Deep Water Ocean Flows in Antarctic Could Slow 40% by 2050

Antarctica's fast-melting ice sheets. Image: NASA

An updated model analyzing the impacts of global heating on ocean water movement in the Southern Hemisphere says what has already happened is already bad and about to get far worse.

It will transform regional climate phenomena permanently, with ripple effects affecting all the oceans and much of marine life within it.

The process that is causing this has its beginnings with the increasing rate of melting of the Antarctic ice sheets as the Earth heats up. When that happens, the ice releases freshwater across the ocean surface. It pools as a far less dense and salty cap atop the underlying saltwater of most of the ocean.

Under normal circumstances without such intense ice melting, warm ocean currents originating northward of Antarctica — part of the Antarctica Circumpolar Current (ACC) — move towards the pole thanks to the Earth’s rotational movement and thermodynamic conditions which move warmth towards the colder polar region. As they grow closer, the warm currents cool, then sink downwards towards the ocean floor, ultimately flipping back on themselves to run deep along the ocean floor. Ultimately the cold underwater currents heat up again as they move northward and are exposed to warmer surface level temperatures. They then reverse direction and head back to the colder waters of the south again.

Think of the image of an Olympic swimmer traversing one length of the pool, then themselves flipping over and propelling their body under water for a bit as they gain momentum.

As these ocean currents continue their cycle, they have many impacts on the ecosystems they pass through. As they pass through the cycle, they stir up and carry a lush nutrient-rich concoction from the seabed upwards. When the cold freshwater melt is released by Antarctica ice sheets because of the climate crisis, that current flow will slow, mostly because of simple thermal interactions.

A view of the Antarctic Circumpolar Current (ACC), showing branches connecting to the larger thermohaline circulation, including currents such as the Atlantic Meridional Overturning Current (AMOC) off the east coast of the United States and Canada. Image: NASA

Steve Rintoul, a fellow at the Australian government’s Commonwealth Scientific and Industrial Research Organization (CSIRO) and a co-lead on the just-published paper analyzing what is happening to the currents near Antarctica, explained how the slowing current can damage ecosystems which interconnect far from the route of these specific currents.

“If we slow the sinking near Antarctica, we slow down the whole circulation and so we also reduce the amount of nutrients that get returned from the deep ocean back up to the surface,” he explained in a recent interview regarding the new paper.

According to Rintoul, under normal circumstance a surprisingly large three-quarters of the global phytoplankton population, creatures which form the base of the entire ocean food chain and provide half the planet's oxygen, are fed by the nutrients stirred up by the Southern Ocean. When those ocean currents are throttled because of intense melting and freshwater release from the Antarctic ice sheets, a sizable percentage of the phytoplankton will not receive the nutrients they need to survive.

The other major impact on the Southern the scientists involved in this study involved how increased warm water infiltration on the underbelly of the vast Antarctic ice sheets will contribute to further melting of the ice from underneath. That will release even more freshwater, creating a feedback loop which is just one of many that impact the region.

Another feedback loop the authors did not comment on but also impacts phytoplankton populations is the increased absorption of carbon dioxide into the top surface waters. That will starve the phytoplankton further, but this time not by nutrients but of oxygen which would otherwise have been available in large quantities.

Based on what even the authors believe are “conservative” assumptions, based on projections of total greenhouse gas emissions releases and associated temperature increases in the air and in the oceans, plus analyses of the current melt rate acceleration near the South Pole, and checking their data backwards in time to validate how well their models predict the current state of the Southern Oceans, the researchers reached the stunning conclusion that the deepwater flow rate of the Southern Ocean could plummet by 40% by mid-century.

The computer model which analyzed this relied heavily on computer simulations and climate models, many of which were developed by the researchers themselves. It took over two years and an estimated 35 million computing hours of time to come up with the final data.

Matthew England, a University of New South Wales (UNSW) professor at that institution’s Climate Research Centreand the coordinator for the new study, said changes to the deep ocean current are currently on “a trajectory which looks headed toward collapse”.

