Ice loss speeds up in major Greenland glacier

A vast ice sheet in northeast Greenland has begun a phase of accelerated ice loss, contributing to destabilisation that will cause global sea-level rise for decades to come, a new study has warned.
Scientists found that since 2012 warmer air and sea temperatures have caused the Zachariae Isstrom ice sheet to "retreat rapidly along a downward-sloping, marine-based bed."
By itself, the Zachariae Isstrom glacier holds enough water to trigger a half-metre rise in ocean levels around the world, researchers said.
"The acceleration rate of its ice velocity tripled, melting of its residual ice shelf and thinning of its grounded portion doubled, and calving is occurring at its grounding line," they said.

"Ice loss is happening fast in glaciological terms, but slow in human terms - not all in one day or one year," said John Paden, from the University of Kansas-based Centre for Remote Sensing of Ice Sheets (CReSIS), who helped analyse data about the thickness of the glacier's ice for the study.
"Within a few generations, ice loss could make a substantial difference in sea levels," Paden said.

"When you add up all the glaciers that are retreating, it will make a difference to a large number of people. Sea level has increased over the last century, but only a small number of people have been affected compared to what is likely to come," said Paden.

Paden crunched data acquired by CReSIS during NASA's Operation IceBridge and previous NASA flights over Greenland, including decades-old measurements of Zachariae Isstrom.
Paden said the "grounding line," or the boundary between land and sea underneath a glacier, is a zone of special interest.
"The grounding line is where the ice sheet starts to float and is where the ice flux was measured," said Paden, who collaborated with researchers from the University of California Irvine and the California Institute of Technology.
While air temperatures have warmed, causing boosted surface runoff, Paden said ice loss from calving off the front of the glacier into the ocean accounts for most of the ice mass reduction from Zachariae Isstrom.

"Ice floating out into the ocean and melting is greater than the ice lost from surface melting," he said.
A neighbouring glacier with an equal amount of ice, named Nioghalvfjersfjorden, is also melting fast but receding gradually along an uphill bed, according to the researchers. Since Zachariae Isstrom is on a downslope, it is disappearing faster.
"The downward slope combined with warming ocean temperatures is what seems to be causing the acceleration now and why we predict it will continue to accelerate over the next few decades," Paden said.
Together, the ice in Zachariae Isstrom and Nioghalvfjersfjorden represent a 1.1-metre rise in sea levels worldwide, researchers said.

The study appears in the journal Science magazine.