Greenland Ice Sheet Thinning--Does a Glacier's Shape Affect it?

COLUMBIA, Md., Dec. 17, 2020 /PRNewswire/ -- Scientists at Universities Space Research Association and their colleagues at NASA, University of Texas and other organizations have determined that the slope of the bedrock underneath glaciers around the Greenland ice sheet can either spread ice thinning farther inland or stall it.

Outlet glaciers are essentially rivers of ice flowing over the bedrock and draining into the ocean.  They retreat and thin as the climate warms and this thinning spreads farther inland toward the center of the ice sheet.

Recently, by investigating the bed topography of 141 outlet glaciers around Greenland, scientists gained a better understanding of which glaciers could have a significant impact on the Greenland Ice Sheet's contribution to sea level rise in the coming decades. They found that steep bedrock features, called "knickpoints", can effectively stall the spread of ice thinning. This often occurs in regions where the bed topography is mountainous. In contrast, in regions with more gentle topography where knickpoints are not present or are less steep, glaciers can transmit ice thinning far into the interior, away from the seacoasts. The results were published in Geophysical Research Letters. 

Universities Space Research Association scientist Dr. Denis Felikson at NASA's Godard Space Flight Center, and the lead author of the study noted, "There are numerous small glaciers flowing over gentle topography in Northwest Greenland that can allow ice thinning to spread to the center of the ice sheet. These smaller glaciers, which have not garnered much attention may play as big a role in future sea level rise as the more well-known and well-studied glaciers."

Glaciers that flow through gentle topography are found to either have gradual knickpoints, or no knickpoint at all. Such glaciers are of interest, and concern, because in spite of their size they have the potential to let thinning expand hundreds of kilometers inland, eroding the heart of the ice sheet. 

Researchers have hypothesized that by allowing thinning of ice sheets to spread far inland, these smaller glaciers without knickpoints will cause the ice sheet to continue to lose ice over a longer period of time than the larger glaciers flowing on a bedrock with knickpoints.

"Only recently have scientists been able to identify the steep changes in topography using digital elevation models to map the topography under the ice. Without that ability, the research team would not have been able to come up with the findings," says Felikson.  The bed topography digital elevation model, known as the BedMachine Dataset--a high resolution model of the bed beneath the Greenland Ice Sheet --created by using airborne data from NASA's Operation IceBridge, was crucial for this research.

Using NASA's remote sensing data, scientists were able to determine whether thinning of the ice sheet can continue to spread upstream from a glacier's ocean boundary by computing a single metric based on the shape of the glacier at every location along a glacier's flow line. This helped them identify a break point between upstream and downstream glacial ice. "Ice below the knickpoint is susceptible to thinning from the glacier's edge. But thinning does not extend upstream so the interior of the ice sheet is not impacted. This means that some parts of the Greenland Ice Sheet interior are more sensitive to what happens at the edge than other parts", says Felikson.

"Small glaciers could be impactful in terms of sea level rise, not because they are big and thick, but because they have access to more ice that they can eat away at," said Felikson. "It will take them a lot longer to respond, but over the long term they could end up contributing significantly to sea level rise, just as the large glaciers do.

According to Felikson, "We are looking for confirmation from models for the theoretical limits on thinning that we have found, and hope that future modeling efforts will further confirm how thinning of ice spreads along glaciers. "

This research was funded by NASA under grant NNX12AP50G and by the Gale White Fellowship at the University of Texas Institute for Geophysics.

Related Resources:

Paper link: https://doi.org/10.1029/2020GL090112.

BedMachine Database Link: BedMachine v3: Complete bed topography and ocean bathymetry mapping of Greenland from multi-beam echo sounding combined with mass conservation, Geophysical Research Letters. 44. . https://doi.org/10.1002/2017GL074954

IceBridge BedMachine Greenland, Version 3. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/2CIX82HUV88Y.

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