For The Past 20 Years Antarctica's Deep Ocean Has Been Heating Up, Scientists Reveal

Deep ocean heat is moving closer to Antarctica, a new decades-long study has revealed, providing clear evidence that the Southern Ocean is already experiencing shifts due to climate change. 

The study, published today in the journal Communications Earth & Environment, warns that the warming threatens the stability of Antarctic ice sheets. Antarctica is one of the fastest warming places on Earth, and as the ocean heats up due to global warming, the rate of ice loss is accelerating, contributing to rising sea levels. In 2022, global average sea level reached a new record high: 4 inches above 1993 levels. The findings are a stark warning that significant climate risks are unfolding in the depths of the Antarctic waters.

“There is a robust signal in observational records of the past decades that these large water masses that contain significant anthropogenic heat and heat in general are encroaching on the Southern Ocean approaching Antarctica, and that is coming at the cost of the deep waters underneath that naturally exist, and Antarctica shrinking,” says Ali Mashayek, professor in the department of earth sciences at the University of Cambridge, and one of the authors of the study. 

It’s the first time that scientists have observed the shift in deep-ocean heat throughout the Southern Ocean. Until now, scientists didn’t have enough data to detect the warming trend. “Observations in this region are really hard to get,” notes Joshua Lanham, the study’s lead author. The researchers relied on data from study sites known as transects, which record things like temperature and nutrients in the water, roughly once every decade. They supplemented this information with publicly available data captured by “Argo floats,” robotic floats that provide continuous data on the upper ocean. They then used machine learning to combine the Argo float data and the long-term changes to create monthly snapshots of the last four decades. 

The data confirms what climate experts have long suspected: that a warm mass known as “circumpolar deep water” has expanded and shifted toward the Antarctic continental shelf over the past 20 years. The ocean absorbs around 90% of the excess heat caused by global warming—with most of it stored in the Southern Ocean. As the waters underneath the ice sheets warm, it risks destabilizing Antarctica's ice shelves, which play a key role in preventing inland ice sheets and glaciers from flowing into the ocean.

“The mechanism which generally destabilizes these ice shelves is melting from warm water below the surface, which can then debuttress the ice shelf, and cause sea level rise. So the implication of those poleward movement in warm water is quite significant from that perspective,” says Lanham. “The collective amount of fresh water in these inland glaciers could raise sea levels by about 58 meters [190 ft.].”

The results, however, do not come as a surprise. “In general, there's extensive, accumulated observational evidence that the ice shelves are more and more in danger,” says Mashayek. “Our work shows that there's also an observational trend that suggests that more heat is getting to Antarctica.”

Global average sea level has risen eight to nine inches since 1880, and the rate is accelerating due to glacier and ice sheet melt—adding new volumes of water to our oceans. Sea level rise due to Antarctic ice melt increases the risk of coastal flooding and can contribute to more intense storms. 

The Antarctic ice sheets and the Southern Ocean are also key in regulating the global climate system—changes to the ice mass of Antarctica could impact climate patterns like El Niño. The study’s findings add to a growing body of recent research documenting significant changes in the world’s polar seas.

Another study published earlier this month found that the Atlantic meridional overturning circulation (AMOC), a system of global ocean currents, is closer to collapse than previously thought, with the system slowing down as air temperatures rise rapidly in the Arctic due to global warming. A collapse in this circulation would trigger a rise in Atlantic sea levels, shift the tropical rainfall belt that millions of people rely on to grow food, and expose parts of Europe to extreme cold winters and summer droughts. 

The AMOC circulation relies on the cold, dense water found in both the Arctic and Antarctic. These cold waters sink down into the ocean’s depths, helping to power a so-called “conveyor belt” of deep currents that circle the globe. As both Antarctic and Arctic waters warm, that circulation is disrupted: warm, fresh water from melted land ice doesn’t sink as easily as cold, salty water. 

“It is a very famous saying that what happens in the Southern Ocean doesn't stay in the Southern Ocean,” warns Mashayek. “The same thing is true about the Arctic. Both have huge global impacts that can be realized relatively quickly on human relevant timescales.”