty, 2009]. Since then, however, altimetry technology has become an important means of determining ice sheet and glacial mass balance—that is, the ledger of gains and losses in ice mass [Pritchard et al., 2010]. While the primary measurement is by laser altimetry (NASA’s ICESAT and the upcoming ICESAT-2), with high accuracy and a very small footprint, radar altimetry also is used, starting with ESA-s ERS-2 ansd Envisat, and especially the recent ESA mission Cryosat-2, optimized for ice work.
Satellite altimetry tracks changes in ice-surface elevation, which, combined with other measurements and adjusting for vertical land motion and ice density, has revealed significant reductions in ice mass for the world’s glaciers as well as for Greenland and at least a portion of the Antarctic ice sheet. Continuous surface-height measurements for nearly all of Greenland, and more than 70 percent of Antarctica, have been conducted since 1992, beginning with ERS-2 and ENVISAT [Pritchard et al., 2010], which were used to expand upon previous measurements showing accelerated thinning of the Pine Island glacier in West Antarctica [Wingham et al., 2009; Rignot et al., 2008].
Radar altimetry also played an important role in characterizing a series of measurable changes that contributed to the collapse of the Larsen A and B ice shelves in 1995 and 2002 [Scambos et al., 2000, 2003].
While radar altimeters measure ice height, the ability of instruments like GRACE to capture gravitational shifts provides a direct means to measure ice mass change and hence another important tool in the quest to understand how varying forces affect glacial mass balance. Longer time-series measurements since GRACE’s launch in 2002—as well as more robust signals because of increased ice loss—also have substantially reduced uncertainties [Vaughan et al., 2013].
Changes over time in ice mass, and hence gravity, allowed GRACE to detect ice loss trends in the Greenland and Antarctic ice sheets within a few years of launch [Velicogna et al., 2014] with a spatial resolution of 300 kilometers (see Velicogna et al. for a discussion of the accuracy of this technique).
Later, as we’ve seen, GRACE data were combined with those yielded by other methods, including laser altimeter measurements from ICESat, to reveal a discrepancy between space and ground-based measurements of glacial mass loss [Gardner et al., 2013].
Satellite synthetic aperture radar interferometry techniques measure the speed at which ice streams move, as well as the position of the grounding line, which separates ice grounded over bedrock from ice floating on the ocean from the same ice stream. One example of the power of this technique was the measurement of the retreat of Antarctic glaciers [Rignot et al., 2014]. The researchers used data from the ERS-1 and 2 synthetic aperture radar (different from the radar altimeters on the same satellites) from 1992 to 2011, finding rapid retreat for the Pine Island, Thwaites, Haynes and Smith-Kohler glaciers in West Antarctica. The differences among sets of radar altimetry measurements for the same time intervals—the “interference” of interferometry—allowed the researchers to compensate for tracking drift caused by the loss of gyroscopes in ERS-2 [Rignot et al., 2014]. The study included color-coded maps of the velocity of recession for the glaciers, which showed movement of the glacial grounding lines. The authors concluded that “this sector of West Antarctica is undergoing a marine ice sheet instability that will signiﬁcantly contribute to sea level rise in decades to centuries to come,” because the bed upon which the ice rests is “retrograde”—that is, it deepens going inland, toward the south.
The same interferometric techniques were used to map the seaward flow of networks of glaciers across the entire continent from 2007 to 2009 [Rignot et al., 2011].
By the 1990s, laser altimetry from aircraft had revealed ice sheets thinning rapidly enough on Greenland’s coastal margins to create an imbalance, as outlet glaciers lost ice mass to the ocean [Abdalati et al., 2001]. Later surveys, again using NASA’s Airborne Topographic Mapper, showed those thinning rates had increased [Thomas et al., 2009].
The largest airborne survey of polar ice is known as Operation IceBridge, a NASA mission that seeks to plug the gap between ICESat, which ceased functioning in 2003, and the planned launch of ICESat-2. Researchers used data from these yearly surveys to reveal that the rate of ice discharge from the two fastest glaciers draining the Greenland and Antarctic ice sheets was increasing [Thomas et al., 2011]. The study also relied on data from NASA’S Airborne Topographic Mapper, as well as the University of Kansas ice-depth sounder, both of which had made almost yearly surveys since 1991 of the Jakobshavn Isbrae glacier in Greenland and, since 2002, of the Pine Island glacier in Antarctica.
Accurately gauging snow levels also is important to the larger picture. The Jet Propulsion Laboratory’s Airborne Snow Observatory flies two instruments over the snowfields of the Sierra Nevada, which are critical to the state’s water supply. An imaging spectrometer captures visible to near-infrared light to measure the albedo, or reflectivity, of the snowpack; the measurements are used to gauge how quickly the snow will melt (darker snow absorbs more radiation, and melts faster). The second instrument, a laser altimeter or lidar, measures snow depth by comparison against a “snow free” profile for the same areas, in order to determine snow depth and the snow-water equivalent within the snowpack [Bormann et al., 2014; NASA-ASO web site].
As with ancient sea level, the collapse and regrowth of ice sheets over the ages can be measured using paleontological proxies. One intriguing study examined coral terraces that, because of tectonic uplift, had been thrust some 20 to 140 meters above present-day sea level in Papua New Guinea [Esat and Yokoyama, 2006]. The researchers linked the episodic rise and fall of sea level, between and 75,000 and 25,000 years ago, to alternating warm and cold periods during the last Ice Age, lasting roughly millennia but transitioning from one to another on the scale of decades. And these bouts of warming and cooling could be read in the patterns of coral growth on the uplifted terraces. The resulting sea level changes were attributed to the partial collapses, and later regrowth, of the Northern Hemisphere ice sheets, causing sea levels to rise, then fall.
The vertical position of coral on the terraces told the story: During periods of falling relative sea level, coral growth was spotty, while rising sea levels allowed the coral to build more robust colonies at higher elevations.