Scientists believe that the deep ocean circulation that forms around Antarctica could be headed for collapse. Such a decline in this ocean circulation will stagnate the ocean floor and have further impacts affecting climate and marine ecosystems for centuries.
The results are detailed in a new study coordinated by Professor of Science Matthew England, Deputy Director of the ARC Center for Excellence in Antarctic Science (ACEAS) at UNSW Sydney. The work, published today in Nature, includes lead author Dr. Qian Li formerly of UNSW and now of the Massachusetts Institute of Technology (MIT) as well as co-authors from the Australian National University (ANU) and CSIRO.
The cold water that sinks near Antarctica drives the deepest overturning circulation flow the networks of currents that span the world’s oceans. Overturning carries heat, carbon, oxygen and nutrients around the world. This affects climate, sea level and the productivity of marine ecosystems.
“Our modeling shows that if global carbon emissions continue at their current rate, then Antarctic overturning will slow by more than 40 percent over the next 30 years – and on a trajectory that looks like it’s headed for collapse,” says Professor England.
Modeling the deep ocean
About 250 trillion tons of cold, salty, oxygen-rich water sinks near Antarctica every year. This water then spreads northward and carries oxygen to the deep Indian, Pacific, and Atlantic oceans.
“If the oceans had lungs, this would be one of them,” says Professor England. An international team of scientists modeled the amount of Antarctic deep water produced under the IPCC’s “high emissions scenario” by 2050.
The model captures details of ocean processes that previous models could not, including how predictions for meltwater from the ice might affect circulation.
This deep ocean current has remained relatively stable for thousands of years, but with rising greenhouse gas emissions, Antarctic overturning is predicted to slow significantly over the next few decades.
Impacts of reduced Antarctic overturning
With the collapse of this deep ocean current, the oceans below 4,000 meters would stagnate. “This would trap nutrients in the deep ocean, reducing the nutrients available to support marine life near the surface of the ocean,” says Professor England.
Co-author Dr. Steve Rintoul from CSIRO and the Australian Antarctic Partnership says model simulations show a slowdown in overturning, which then leads to rapid warming of the deep ocean.
“Direct measurements confirm that warming of the deep ocean is already taking place,” says Dr. Rintoul. The study found that melting ice around Antarctica is reducing the density of waters in the nearby ocean, slowing the Antarctic’s overturning circulation. The melting of the Antarctic and Greenland ice sheets is expected to continue to accelerate as the planet warms.
“Our study shows that melting ice sheets have 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’re talking about the potential long-term demise of an iconic body of water,” says Professor England.
“Such profound changes in the transformation of heat, freshwater, oxygen, carbon and nutrients in the oceans will have significant adverse impacts on the oceans for centuries.”
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