Rapidly Melting Glaciers Threaten Collapse of Crucial Ocean Circulation Systems: Study

Scientists from the United States and Australia on Wednesday warned in a new study that the current rate of greenhouse gas emissions and the resulting rapid melting of Antarctic glaciers is placing a vital deep ocean current "on a trajectory that looks headed towards collapse" in the coming decades.

As Common Dreams has reported, Antarctic ice is melting at an unprecedented rate, and the melting is causing fresh water to enter the ocean—reducing the salinity and density which is needed to drive the "overturning circulation" of water deep in the world's oceans.

Normally, dense water flows toward the ocean floor and helps transport heat and and vital nutrients through the planet's oceans. The circulation helps support marine ecosystems and the stability of ice shelves.

With carbon emissions continuing to rise despite clear warnings from energy and climate experts about the urgent need to draw down emissions, the deep ocean current is projected to slow by 40% by 2050, according to the study, which was published in Nature.

The current slowdown would "profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon, and nutrients, with impacts felt throughout the global ocean for centuries to come," according to the study.

The researchers, who study at the Massachusetts Institute of Technology, Australian National University, and Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO), examined models and simulations over two years to determine how fast the deep ocean current more than 13,000 feet below the surface is expected to slow down as fresh water rapidly enters the ocean.

Circulation deep in the ocean could weaken twice as fast as the Atlantic Meridional Overturning Circulation (AMOC), which carries warm water from the tropics to the North Atlantic Ocean. The collapse of the AMOC has previously been identified as "one of the planet's main potential tipping points," as Common Dreams reported in 2021, but the Antarctic overturning circulation has been less studied until now.

The deep ocean current allows nutrients to rise from the bottom of the ocean, supporting about three-quarters of phytoplankton production and forming the basis of the global food chain.

"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," Stephen Rintoul, a fellow at CSIRO and co-author of the study, told Al Jazeera.

The ocean would also be left with a limited ability to absorb carbon dioxide due to the stratification of its upper layers, and warm water could increasingly intrude on the western Antarctic ice shelf, creating a feedback loop and even more melting of glaciers.

The study is "actually kind of conservative" in that it doesn't go into detail regarding that "disaster [scenario]," Alan Mix, a co-author of the latest Intergovernmental Panel on Climate Change (IPCC) report who was not involved in Wednesday's study, told Al Jazeera.

Matthew England, another co-author of the study, told The Guardian that deep ocean circulations have "have taken more than 1,000 years or so to change, but this is happening over just a few decades."

"It's way faster than we thought these circulations could slow down," England added. "We are talking about the possible long-term extinction of an iconic water mass."

The study is the latest sign that action to reduce planet-heating fossil fuel emissions is happening far too slowly, said climate scientist Bill McGuire.

"It seems almost certain that continuing on a high greenhouse gas emission pathway will lead to even more profound effects on the ocean and the climate system," John Church, an emeritus professor at University of New South Wales in Australia, told Al Jazeera. "The world urgently needs to drastically reduce our emissions to get off the high-emission pathway we are currently following."