Climate change slowing Antarctica's deep, dense waters

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Antarctic deep ocean currents have slowed in the past three decades, which scientists say will reduce oxygen levels across the world's deep seas.

The research, published in the journal Nature Climate Change on Friday, was conducted by scientists working at the ARC Australian Centre for Excellence in Antarctic Science (ACEAS) and CSIRO Environment.

Lead author Dr Kathy Gunn said the dense water that sinks near the frozen continent is known as Antarctic bottom water (AABW) and is a crucial component of a global network of ocean currents transporting heat, carbon, oxygen and nutrients around the world.

She said with the warming climate melting more ice in the region, glacial meltwater makes the waters of Antarctica fresher, and more buoyant.

This high amount of fresh water keeps the denser waters from sinking properly, reducing the amount of dense, oxygen-rich water that can sink to the deep sea, slowing the deep circulation.

Dr Gunn said this will have devastating knock-on effects on climate, sea level rise and deep-sea oxygen levels for centuries to come.

"We knew things were changing but our observations of the deep ocean are scarce, so it has been difficult to determine how and why the deep ocean was changing before now," she said.

Deputy director of ACEAS and report co-author Professor Matthew England said other studies predicting a 40 per cent reduction in the circulation of the deep waters by 2050 were based on estimates, but this research used existing data taken over 30 years.

"These latest observations suggest that the projected slowdown is in fact already well underway," he said.

"The process of dense water sinking around Antarctica pumps oxygen into the deep sea, much like a lung pumps oxygen into our blood.

"As this pump slows down, it reduces oxygen levels in the world's deep ocean."

Dr Steve Rintoul from CSIRO Environment and the Australian Antarctic Program Partnership said the freshening around Antarctica will continue, even accelerate, as the Antarctic ice sheet responds to a rapidly warming climate.

Dr Gunn said because of this, scientists expect a continuing decline of the deep ocean circulation and its oxygen levels.

"These declines are already significantly changing the deep ocean's structure and chemistry," she said.

Monash University's Dr Ariaan Purich, who is part of the ARC special research initiative Securing Antarctica's Environmental Future, said the new study is significant.

"It provides further support including observational evidence that the melting Antarctic ice sheet and shelves will impact the global ocean overturning circulation, with important impacts on the ocean uptake of heat and carbon," she said.

"While they might seem far away, changes occurring around the Antarctic margins will affect the climate and sea levels experienced here in Australia for decades to come."