A colossus iceberg, named A23a weighing a whopping trillion tonnes that calved out of the Antarctica’s Filchner Ice Shelf in 1986 resumed its journey in the Southern Ocean in 2024. At almost 4,000 sq. km in area, it’s essentially an ice island twice the size of Greater London. Earlier, iceberg A68 measuring 6000 sq. km calved out from Larson C Ice Shelf of Antarctic Peninsula in 2017 melted away into the warm waters of South Atlantic. Another iceberg A76A measuring about 3500 sq. km that calved out of Antarctica’s Ronne Ice Shelf in May 2021 is presently drifting in the Drake Passage and is on course to the graveyard of icebergs in the warm waters of South Atlantic Ocean.
Ice calving is an ablation of an advancing glacier when the accumulation exceeds a critical level and breaks down. Ice built up in the interior part of a landmass get pushed out to the edges like a frozen river moving slowly. In case the edges break off into the ocean it become floating icebergs. The question is, will the melting of icebergs lead to sea level rise? The answer is not straight forward. Since, icebergs are floating on the sea surface, it should not raise sea level when they melt for the same reason that water doesn’t overflow from a glass when the ice cubes melt. But several studies have shown that fresh water floating ice actually increases sea level slightly when it melts into denser salty sea water. This happens because when ice melts and become liquid it takes up more volume than the seawater it displaces when it was ice, raising the sea level.
The fresh water on Earth is locked up in the polar ice sheets of Arctic Ocean, Greenland, Antarctic peninsula and other global inland glaciers. The source of ice in the polar regions is snowfall and the low temperatures preserve the accumulated snow as compact ice sheets. Since, snow is made of freshwater, the compact ice retains its freshwater characteristics and when icebergs calve out from the ice sheets it tend to float in denser salty water of the oceans. While the Antarctica ice is over continental landmass, the Arctic ice is mainly sea ice floating in the Arctic Ocean. NASA’s GRACE satellite data shows that the average annual loss of ice mass in Antarctica is about 150 billion tons and Greenland about 270 billion tons that adds to sea level rise. But the rise is not even due to ocean dynamics and Earth’s uneven gravity field. Ocean currents driven by wind, heating, evaporation and precipitation lead to redistribution of mass and since the Earth’s mass is uneven, the gravity is also uneven making ocean’s surface a bumpy surface. When the land-based ice sheets of Greenland and Antarctica continue to unload their mass in the form of icebergs from far above sea level and far from the tropics, that mass reaches the sea in the form of melt water that is then redistributed along Earth’s gravity field.
– Dr. R K Chadha
