THE IMMENSE AND FORBIDDING Southern Ocean is famous for howling gales and devilish swells that have tested mariners for centuries. But its true strength lies beneath the waves.
The ocean’s dominant feature, extending up to two miles deep and as much as 1,200 miles wide, is the Antarctic Circumpolar Current, by far the largest current in the world. It is the world’s climate engine, and it has kept the world from warming even more by drawing deep water from the Atlantic, Pacific and Indian oceans, much of which has been submerged for hundreds of years, and pulling it to the surface. There, it exchanges heat and carbon dioxide with the atmosphere before being dispatched again on its eternal round trip.
Without this action, which scientists call upwelling, the world would be even hotter than it has become as a result of human-caused emissions of carbon dioxide and other heat-trapping gases.
“From no perspective is there any place more important than the Southern Ocean,” said Joellen L. Russell, an oceanographer at the University of Arizona. “There’s nothing like it on Planet Earth.”
For centuries this ocean was largely unknown, its conditions so extreme that only a relative handful of sailors plied its iceberg-infested waters. What fragmentary scientific knowledge was available came from measurements taken by explorers, naval ships, the occasional research expeditions or whaling vessels.
But more recently, a new generation of floating, autonomous probes that can collect temperature, density and other data for years — diving deep underwater, and even exploring beneath the Antarctic sea ice, before rising to the surface to phone home — has enabled scientists to learn much more.
They have discovered that global warming is affecting the Antarctic current in complex ways, and these shifts could complicate the ability to fight climate change in the future.
As the world warms, Dr. Russell and others say, the unceasing winds that drive the upwelling are getting stronger. That could have the effect of releasing more carbon dioxide into the atmosphere, by bringing to the surface more of the deep water that has held this carbon locked away for centuries.
In addition, the Southern Ocean is getting warmer, and that has another important climate effect. Some of this upwelling water, which is already relatively warm, flows beneath ice shelves on the Antarctic coast that help keep the continent’s vast, thick ice sheets from reaching the sea more quickly.
In effect, “Antarctica is melting from the bottom,” said Henri Drake, an oceanographer at the Massachusetts Institute of Technology.
That, scientists say, is already adding to sea level rise. Over time it could contribute much more, potentially swamping coastlines in the next century and beyond.
While the potential magnitude of all these effects remains unclear, oceanographers and climate scientists say that it is increasingly urgent to understand this interplay of powerful forces and how human activity is transforming them. “There’s lots of questions left,” said Lynne Talley, an oceanographer at Scripps Institution of Oceanography in La Jolla, Calif.
Beginning in the late 19th century, whaling ships began heading southward, to the Antarctic, in growing numbers as whale populations in the more hospitable waters of the Atlantic and Pacific oceans declined from overhunting. Hundreds of ships sailed the violent southern waters on voyages that could last a year or longer.