Antarctica holds approximately 90 percent of the world’s freshwater, locked away in ice. The mean thickness of the Antarctic ice is 2.2 km (just under 1.4 miles). However, along the coasts of the continent, the Antarctic ice sheets extend out over the coastal seas, forming thick shelves of “floating “ice. Several of these prominent ice shelves collapsed during the past 50 years, and conventional thinking was that atmospheric warming was the primary culprit for these collapses. Now, there is mounting evidence that the Antarctic ice shelves are under attack from above by atmospheric warming and undermined below by oceanic warming.
Ocean warming is a relative term. In the case of Antarctic waters, a temperature of 0 degrees Celsius (32 degrees Fahrenheit) is warm. Yes, this is the freezing point of water, but it only applies to freshwater. Seawater freezes at -2 degrees Celsius (28.4 degrees Fahrenheit), due to its salinity. So, the temperature dynamics at the base of an ice shelf require that the
Thwaites Glacier of Antarctica
A multimillion-dollar research project recently drilled through 610 meters (2000 feet) of ice and measured ocean temperatures below the Thwaites Glacier. The drilling project investigated a spot at the “grounding line” of the glacier. A “grounding line” is the location where the glacier passes from the land and extends out over the ocean. The temperature of the water below the Thwaites ice was zero degrees Celsius; warm enough to induce melting.
Typical ocean ice forms from heat transfer when the ocean is warmer than the atmosphere during the winter. The heat transfers from the warm water to the colder air. This heat transfer lowers the heat energy, and hence the temperature, at the immediate surface of the water and ice forms. However, as ice thickens on the ocean surface, it inhibits this heat transfer and acts as an insulator. When the ice is thick enough to impede any transfer of heat (about 3 meters), it stops growing.
Antarctica’s thick ice shelves are not the result of normal sea ice formation. Instead, they are the result of onshore glaciers extruding ice from the continent onto the ocean’s surface. Those beautiful pictures we see of towering ice cliffs with large sections calving into the ocean, mark the endpoint of a glacier. There, local conditions are such that the glacier no longer supports itself, and it collapses into the sea. An ice shelf collapse occurs when the entire shelf cannot sustain itself, and the glacier edge retreats to the coastline.
Simultaneous melting from above and below hastens the collapse of individual ice sheets. This observation highlights the importance of understanding the mechanisms controlling ocean warming below ice shelves.
Causes of warming:
Three primary sources of heat affect coastal water temperatures around Antarctica. The first and most visible source is sunlight, solar radiation. During the summer, the dark ocean waters absorb a lot of solar heat. So oceans try to dissipate this heat through mixing with colder waters. However, salinity and temperature differences between shallow and deep waters create a stratification of the water column, and this layering may disrupt attempts at mixing. Then the surface waters can overheat and enhance melting at the base of ice sheets.
The second source of warmer water is glacial meltwater runoff from land. Commonly, glacier melt will create streams at the base of ice sheets where they rest on solid ground. In the case of costal glaciers, these meltwater streams flow beneath the ice and into the ocean. This process introduces warmer waters that mix with the existing ocean water and raise temperatures. There is evidence that this mechanism operates in some areas of the Antarctic, but the data is sparse.
The third, and probably most important, source of warm water is from the deep ocean. The Antarctic Circumpolar Current dominates the Southern Ocean. Driven by westerly winds, this current endlessly circles Antarctica, and it facilitates upwelling, so deep ocean waters are driven to the surface. These waters are cold, but warmer than the Arctic coastal waters. Hence, the shallow oceans warm as deep-ocean water mixes with the Antarctic coastal water.
Data is sparse in the Antarctic Ocean because of the harsh conditions that discourage direct sampling and physical conditions that make satellite measurements less reliable. However, despite the difficulties of data collection, there is mounting evidence that Antarctic ice is melting more rapidly than originally expected, driven in part by warming in the Antarctic coastal ocean.
ArcheanWeb:
Ocean warming dwarfs atmospheric warming (Source: ArcheanWeb) – https://archeanweb.com/2020/01/21/ocean-warming-dwarfs-atmospheric-warming/ Also:
The Antarctic Circumpolar Current: An Ouroboros (Source: ArcheanWeb) – https://archeanweb.com/2020/01/29/the-antarctic-circumpolar-current-an-ouroboros/ Also:
Sources:
Southern Ocean Warming (By Jean-Baptiste Sallée; Oceanography) – https://tos.org/oceanography/article/southern-ocean-warming Also:
Ocean temperature impact on ice shelf extent in the eastern Antarctic Peninsula (By Johan Etourneau, Giovanni Sgubin, Xavier Crosta, Didier Swingedouw, Verónica Willmott, Loïc Barbara, Marie-Noëlle Houssais, Stefan Schouten, Jaap S. Sinninghe Damsté, Hugues Goosse, Carlota Escutia, Julien Crespin, Guillaume Massé & Jung-Hyun Kim; Nature Communications) – https://www.nature.com/articles/s41467-018-08195-6 Also:
Glacial meltwater dynamics in coastal waters west of the Antarctic peninsula (By Heidi M. Dierssen, Raymond C. Smith, and Maria Vernet; PNAS February 19, 2002 99 (4) 1790-1795) – https://www.pnas.org/content/99/4/1790 Also:
Unprecedented data confirms that Antarctica’s most dangerous glacier is melting from below (By Chris Mooney; Washington Post) – https://www.washingtonpost.com/climate-environment/2020/01/30/unprecedented-data-confirm-that-antarcticas-most-dangerous-glacier-is-melting-below/ Also:
Can the ocean freeze? (NOAA) – https://oceanservice.noaa.gov/facts/oceanfreeze.html Also:
Feature Image: Ice Shelf Antarctica (By Georges Nijs) (Modified) – https://commons.wikimedia.org/wiki/File:Ice_Shelf_Antarctica_13.jpg – This file is licensed under the Creative Commons Attribution 2.0 Generic license. – https://creativecommons.org/licenses/by/2.0/deed.en
