Scientists monitoring the giant A68A Antarctic iceberg from space reveal
that a huge amount of fresh water was released as it melted around the
sub-Antarctic island of South Georgia.
Satellites revealed that 152 billion tonnes of fresh water entered the seas
around the sub-Antarctic island of South Georgia when the megaberg A68A
melted over 3 months in 2020/2021, according to a new study.
152 billion tonnes of water is equivalent to 20 times the amount of water in
Loch Ness or 61 million Olympic sized swimming pools.
In July 2017, the A68A iceberg snapped off the Larsen-C Ice Shelf on the
Antarctic Peninsula and began its 3.5 year, 4000 km journey across the
Southern Ocean. At 5719 square kilometres – quarter the size of Wales –, it
was the biggest iceberg on Earth when it formed and the sixth largest on
record.
Around Christmas 2020, the berg received widespread attention as it drifted
worryingly close to South Georgia, raising concerns it could harm the
island’s fragile ecosystem.
Researchers from the University of Leeds, Centre for Polar Observation and
Modelling (CPOM) and British Antarctic Survey (BAS) used satellite
measurements to chart the A68A iceberg’s area and thickness change
throughout its life cycle.
Their findings show that the berg had melted enough as it drifted to avoid
damaging the sea floor around South Georgia. However, a side effect of the
melting was the release of a colossal 152 billion tonnes of fresh water in
close proximity to the island – a disturbance that could have a profound
impact on the island’s marine habitat.
Anne Braakmann-Folgmann, a researcher at CPOM and PhD candidate at the
University of Leeds’ School of Earth and Environment, is lead author of the
study. She said: “This is a huge amount of melt water, and the next thing we
want to learn is whether it had a positive or negative impact on the
ecosystem around South Georgia.
“Because A68A took a common route across the Drake Passage, we hope to learn
more about icebergs taking a similar trajectory, and how they influence the
polar oceans.”
For the first two years of its life, A68A stayed close to Antarctica in the
cold waters of the Weddell Sea and experienced little in the way of melting.
However, once it began its northwards journey across Drake Passage it
travelled through increasingly warm waters and began to melt.
Altogether, the iceberg thinned by 67 metres from its initial 235 m
thickness, with the rate of melting rising sharply as the berg drifted in
the Scotia Sea around South Georgia.
Laura Gerrish, GIS and mapping specialist at BAS and co-author of the study
said:“A68 was an absolutely fascinating iceberg to track all the way from
its creation to its end. Frequent measurements allowed us to follow every
move and break-up of the berg as it moved slowly northwards through iceberg
alley and into the Scotia Sea where it then gained speed and approached the
island of South Georgia very closely.”
If an iceberg’s keel is too deep it can get stuck on the sea floor. This can
be disruptive in several different ways; the scour marks can destroy fauna,
and the berg itself can block ocean currents and predator foraging routes.
However, this new study reveals that A68A collided only briefly with the sea
floor and broke apart shortly afterwards, making it less of a risk in terms
of blockage.
By the time it reached the shallow waters around South Georgia, the
iceberg’s keel had reduced to 141 metres below the ocean surface, shallow
enough to avoid the seabed which is around 150 metres deep.
Nevertheless, the ecosystem and wildlife around South Georgia will certainly
have felt the impact of the colossal iceberg’s visit. When icebergs detach
from ice shelves, they drift with the ocean currents and wind while
releasing cold fresh meltwater and nutrients as they melt.
This process influences the local ocean circulation and fosters biological
production around the iceberg. At its peak, the iceberg was melting at a
rate of seven metres per month, and in total it released a staggering 152
billion tonnes of fresh water and nutrients.
The journey of A68A has been charted using observations from five different
satellites. The iceberg’s area change was recorded using a combination of
Sentinel-1, Sentinel-3, and MODIS imagery. Meanwhile, the iceberg’s
thickness change was measured using CryoSat-2 and ICESat-2 altimetry. By
combining these measurements, the iceberg’s area, thickness, and volume
change were determined.
Tommaso Parrinello, CryoSat Mission Manager at the European Space Agency,
said: “Our ability to study every move of the iceberg in such detail is
thanks to advances in satellite techniques and the use of a variety of
measurements. Imaging satellites record the location and shape of the
iceberg and data from altimetry missions add a third dimension as they
measure the height of surfaces underneath the satellites and can therefore
observe how an iceberg melts.”
Reference:
Braakmann-Folgmann A, Shepherd A, Gerrish L, Izzard J, Ridout A. Observing
the disintegration of the A68A iceberg from space. Remote Sens Enviro.
2022;270:112855.
DOI: 10.1016/j.rse.2021.112855
Tags:
Planet and Environment