Fresh water from melting Antarctic ice is projected to weaken the world’s most powerful ocean current by 20 percent in the next quarter century, an international team of scientists concluded in a study published today in the journal Environmental Research Letters.

A weakening of the Antarctic Circumpolar Current—one of Earth’s strongest climate engines—would have dire consequences, including “more climate variability, with greater extremes in certain regions, and accelerated global warming,” said lead author Bishakhdatta Gayen, an associate professor of fluid mechanics at the University of Melbourne.

For the past two years, human-caused warming has driven the planet’s temperature past the limit set by the Paris Agreement, and if the new projections of an ACC slowdown are accurate, even more dangerous extremes are likely in the future, he said.

The ACC is the only ocean current to flow around the entire planet unimpeded, carrying more than 100 times more water than all the world’s rivers combined. It reaches 100 to 200 miles wide and as deep as three miles as it circles Antarctica from west to east, mixing water from the planet’s largest ocean basins—the Pacific, Indian and Atlantic.

The ACC streams off the shore of southern Australia and southern Africa and flows through the channel between the southern tip of South America and the Antarctica Peninsula, forming the “main mechanism for the exchange of heat, carbon dioxide, chemicals and biology across these ocean basins,” the researchers wrote.

“Without that mixing, if the redistribution stops, you can start getting hot spots or cold spots,” Gayen said. If the ocean becomes more stagnant, the likelihood of marine heatwaves and associated impacts like toxic algal blooms increases. As ocean regions warm, they also expand so a weaker ACC could accelerate sea level rise, he added.

In the Southern Hemisphere, the ocean off the coast of eastern Australia is currently about 3 to 7 degrees Fahrenheit warmer than average, and the record ocean temperature has resulted in severe coral reef damage in the region during the last few months. In recent years, there have been record-breaking ocean heat waves in the Tasman Sea and the South Pacific around New Zealand.

The emergence of new heat and density patterns in the Southern Ocean to some degree reflects changes documented in and near the Arctic Ocean, where disturbances in other ocean currents threaten coastal areas, including the eastern United States, with accelerating sea level rise.

In the Northern Hemisphere, scientists have chemically analyzed fossilized plankton remnants and sediment grains that document how shifts in ocean patterns of density and salinity can drive changes to major currents. Because the South Ocean is so expansive, scientific sampling of ocean patterns can be challenging. A study published in Nature last year suggests that cycles of ACC strength have been linked to changing climate cycles over the past 5 million years. 

Gayen said the ACC is also an important physical barrier that isolates Antarctica’s land, coastal and ocean ecosystems from the rest of the world. If it weakens, more species, including those that can have harmful invasive effects, may be enabled to move toward the still-frozen continent, according to scientists who studied how driftwood lands on the icy shores of Antarctica.

Gayen said that a 2023 paper in the journal Nature Climate Change documented how freshwater from melting ice has already been weakening the overturning, or vertical circulation, of Antarctic shelf waters, which also reduces oxygen in the deep ocean.

“Analyzing it for the Southern Ocean, we found such a remarkable outcome, that the ACC will slow down 20 percent in the next 30 years,” he said. “This is a very rich model in terms of resolving eddies,” he added, which is important because the full effects of the thousands of loops and swirls along the edges of the ACC across thousands of miles of ocean have not been accurately represented by climate models.

Co-author Taimoor Sohail, a postdoctoral research associate at the School of Geography, Earth and Atmospheric Science at the University of Melbourne, said previous studies show that a growing north-south temperature gradient in the Southern Ocean would speed up the ACC.

The new study, along with other recent research, strongly suggests that the salinity changes caused by freshwater from melting ice “far outweigh” temperature effects on the ACC.

Gayen said one of the big uncertainties is exactly how much ice is melting. Some recent research estimates the thawing is producing 28.8 trillion gallons of meltwater per year. 

“We don’t really know much about East Antarctica,” he said. “But one thing is clear, there is a connection between melting and an ACC slowdown.”

Bob Berwyn

Reporter, Austria

Bob Berwyn an Austria-based reporter who has covered climate science and international climate policy for more than a decade. Previously, he reported on the environment, endangered species and public lands for several Colorado newspapers, and also worked as editor and assistant editor at community newspapers in the Colorado Rockies.

• 
  @bberwyn
  
• 
  [email protected]