NASA research finds new connection between Greenland glaciers’ slopes and ice sheet thinning

Outlet glaciers like these are rivers of ice flowing within the cracks of the bedrock, seeping into the surrounding sea. As and when the climate warms up, these glaciers begin shrinking and thinning, right up to the center of the ice sheet. Analyzing the bed topography beneath the ice gives scientists a better understanding of which glaciers could significantly impact the Greenland Ice Sheet’s contribution to a rise in sea-levels in the coming years.

NASA research finds new connection between Greenland glaciers’ slopes and ice sheet thinning
NASA research finds new connection between Greenland glaciers’ slopes and ice sheet thinning

NASA research finds new connection between Greenland glaciers’ slopes and ice sheet thinning

Recent research into polar glaciers in Greenland, published in Geophysical Research Letters, has found a new connection between a glacier’s slope and the thinning of ice sheets, contributing to a future rise in sea levels.

The research analyzed 141 outlet glaciers on the Greenland Ice Sheet to theorize how deep the thinning could spread along the flow lines, starting at the ocean’s edge.

Outlet glaciers like these are rivers of ice flowing within the cracks of the bedrock, seeping into the surrounding sea. As and when the climate warms up, these glaciers begin shrinking and thinning, right up to the center of the ice sheet. Analyzing the bed topography beneath the ice gives scientists a better understanding of which glaciers could significantly impact the Greenland Ice Sheet’s contribution to a rise in sea-levels in the coming years.

The research has found that certain glaciers flowing over gentler slopes could have a higher impact as they allow the thinning to spread deeper into the interior, while glaciers with steep slopes limit the spread of the interior thinning.

“What we discovered is some glaciers flow over these steep drops in the bed, and some don’t,” said lead author Denis Felikson with NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the Universities Space Research Association (USRA). “For the glaciers that do have that steep drop in the bed, thinning can’t make its way past those drops.” Borrowing a term from geomorphology – the study of Earth’s physical features – they coined these steep drop features “knickpoints.”

When a river flows over a knickpoint, it often creates a waterfall or a lake. But glacier dynamics are much different, and steep translates to just about three degrees of incline.

The researchers were able to identify these “steep” changes in topography using digital elevation models of the ice sheet bed and surface topography, taken from the Greenland Ice Mapping Project. The project was created using NASA’s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument that flies aboard NASA’s Terra satellite, along with data from NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) mission.

The bed topography digital elevation model, known as the BedMachine data set, is a high-resolution model of the bed beneath the Greenland Ice Sheet, created using data from NASA’s Operation IceBridge airborne surveys of polar ice.

“This bed topography data set was critical to us doing our work,” Felikson said. “And it is thanks to NASA remote sensing, namely the Operation IceBridge surveys, that we were able to do this.” Using the remote sensing data, scientists were able to compare topography measures to produce a single metric along a glacier’s flow line. This helped them identify a break point between the upstream and downstream parts of the glacial ice.

Ice below the knickpoint is susceptible to thinning from the glacier’s edge. But the thinning does not extend beyond this point upstream, so the interior of the ice sheet is not impacted.

Of all the glaciers observed, a majority (65 percent) had discernable knickpoints. Especially steep knickpoints were found in the more mountainous regions of Greenland, where many of the biggest and fastest-moving glaciers also show knickpoints closer to the coast. Their sheer size itself means these glaciers could contribute significantly to the thinning and melting of ice sheets. But since their knickpoints are closer to the coast, the thinning is not expected to spread far inland.

However, glaciers that flow through gentle topography have gradual knickpoints, or none at all. However, despite being smaller in size, they could let the thinning expand hundreds of kilometers inland, eroding the very heart of the ice sheet.

“They could be impactful in terms of sea-level rise, not because they are big and deep, but because they have access to more ice that they can eat away,” said Felikson. “It will take them a lot longer to respond, but over the long term they could end up contributing just as much to sea-level rise, maybe, as the big glaciers.”

This work was started at the University of Texas as part of Felikson’s dissertation and has continued throughout his time at NASA Goddard. The origins of knickpoints and their implications for long-term thinning, as well as Greenland’s overall contribution to sea-level rise, comprise his future research.