Thanks to satellite measurements provided by the European Space Agency’s (ESA) CryoSat-2 mission, the actual dimensions of the world’s largest iceberg, known as A23a, and its trajectory are now known with greater precision.

Current surveys have revealed that the colossal ice block, more than twice the size of Greater London, has an average total thickness of just over 280 meters, equivalent to the height of London’s 22 Bishopsgate skyscraper, which is 278 meters tall. This measurement, combined with its already known area of 3,900 km², results in a volume of approximately 1,113 cubic km and a mass of 950 billion tonnes.

The data was collected by the veteran spacecraft equipped with a radar altimeter capable of detecting how much of an iceberg’s mass is above the waterline. Based on information about the density of ice, it then determines how much mass is submerged.

Altimetric satellites like CryoSat-2, which measure the distance from the surface of the iceberg and the sea surface, allow us to monitor the thickness of the iceberg from space,” said Anne Braakmann-Folgmann of the University of Tromso – Arctic University of Norway, speaking to the BBC.

“They also enable us to observe the thinning of the iceberg as it is exposed to warmer ocean waters. And, combined with knowledge of the seafloor topography, we know where an iceberg will become lodged or when it has thinned enough to be released again,” added the expert.

Born from a mass outflow of icebergs from the Filchner Ice Shelf in the southern Weddell Sea, A23a almost immediately became stuck in the shallow seabed mud to become an “ice island” for over three decades. Now, CryoSat data can explain why: the iceberg is not a uniform block, and some parts are thicker than others. According to the same source, a specific section has a very deep keel, which in 2018 had a draft – the submerged part of an iceberg – of almost 350 meters, anchoring it for so long.

“In the last ten years, we have witnessed a steady decrease in thickness of 2.5 meters per year, which is what one would expect given the water temperatures in the Weddell Sea,” explained Dr. Andy Ridout, senior researcher at the Centre for Polar Observation and Modelling (CPOM) at University College London.

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