A layer of warming water is rising from the subsurface, threatening to speed up Antarctic ice melt
By Andrew FreedmanJan. 21, 2021
The Southern Ocean is one of the most important yet least explored and understood regions of the planet when it comes to determining how global warming may affect the future of humanity, thanks to its capacity to absorb huge quantities of heat and carbon dioxide, and melt swaths of the Antarctic ice sheet.
In addition, this vast ocean, part of which separates Australia and Antarctica and also circles the frozen continent, is where global ocean currents get started, as heat is exchanged between the ocean and atmosphere, salinity differences arise in various layers of the deep, storm-churned waters, and currents reaching the North Atlantic Ocean and beyond are powered.
In recent years, understanding how the Southern Ocean is changing as a result of increased greenhouse gas emissions has taken on greater urgency as scientists have learned more about the fragility of large parts of the Antarctic ice sheet, since glaciers extending into the ocean are being eroded by relatively mild waters below. Like removing a doorstop, the collapse of these ice shelves can free up inland ice to move into the ocean, raising global sea levels and harming coastal communities.
Now a new study, published Thursday in the journal Nature Communications, finds that beneath the surface layer of waters circling Antarctica, the seas are warming much more rapidly than previously known. Furthermore, the study concludes, this relatively warm water is rising toward the surface over time, at a rate three to 10 times what was previously estimated.
This means that there is a greater potential for the waters of the Southern Ocean, which are absorbing vast quantities of added heat and carbon dioxide from the atmosphere as a result of human activities, may soon help destabilize parts of the Antarctic Ice Sheet.
The observations in the study, conducted by researchers at institutions in France and Australia, come from 25 years of temperature measurements taken aboard the French Antarctic resupply vessel L’Astrolabe, from the surface down to about 2,600 feet deep. The data, which includes more than 10,000 vertical temperature profiles, was gathered during multiple transits per year from Hobart in Tasmania to the Dumont d’Urville research station in Antarctica.
The researchers found that warming under the sea surface within waters near Antarctica stands out from naturally occurring trends, with temperatures increasing at a rate of about 0.072 degrees Fahrenheit (0.04 Celsius) per decade. At the same time, the relatively warm water — usually located under a colder layer — is rising toward the ocean’s surface at the rate of about 130 feet per decade. While the temperature change within waters that move from west to east around Antarctica may appear small, the study indicates it is a “radical” change from its average state and is enough to threaten ice stability where glaciers empty into the sea via fragile floating ice shelves.
Part of the reason for concern is because within these waters, seemingly small shifts in temperature can have dramatic implications. Subsurface waters flowing around Antarctica are typically below freezing, but because of the pressures involved at depth and the water’s salt content, the freezing point can be lower than 32 degrees Fahrenheit (0 degrees Celsius). This makes otherwise cold waters capable of melting ice.
The study states that the temperature changes found in the ship-based data matches the magnitude of changes found in the Amundsen-Bellingshausen Seas in West Antarctica, where the continent’s most dramatic ice loss is already happening.