The graphic shows the water temperatures around the Antarctic. On the western side of Antarctica, water temperature at the sea floor is approaching 2 degrees C—and that is warm enough to melt the ice that is flowing on top of it. Sea temperatures on the eastern flank are colder. Credit: Dr Benjamin Davison, University of Leeds
According
to scientists, 71 of the 162 ice shelves that surround Antarctica have reduced
in volume over 25 years from 1997 to 2021, with a net release of 7.5 trillion
metric tons of meltwater into the oceans. The work is published in the journal Science
Advances.
They found that almost all the ice
shelves on the western side of Antarctica experienced ice loss. In contrast, most of the ice shelves on the eastern
side stayed the same or increased in volume.
Over the 25 years, the scientists
calculated almost 67 trillion metric tons of ice was exported to the ocean,
which was offset by 59 trillion metric tons of ice being added to the ice
shelves, giving a net loss of 7.5 trillion metric tons.
Dr. Benjamin Davison, a research fellow
at the University of Leeds who led the study, said, "There is a mixed
picture of ice-shelf deterioration, and this is to do with the ocean
temperature and ocean currents around Antarctica.
"The western half is exposed to
warm water, which can rapidly erode the ice shelves from below, whereas much of
East Antarctica is currently protected from nearby warm water by a band of cold water at the coast."
Antarctica is a vast continent—50 times the size of the UK—and the seas on the western side experience
different currents and winds than on the east, and this is driving warmer water
underneath the ice shelves on the western flank.
The video animation shows what has happened to
ice around the Antarctic over the last 25 years—and summarizes the findings of
this research project. Credit: Planetary Visions/European Space Agency
Dr. Davison, an expert in Earth
Observation of polar regions in the School of Earth and Environment, said,
"We expected most ice shelves to go through cycles of rapid, but
short-lived shrinking, then to regrow slowly. Instead, we see that almost half
of them are shrinking with no sign of recovery."
He believes human-induced global
warming is likely to be a key factor in the loss of the ice. If it was due to
natural variation in climate patterns, there would have been some signs of ice
regrowth on the western ice shelves.
Ice shelves float on the seas
surrounding Antarctica and are extensions to the ice sheet that covers much of
the continent. The ice shelves act as giant "plugs" at the end of
glaciers, slowing down the flow of ice draining into the oceans.
When the ice shelves thin or reduce
in size, these plugs weaken with the result that the rate of ice lost from the
glaciers increases.
Getz Ice Shelf
Some of the biggest ice losses were observed on the Getz Ice Shelf, where 1.9 trillion metric tons of ice were lost over the 25-year study period. Just 5% of that was due to calving, where large chunks of ice breakaway from the shelf and move into the ocean. The rest was due to melting at the base of the ice shelf.
A
satellite image showing the Getz ice shelf. Getz lost 1.9 trillion tonnes of
ice over the 25-year study period. The image is a composite made from satellite
data recorded between January and September 2023. Credit: European Space Agency
Similarly on the Pine Island Ice
Shelf, 1.3 trillion metric tons of ice were lost. Around a third of that
loss—450 billion metric tons—was due to calving. The rest due to melting from
the underside of the ice shelf.
In contrast, the Amery Ice Shelf—on
the other side of Antarctica—gained 1.2 trillion metric tons of ice. It is
surrounded by much colder waters.
Major assessment of Antarctica
The researchers analyzed over
100,000 satellite radar images to produce this major assessment of the
"state of the health" of the ice shelves.
If the ice shelves disappear or
even diminish, there will be major knock-on effects for the ice system on
Antarctica and for global ocean circulation, the giant "conveyor
belt" which moves nutrients as well as heat and carbon from this sensitive
polar ecosystem.
Potential disruption to ocean circulation
Water released into the ocean from
the ice shelves and from the glaciers is freshwater. Over the 25-year study
period, the researchers estimated that 66.9 trillion metric tons of freshwater
went into the Southern Ocean around Antarctica from the ice shelves alone.
In the Southern Ocean, dense salty
water sinks to the ocean floor as part of the global ocean conveyor belt. This
sinking of water acts as one of the engines that drive the ocean conveyor belt.
Freshwater from Antarctica dilutes
the salty ocean water, making it fresher and lighter, which takes longer to
sink and this can weaken the ocean circulation system
A different study, published in the
journal Nature Climate Change, shows that this process may already be underway.
'Steady attrition'
Professor Anna Hogg, also from the
University of Leeds and a co-author of the study, said, "The study has
generated important findings. We tend to think of ice shelves as going through
cyclical advances and retreats. Instead, we are seeing a steady attrition due
to melting and calving.
"Many of the ice shelves have
deteriorated a lot: 48 lost more than 30% of their initial mass over just 25
years.
"This is further evidence that
Antarctica is changing because the climate is warming.
"The study provides a baseline
measure from which we can see further changes that may emerge as the climate
gets warmer."
Satellite monitoring
Information about what is happening
in Antarctica in recent years has largely come from the CryoSat-2 and Sentinel-1 satellites, that can monitor Antarctica even
when it is cloudy and through the long polar nights.
CryoSat-2, launched in 2010, was
the first European Space Agency
Explorer mission
and the first dedicated to monitoring the Earth's polar ice sheets and
glaciers.
Professor Noel Gourmelen, from the
University of Edinburgh and Earthwave and co-author of the study, said,
"CryoSat-2 has been an incredible tool for monitoring the polar
environment. Its ability to precisely map the erosion of ice shelves by the
ocean below enabled this accurate quantification and partitioning of ice shelf
loss, but also revealed fascinating details on how this erosion takes
place."
Details captured by these satellite
sensors are so great scientists have been able to track year-by-year changes in
Antarctica.
Dr. Mark Drinkwater, Head of Earth
and Mission Science at the European Space Agency (ESA), said, "Monitoring
and tracking climate change across the vast Antarctic continent requires a
satellite system that captures data routinely throughout the year.
"The European Copernicus
Program's Sentinel-1 satellite mission has fulfilled this need. Together with
the historical data acquired by its ESA predecessors ERS-1, -2 and
Envisat, Sentinel-1 has revolutionized our ability to take stock of
floating ice shelves, as a bellwether for mass balance and the health of
the Antarctic ice sheet.
"In the near future, we will further augment Antarctic monitoring with three new polar-focused missions CRISTAL, CIMR and ROSE-L."
Source: Over 40% of Antarctica's ice shelves reduced in volume over 25 years, scientists say (phys.org)
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