New alarms over consequences of Antarctic ice melt

Changes to circulation in parts of the Southern Ocean are taking place faster than expected, with potentially profound implications for the global climate and marine life, a new study finds.

Why it matters: The Southern Ocean is home to the engine that powers the world's deep ocean currents, and regulates the climate. It governs the exchange of heat, carbon, oxygen and nutrients between ocean layers.

The big picture: There are only two main regions on the planet where cold, salty and dense water forms and sinks to the bottom of the sea, before slowly moving outward thousands of miles. This water eventually rises again to the surface, often years later.

• One such spot is in the North Atlantic, near Greenland, and the other is in the Southern Ocean, in pockets surrounding Antarctica.
• Both are exhibiting signs of stress as climate change melts land-based ice sheets, pouring lightweight freshwater into the upper layers of the ocean.
• This disrupts ocean currents that are driven by the vertical exchange of waters.
• A key part of such currents is Antarctic "bottom water" that forms off the frigid — though rapidly warming — continent's coasts.

Of note: Antarctic bottom water makes up nearly half the volume of the global ocean, and helps power what the late geoscientist Wallace "Wally" Broecker described as the Great Ocean Conveyor Belt.

Zoom in: The new study, published May 25 in the journal Nature Climate Change, finds evidence that at least one area of the Southern Ocean that drives this cycle is changing faster than previously anticipated.

• The results raise questions about the long-term vitality of this delicately interconnected series of ocean currents in the face of melting ice sheets.
• The research uses observational data to show that the overturning circulation has slowed by nearly 30% in the Australian Antarctic Basin between 1994 and 2017; deep ocean oxygen levels are declining there too.
• This area lies downstream of the Amundsen Sea, an area of large ice mass loss from increasing air and sea temperatures.

Between the lines: Lead author Kathryn Gunn of the Australian science agency CSIRO and Southampton University told Axios that observations don't yet show the same rapid trends occurring in other spots around Antarctica.

• "This basin gives an indication for what could happen in other basins," Gunn said via email. "This is further motivation to reduce greenhouse gas emissions as fast as possible."
• She noted that widely used climate projection models show far slower consequences for deep ocean circulation than what is actually occurring in the Southern Ocean.

What they're saying: The study's results, said co-author Matthew England of the University of New South Wales, are a concerning sign of an amplifying climate feedback, which may in turn speed up climate-induced ice melt.

• "Dense shelf water overturning keeps the Antarctic continental shelves cold. If we lose that, we’d expect those shelves to warm," he told Axios in an email.
• "That’s bad news as it would drive further ice melt."

Flashback: A previous study involving some of the same authors and published in Nature in late March, combined observational data from the 1990s through the 2000s with computer modeling experiments.

• It predicted just over a 40% net slowdown in the global overturning circulation by 2050 due to increased meltwater, with impacts that "could last for centuries."
• Gunn said the new research is consistent with that modeling.
• In concert with the Antarctic findings, recent research has also shown that the Atlantic Meridional Overturning Circulation is slowing as a result of freshwater floes from the Greenland Ice Sheet.

The bottom line: The new study is a warning sign of potential future changes to a crucial ocean cycle as ice melt continues, barring rapid and steep cuts to greenhouse gas emissions.