Significance of Antarctic Meltwater in Setting Ocean Overturning

Over the next 30 years, Antarctic circulation might slow by more than 40%, with serious consequences for the oceans and climate.

According to researchers, the deep ocean circulation around Antarctica may be on its way out.

Such a decline would cause the oceans’ floors to become stagnant, affecting climate and marine ecosystems for centuries.

The findings are reported in research led by Scientia Professor Matthew England, Deputy Director of UNSW Sydney’s ARC Centre of Excellence in Antarctic Science (ACEAS). The research, which was published in Nature included lead author Dr Qian Li, who previously worked at UNSW and is now at the Massachusetts Institute of Technology (MIT), and co-authors from the Australian National University (ANU) and CSIRO.

The deepest flow of the overturning circulation—a network of currents that spans the world's oceans—is driven by cold water sinking near Antarctica. The overturning transfers heat, carbon, oxygen, and nutrients worldwide. This has an impact on the climate, sea level, and productivity of marine ecosystems.

Our modeling shows that if global carbon emissions continue at the current rate, then the Antarctic overturning will slow by more than 40% in the next 30 years—and on a trajectory that looks headed towards collapse.

Matthew England, Professor and Deputy Director, ARC Centre of Excellence in Antarctic Science, UNSW Sydney

Modeling the Deep Ocean

Every year, approximately 250 trillion tons of cold, salty, oxygen-rich water sinks around Antarctica. This water then moves north, carrying oxygen into the deep Indian, Pacific, and Atlantic Oceans.

“If the oceans had lungs, this would be one of them,” Prof England notes.

The international group of experts calculated the quantity of deep water produced in Antarctica under the IPCC’s “high emissions scenario” until 2050.

The model contains details of ocean processes that earlier models could not, such as how meltwater from ice might influence circulation.

For thousands of years, this deep ocean current has remained relatively steady, but with rising greenhouse gas emissions, Antarctic overturning is expected to slow dramatically over the next several decades.

Impacts of Reduced Antarctic Overturning

The oceans below 4000 m would become stagnant if this deep ocean current failed.

This would trap nutrients in the deep ocean, reducing the nutrients available to support marine life near the ocean surface.

Matthew England, Professor and Deputy Director, ARC Centre of Excellence in Antarctic Science, UNSW Sydney

According to Co-Author Dr. Steve Rintoul of CSIRO and the Australian Antarctic Program Partnership, the model simulations suggest a slowing of the overturning, which results in rapid warming of the deep ocean.

“Direct measurements confirm that warming of the deep ocean is indeed already underway,” says Dr. Rintoul.

The research found that melting ice surrounding Antarctica reduces the density of neighboring ocean waters, slowing the Antarctic overturning circulation. As the earth warms, the melting of the Antarctic and Greenland ice sheets is predicted to intensify.

“Our study shows that the melting of the ice sheets has a dramatic impact on the overturning circulation that regulates Earth’s climate,” says Dr. Adele Morrison, also from ACEAS and the ANU Research School of Earth Sciences.

We are talking about the possible long-term extinction of an iconic water mass. Such profound changes to the ocean’s overturning of heat, freshwater, oxygen, carbon, and nutrients will have a significant adverse impact on the oceans for centuries to come.

Matthew England, Professor and Deputy Director, ARC Centre of Excellence in Antarctic Science, UNSW Sydney

Video Credit: University of New South Wales

Journal Reference:

Li, Q., et al. (2023). Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature. doi.org/10.1038/s41586-023-05762-w.