https://www.zmescience.com/ecology/environmental-issues/antarctic-ice-sheet-melting-astonishing-rate/
Antarctic ice sheet is melting underwater, and this is a big problem
Last updated on April 4th, 2018 at 6:38 pm by Elena Motivans
Antarctica’s ice sheet is melting from below– warm ocean water is
causing melting that can’t really be seen from the surface. To visualize
this below the surface melting, researchers from the UK Center for Polar
Observation and Modeling at the University of Leeds have used satellite
images. Some glaciers on the continent are retreating by 125 m (410
feet) per year!
Ice sheets have a leading edge, called a “grounding line.” It is where
the base of the glacier leaves the sea floor and starts to float. It
usually lies a kilometer or more below the sea level.
“Ice that is taken from the Antarctic continent and melts into the
world’s oceans, will raise global sea levels. This is not the case for
ice that is already floating on the ocean. The grounding line exactly
separates the floating parts, so-called ice shelves, from the grounded
parts of Antarctica, which rest on the seafloor or on the rock above sea
level. So technically, one can say that only a shrinking ice volume
inside the grounding line will raise sea levels. Retreating grounding
lines then mean that the area, in which Antarctica rests firmly on the
ground, is getting smaller, and by that also its capacity to store water
and prevent it from going into the ocean in the future,” said leading
author Dr Hannes Konrad from the University of Leeds to ZME Science.
Even submersibles have trouble accessing the grounding line so remote
sensing data was used in this study. The European Space Agency’s
CryoSat-2 was used to track 16,000 km of Antarctic coast. The satellite
works by measuring changes in the elevation of the ice sheet. Using sea
floor geometry, the principle of buoyancy, and knowledge of the glacier,
this information can be used to estimate the thickness of floating ice.
“We were delighted at how well CryoSat-2 is able to detect the
motion of Antarctica’s grounding lines. They are impossible places to
access from below, and usually invisible on the ground, so it’s a
fantastic illustration of the value of satellite measurements for
identifying and understanding environmental change,” said study
co-author Professor Andy Shepherd, from the School of Earth and
Environment at Leeds.
Between 2010 and 2016, 1, 463 km2 (564.86 square miles) of underwater
ice shelves melted. The melting has been extremely fast at eight of the
sixty-five biggest glaciers. West Antarctic was the most strongly
affected. The rate of ice melt at these locations is much higher than it
has been previously since the last ice age—up to five times quicker! The
baseline-melting rate of glaciers is about 25 meters (82 feet) per year.
“Our study provides clear evidence that retreat is happening across
the ice sheet due to ocean melting at its base, and not just at the few
spots that have been mapped before now. This retreat has had a huge
impact on inland glaciers, because releasing them from the sea bed
removes friction, causing them to speed up and contribute to global sea
level rise,” said Konrad.
Once ice shelves have melted at the bottom, it speeds up the melting
process for the rest of the glacier. This piece of news identifies
another factor that influences the melting of the Antarctic ice sheet,
and sea levels.
“Grounding lines of a number of indidual ice streams in Antarctica
have been studied before and found retreating, so our finding of a net
retreat in Antarctica does not come as a big surprise. One thing that
maybe wasn’t so clear before is how grounding lines react to changes in
the enviroment very differently in different places and at different
times. For example, the grounding line of Pine Island Glacier (a massive
ice stream and one of the largest single contributors to sea level rise
in recent years) which has retreated extremely fast since the early
1990s, at about 1 km per year or more, has been almost stable since
2011. Or in East Antarctica, one can find one ice stream retreating and
the neighbouring one advancing,” said Konrad to ZME Science.
The ice sheet behaves in a complex manner—although the melting of
grounding lines has increased, there are some areas where it has slowed
down. The Pine Island Glacier was one of the quickest retreating
glaciers on Antarctica, but now the below the surface melting has come
to a halt.
Now we know of a new factor that influences the melting of ice sheet and
help us to understand it better.
Journal reference: Konrad et al (2018) Net retreat of Antarctic glacier
grounding lines. Nature Geoscience.
http://dx.doi.org/10.1038/s41561-018-0082-z
