The new research, published Tuesday in the journal Nature Communications, focuses on a process known as Antarctic bottom water production. When seawater in the Southern Ocean freezes into sea ice, it expels the salt it used to contain, and this salt makes its way into the water column below. The saltier the water becomes, the denser it gets, and this cold, dense water eventually sinks down to the bottom of the ocean.
The downward sinking motion of this dense water is “one of the driving cogs in the global conveyor belt of the world ocean,” explained Guy Williams, a polar oceanographer at the University of Tasmania and the new study’s lead author. As the dense water sinks to the bottom, it forces the water already there to move forward, creating a current that runs toward the equator. Eventually, this water warms up and rises to the surface, and the warmer surface water then runs back toward the poles. (This handy NASA video is a good illustration of the process.)
Worldwide, it’s a process known as “thermohaline circulation,” or sometimes generally just “overturning circulation,” and it’s responsible for bringing cool water up to the tropics and warm water out to the poles — an important component of the Earth’s climate and weather systems. It also helps drive major currents like the Gulf Stream, which runs along the East Coast of the United States.
But lately, scientists have begun to worry that the changes happening at the Earth’s poles could actually weaken this global circulation, with potentially dire consequences.
The idea is that as polar glaciers melt, they release an influx of fresh water into the ocean. This makes the water less salty, and therefore less dense, so it doesn’t sink to the bottom as readily. If this happens, it could slow down the great circulation, causing changes in ocean currents that might lead to increased sea-level rise in some places, changes in local weather patterns and other regional and global consequences.
Now, the new study provides fresh evidence that melting glaciers can, in fact, hinder the production of dense water in the Southern Ocean. The new research shows that it’s already happening in a region of East Antarctica known as Prydz Bay.
Prydz Bay happens to sit close by an area of Antarctica where a lot of bottom water production has been observed. In fact, this region is one of just four areas in Antarctica where conditions are right for the process to occur.
Scientists know that a lot of the dense, salty water has been coming from a nearby sea ice-rich location called Cape Darnley. But for years, they’ve also suspected that Prydz Bay might also be contributing some of its own dense water as well. The new study aimed to investigate.
The study relied on data collected through a program called Marine Mammals Exploring the Oceans Pole to Pole, or MEOP — this is where the elephant seals come into play. This program, a cooperative effort among multiple scientific institutions, has non-invasively outfitted diving animals, including elephant seals, with scientific instruments that allow them to take measurements in otherwise difficult-to-access parts of the ocean.
The program has resulted in hundreds of thousands of temperature and salinity profiles in the world’s oceans, which have aided in nearly 100 research papers so far. (Earlier this summer, for instance, MEOP data helped support research that suggested warm, deep ocean water is melting Antarctic glaciers from the bottom up.)
MEOP data, acquired in 2012 and 2013, confirmed that Prydz Bay does make a substantial contribution to the Antarctic bottom water produced in the area — less so than Cape Darnley, but enough to be significant. But the researchers also noticed a complex, and potentially worrying, series of interactions going on between the ocean water at Prydz Bay and some of the nearby glaciers.
It appears that melting ice shelves at Prydz Bay, which are pouring fresh water into the ocean, are suppressing the formation of dense shelf water to a certain extent. It’s not enough to halt the region’s bottom water contributions, but it’s noticeably hindering the process.
“There’s sort of a fine dance going on,” Williams said. “In this case of Prydz Bay, the salinity is winning if you like, and it’s still capable of producing bottom water — but we do detect this suppression that’s going on.”
While the new study only focuses on Prydz Bay and can’t say for sure what’s going on in other regions of Antarctica, it’s an issue that’s been recently explored in other papers as well.
Last year, a study in Nature Climate Change, led by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, suggested there’s been a slowdown in the Atlantic component of the global ocean’s overturning circulation during the 20th century. Furthermore, the paper suggested that this slowdown may have been influenced by ice melt in Greenland, which could further weaken circulation as the melting accelerates in the coming decades.
Should this happen, consequences could include sea-level rise in the North Atlantic, along the U.S. East Coast, as well as cooling in Europe and potential changes in weather patterns (although the researchers suggested that this cooling effect would likely be offset by future global warming). And there could be potential changes to marine ecosystems and fisheries as well.
The new study “significantly improves our understanding of the details of bottom water production around Antarctica,” said Rahmstorf, who was not involved in the new research, by email. “Scientists have long feared that global warming will slow down this vital process of deep and bottom water production, both in the North Atlantic and in Antarctic waters. With too much global warming, a critical threshold could be crossed where this process grinds to a halt, with incalculable and potentially catastrophic consequences for marine life and climate.”
Rahmstorf isn’t the only researcher concerned about this issue, either. It’s a key component of a recent paper led by former NASA scientist James Hansen, now at Columbia University’s Earth Institute. The paper outlines a dire scenario in which even 2 degrees Celsius of warming above pre-industrial levels could lead to “dangerous” global consequences.
One of the paper’s key points is that rapid melting of both the Antarctic and Greenland ice sheets may not only contribute to dramatic sea-level rise in the next century, but also affect the world’s oceans in profound ways — including freshening the water at the poles and contributing to a slowdown of the oceans’ overturning circulation.
The new paper “tends to confirm one of the principal phenomena that we were drawing attention to: the effect of freshwater from ice shelves reducing [Antarctic bottom water] formation,” Hansen told The Post by email. “We concluded that this process, slowing down on Antarctic bottom water formation, has already begun.”
For its part, the new study doesn’t actually demonstrate a slowdown in ocean circulation, and it also doesn’t make any clear predictions about what might happen under future climate change scenarios. It merely shows that bottom water formation in one location in Antarctica can be hindered by glacier melt. Long-term monitoring will be the only way to find out for sure whether this process is occurring in other locations and how it might be changing over time, Williams noted.
“While this particular area may not be the hotspot for this kind of activity, the fact that we have all the main players makes it a very unique lab experiment to try to understand how it works,” Williams said. “It provides observational evidence which should renew efforts to look for this happening in more key areas of Antarctica where we do know there’s accelerating melt occurring and where bottom water production is important as well.”