Increasing ocean temperature threatens Greenland's ice sheet

Date:
January 25, 2021
Source:
University of California - Irvine
Summary:
Scientists have for the first time quantified how warming
coastal waters are impacting individual glaciers in Greenland's
fjords. Their work can help climate scientists better predict
global sea level rise from the increased melting.



FULL STORY ========================================================================== Scientists at the University of California, Irvine and NASA's Jet
Propulsion Laboratory have for the first time quantified how warming
coastal waters are impacting individual glaciers in Greenland's
fjords. Their work is the subject of a study published recently in
Science Advances.


========================================================================== Working under the auspices of the Oceans Melting Greenland mission for
the past five years, the researchers used ships and aircraft to survey
226 glaciers in all sectors of one of Earth's largest islands. They
found that 74 glaciers situated in deep, steep-walled valleys accounted
for nearly half of Greenland's total ice loss between 1992 and 2017.

Such fjord-bound glaciers were discovered to be the most subject to undercutting, a process by which warm, salty water at the bottom of
the canyons melts the ice from below, causing the masses to break apart
more quickly than usual. In contrast, the team found that 51 glaciers positioned in shallower gullies experienced less undercutting and
contributed only about 15 percent of the total ice loss.

"I was surprised by how lopsided it was. The biggest and deepest glaciers
are undercut much faster than the smaller glaciers in shallow fjords,"
said lead author Michael Wood, a post-doctorate researcher at NASA's Jet Propulsion Laboratory in Southern California, who began this research
as a doctoral student at UCI. "In other words, the biggest glaciers are
the most sensitive to the warming waters and those are the ones really
driving Greenland's ice loss." The study highlighted the dynamic whereby deeper fjords allow the intrusion of warmer ocean water than shallow
ones, hastening the process of undercutting with some of Greenland's
largest glaciers.

Greenland is home to one of Earth's only two ice sheets, the largest
being Antarctica's. The ice in Greenland is more than two miles (three kilometers) thick in places. At the edges of the land mass, the vast
glaciers extending from the ice sheet travel slowly down valleys like
icy conveyor belts, which inch into the fjords and then melt or break
off as icebergs. The ice is replenished by snowfall that is compressed
over time into the ice pack.



==========================================================================
If the ice sheet were in balance, the amount of snow accumulating on the
top would roughly equal the ice lost from melt, evaporation and calving -- chunks breaking free from anchored masses and floating off into the ocean.

But the ice sheet has been out of balance since the 1990s. Melt has
accelerated and calving has increased, causing glaciers that extend into
the sea to retreat back toward land. Together, these are resulting the
ice sheet shrinkage.

According to the research team, the build-up of warm salty water at the
bottom of fjords has been accelerated by increasing temperatures in the
summer months, which heat the surfaces of glaciers, creating pools of meltwater. This liquid leaks through cracks in the ice to form subsurface freshwater rivers which flows into the sea where it interacts with salty
water beneath fjords.

Glacier meltwater is free of salt, so it is lighter than seawater and
rises to the surface as a plume, dragging up warm water and putting it in contact with the bottoms of glaciers. Fjord depth is a fairly immutable
factor, but other factors such as seawater temperature and the amount
of meltwater from glaciers surfaces are greatly impacted by climate
warming. All three factors combine to cause accelerated deterioration
of Greenland's ice sheet, the researchers said.

As the water temperature around Greenland's coastline is predicted to
continue to increase in the future, these findings suggest that some
climate models may underestimate glacial ice loss by at least a factor
of two if they do not account for undercutting by a warm ocean.

The study also lends insight into why many of Greenland's glaciers
never recovered after an abrupt ocean warming between 1998 and 2007,
which caused an increase in ocean temperature by nearly 2 degrees
Celsius. Although ocean warming paused between 2008 and 2017, the glaciers
had already experienced such extreme undercutting in the previous decade
that they continued to retreat at an accelerated rate.

"We have known for well over a decade that the warmer ocean plays a major
role in the evolution of Greenland glaciers," said OMG deputy principal investigator Eric Rignot, also of JPL and UCI. "But for the first time,
we have been able to quantify the undercutting effect and demonstrate
its dominant impact on the glacier retreat over the past 20 years."

========================================================================== Story Source: Materials provided by
University_of_California_-_Irvine. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Michael Wood, Eric Rignot, Ian Fenty, Lu An, Anders Bjo/rk,
Michiel van
den Broeke, Cilan Cai, Emily Kane, Dimitris Menemenlis, Romain
Millan, Mathieu Morlighem, Jeremie Mouginot, Brice Noe"l, Bernd
Scheuchl, Isabella Velicogna, Josh K. Willis, Hong Zhang. Ocean
forcing drives glacier retreat in Greenland. Science Advances,
2021; 7 (1): eaba7282 DOI: 10.1126/sciadv.aba7282 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210125191851.htm

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