Laser Eyes Spy a Big Melt in the Arctic
Airborne altimeters yield a disturbing picture of polar ice loss
Scientists from the U.S. National Oceanic and Atmospheric Administration didn’t pull any punches when they released the latest Arctic Report Card in early December. In 2012, levels of snow cover and sea ice were both at record lows, while surface melting of the massive Greenland ice sheet increased, despite temperatures in the region that were “unremarkable relative to the last decade.” In short, the Arctic seems to be deteriorating faster than scientists thought it would.
The latest results rely heavily on a NASA mission called IceBridge and its suite of sensors. IceBridge, an operation involving the P-3B, the DC-8, and other airplanes, is intended to “bridge” the gap between ICESat, a satellite that operated between 2003 and 2009, and ICESat-2, scheduled for launch in 2016.
The primary instrument used by both ICESat missions, as well as IceBridge aircraft, are laser altimeters. IceBridge uses several versions of such altimeters; the Airborne Topographic Mapper, for example, on both P-3B and DC-8 planes, can measure topography below the plane to within 10 centimeters. (ICESat-2 will do this one better with a multibeam laser altimetry system, which NASA says will improve ice elevation measurements as well as the detection of ice leads, which are basically cracks in sea ice.)
A more advanced instrument, a scanning lidar, can collect 10 000 data points every second as it scans a wide swath of ice from beneath a smaller aircraft. A different lidar device, used in IceBridge by researchers from the University of Texas at Austin, raises that to more than 2 million data points per second but is more constrained in its scanning width.
The planes also feature a number of radar instruments that let scientists characterize layers of ice all the way down to the bedrock beneath. The same trick doesn’t work over floating ice, so each aircraft carries a gravimeter to determine how much water is below the ice.
All this instrumentation has yielded some disturbing results recently. The extent of sea ice this year dropped down to 3.41 million square kilometers on 16 September, a remarkable 18 percent lower than the previous record set, in 2007. Further, the 2012 minimum was a full 49 percent below the 1979–2000 average sea ice cover. September ice levels seem to be dropping faster than those in the computer models, according to Andrew Barrett, a research scientist at the National Snow and Ice Data Center at the University of Colorado, in Boulder. However, Barrett says, this discrepancy is not statistically significant when the models are considered as a group.
In discussing some of the recent IceBridge data at the American Geophysical Union (AGU) meeting in December, Barrett asked what he called a “rather crass” and “somewhat simplistic” question: “When are we going to see an ice-free Arctic Ocean?” He then answered his own question, saying that fewer than 50 percent of the models show an ice-free Arctic before 2060. But he suggests that 2030 may be a reasonable guess, given most recent observations. (Researchers treat anything below 1 million km2 as “ice-free.”)
The question of when the ice will disappear entirely is not purely academic. It affects marine shipping routes, as well as coastal communities and wildlife.
If you’re looking for more globally relevant implications of the remote sensing data, look to the melting Greenland and Antarctic ice sheets, which, unlike Arctic sea ice, will play the primary role in sea level rise in coming years. Modeling ice sheets is much more difficult than modeling sea ice; just in recent months, studies have suggested both an acceleration and a slowing of Greenland’s sheet, and an accounting based on aerial photographs yielded the conclusion that the sheet has periods of rapid melting that come and go.
Satellite data suggest, however, that the last decade or so has seen some fairly dramatic melting. John Robbins, a principal scientist at NASA, presented research at the AGU that uses ICESat laser altimetry data to attempt to quantify ice loss from the “ice caps” around the Greenland sheet. The ice caps, he says, are “the little bits and pieces that are around the edges of Greenland. When we’ve analyzed ICESat up to this point, we’ve been focused mostly on the main sheet.”
His analysis of more than 25 ice caps—based on changes in elevation of the caps measured by ICESat’s Geoscience Laser Altimeter System—shows a combined loss of around 15 gigatons per year of ice (or about 16.7 cubic kilometers of volume). There are other ice caps to measure, so the final answer is most likely a bit higher.
“As an approximation, Greenland as a whole is losing something on the order of 200 gigatons per year,” Robbins says. “So this is something on the order of maybe 8 to 10 percent of the total loss going on in the whole of Greenland.” Only one of the 25 ice caps he initially sampled was found to be gaining any mass.
Combining IceSat and IceBridge has yielded a data set that shows an acceleration in Greenland ice loss, according to Beata Csatho, a geophysicist at the University of Buffalo, in New York. She estimates that the mass loss in the Greenland sheet will accelerate 22.1 km3 per year each year. The acceleration is probably slowing slightly, she says, but “there really is a potential for rapid ice loss.”