“In the past, these circulations have taken more than 1,000 years or so to change, but this is happening over just a few decades,” he explained.

“It’s way faster than we thought these circulations could slow down,” he continued. “We are talking about the possible long-term extinction of an iconic water mass.”

This is far from the only place where global heating is already significantly impacting the rate of flow of ocean currents which are critically important to the planet.

In the Atlantic Ocean, researchers are watching with high concern the slowdown of one of the other oceanic flow “giants”, the Atlantic Meridional Overturning Current (AMOC), which also includes the Gulf Stream.

When that one flows normally, it moves first northwards then more northeasterly towards the United Kingdom, by forces related to the Earth’s rotation, and by thermodynamic effects as it encounters colder waters. As with the Southern Ocean currents, when the AMOC gets closer to the end of its route, it cools, drops down under the far colder “caps” along the top, and flips downwards on a deep dive. As the top warmer part of the cyclical current continues to drive towards Europe and the UK, the lower cooler part flows down closer to the ocean floor. It too stirs up nutrients, assists in overall water mixing, and distributes food in the form of seabed deposits throughout the marine cycle.

In research published in September 2020, scientists revealed that direct heating of the Atlantic has caused ocean surface temperatures to rise significantly, especially in the northeastern Atlantic, and mostly just within the last six decades. That has caused the Atlantic Meridional Overturning Current to slow measurably, with multiple side-effects.

As in the case of the Southern Ocean, the decreased rate of flow has caused less churning along the ocean bottom, along with less nutrient mixing and redistribution along its route. As the water moves slower, direct solar and atmospheric heating of the Atlantic is able to penetrate to deeper levels, especially in the first 650 feet below the surface. As that happens, the ocean becomes less turbulent and stratified, allowing faster and deeper heating in a feedback loop. As the ocean surface grows both more stable and hotter, it also provides an increasingly more dangerous “power source” for hurricanes to gain power and travel longer distances.

Besides the near-term implication that the slowdown of the AMOC will disrupt vast ranges of marine ecosystems, including fishing potential and more. A stagnating AMOC will contribute to more events like the hurricane which struck Portugal in 2018, the first since 1842, and high pressure forces which are part of why severe drought and wildfires have plagued the entire Iberian peninsula in the last few years. It also will prevent warmer waters from reaching the UK, resulting in the paradoxical climate shift to much colder than normal weather in the next few decades in southern England.

The conclusion of the AMOC study was in fact more dramatic than the current one regarding the Southern Ocean between Australia and Antarctica. It concluded the AMOC current could stall completely because of global heating in less than a decade.

What both studies have in common is the devastating nature of what the change in their currents will have on the planet.

As the current paper regarding the Southern Ocean concludes, the rapid slowdown of currents approaching the Antarctic coast will “profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come.”

These ocean impacts will include much more severe weather, a massive collapse of most marine life and a steady reduction of oxygen levels across the planet.

And at this stage, nothing we do can stop it. We could have possibly prevented the worst had we gotten off fossil fuels and stopped deforestation 40 years ago, when scientists first started publicly warning us of what was coming.

Had we not been so collectively stupid, we would have contemplated the consequences of the greenhouse effect when it was first discovered by Irish physicist John Tyndall back in 1859. Or we could have taken heed in 1959 when physicist Edward Teller warned the American Petroleum Institute of the climate crisis that would result from continuing to burn oil and coal.

Now, only a small portion of humanity is preparing to live on an increasingly inhospitable planet and cope with the mass extinction that is already well underway, while most others desperately try to convince themselves that it isn't happening or that buying an electric car will somehow make it all OK.

“Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater,” the paper explaining the details of this study and authored by Qian Li, Matthew England, Andrew McC. Hogg, Stephen R. Rintoul, and Adele K. Morrison, was published March 29, 2022, in Vol. 615 of the journal Nature.