Two decades of changes in Helheim Glacier, compiles NASA

Named after the Vikings’ world of the dead, Helheim has kept scientists on their toes for the past two decades. Between 2000 and 2005, Helheim quickly increased the rate at which it dumped ice to the sea, while also rapidly retreating inland- a behavior also seen in other glaciers around Greenland, read the NASA website

Since then, the ice loss has slowed down and the glacier’s front has partially recovered, readvancing by about 2 miles of the more than 4 miles it had initially ­retreated.

NASA has compiled a time series of airborne observations of Helheim’s changes into a new visualization that illustrates the complexity of studying Earth’s changing ice sheets.

NASA uses satellites and airborne sensors to track variations in polar ice year after year to figure out what’s driving these changes and what impact they will have in the future on global concerns like sea level rise.

Since 1997, NASA has collected data over Helheim Glacier almost every year during annual airborne surveys of the Greenland Ice Sheet using an airborne laser altimeter called the Airborne Topographic Mapper (ATM). Since 2009 these surveys have continued as part of Operation IceBridge, NASA’s ongoing airborne survey of polar ice and its longest-running airborne mission. ATM measures the elevation of the glacier along a swath as the plane files along the middle of the glacier. By comparing the changes in the height of the glacier surface from year to year, scientists estimate how much ice the glacier has lost.

The animation begins by showing the NASA P-3 plane collecting elevation data in 1998.

The laser instrument maps the glacier’s surface in a circular scanning pattern, firing laser shots that reflect off the ice and are recorded by the laser’s detectors aboard the airplane.

The instrument measures the time it takes for the laser pulses to travel down to the ice and back to the aircraft, enabling scientists to measure the height of the ice surface. In the animation, the laser data is combined with three-dimensional images created from IceBridge’s high-resolution camera system.

The animation then switches to data collected in 2013, showing how the surface elevation and position of the calving front (the edge of the glacier, from where it sheds ice) have changed over those 15 years.

Helheim is about 4 miles wide on average, but IceBridge only collects data along an 820-foot swath in the center of the glacier.

“Because we can measure only about 4 percent of the width of the glacier, we fly the center line, which we know, from other glaciers, is reasonably representative of the glacier as a whole,” said Kristin Poinar, a polar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We have navigation systems on board that help us fly over the same center line each year, so we get a reliable overlap of measurements.”

Helheim’s calving front retreated about 2.5 miles between 1998 and 2013. It also thinned by around 330 feet during that period, one of the fastest thinning rates in Greenland.

“The calving front of the glacier most likely was perched on a ledge in the bedrock in 1998 and then something altered its equilibrium,” said Joe MacGregor, IceBridge deputy project scientist. “One of the most likely culprits is a change in ocean circulation or temperature, such that slightly warmer water entered into the fjord, melted a bit more ice and disturbed the glacier’s delicate balance of forces.”

“As the front of the ice retreated, it showed more and more of its face to the warm ocean and this became a vicious cycle of retreat,” Poinar said. “We see this behavior over and over again in glaciers that flow all the way into the ocean. As such a glacier starts to move faster, it sucks more ice into the ocean and the net result is that the glacier gets thinner and retreats farther. This whole process continues until the glacier can find another ledge to anchor to and restabilize.”

As the glacier evolved, so did the ATM instrument flown aboard the aircraft. At the beginning of the survey, the whole system weighed over 4,000 pounds – now it’s only around 400 pounds, so scientists can deploy it in smaller planes when needed. Back in the 1990s, the laser fired 2,000 pulses per second; now it transmits 10,000 pulses per second and the pulses themselves are also almost ten times shorter, which allows for denser, more precise measurements.

“The accuracy of the laser’s elevation measurements has improved from about 6 inches in the 1990s to less than 2 inches during the IceBridge era because of improvements in the technology,” said Michael Studinger, principal investigator for the laser instrument team. “That allows us to do more science: We can now go look at areas of Greenland that are experiencing smaller changes because we know that our measurements resolve much finer details than in the old days.”

“We now have a two-decade-long, reliable time series of elevation measurements in Greenland that allows us to link to the data from many other instruments, such as NASA’s Ice, Cloud, and land Elevation Satellite missions or the European Space Agency’s CryoSat-2 satellite,” Studinger said. “Having such a long time series is important when you look at changes in the ice sheets, and the ATM’s is the longest and most consistent elevation time series that’s out there.”

Image: NASA website