Deep ocean currents around Antarctica headed for collapse, new study

Scott Base - The deep ocean circulation that forms around Antarctica could be headed for collapse, scientists say.

Such decline of this ocean circulation will stagnate the bottom of the oceans and generate further impacts affecting climate and marine ecosystems for centuries to come, a University of New South Wales study says.

Cold water that sinks near Antarctica drives the deepest flow of the overturning circulation, a network of currents that spans the world's oceans. The overturning carries heat, carbon, oxygen and nutrients around the globe. This influences climate, sea level and the productivity of marine ecosystems.

Their modelling shows that if global carbon emissions continue at the current rate, then the Antarctic overturning will slow by more than 40 per cent in the next 30 years and on a trajectory that looks headed towards collapse, the researchers say.

About 250 trillion tonnes of cold, salty, oxygen-rich water sinks near Antarctica each year. This water then spreads northwards and carries oxygen into the deep Indian, Pacific and Atlantic Oceans.

The international team of scientists modelled the amount of Antarctic deep water produced under the IPCC high emissions scenario, until 2050.

The model captures detail of the ocean processes that previous models haven't been able to, including how predictions for meltwater from ice might influence the circulation.

This deep ocean current has remained in a relatively stable state for thousands of years, but with increasing greenhouse gas emissions, Antarctic overturning is predicted to slow down significantly over the next few decades.

With a collapse of this deep ocean current, the oceans below 4000 metres would stagnate.

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

The  Australian Antarctic programme partnership says the model simulations show a slowing of the overturning, which then leads to rapid warming of the deep ocean.

Direct measurements confirm that warming of the deep ocean is indeed already underway. The study found melting ice around Antarctica makes the nearby ocean waters less dense, which slows the Antarctic overturning circulation. The melt of the Antarctic and Greenland ice sheets is expected to continue to accelerate as the planet warms.

Their research shows that the melting of the ice sheets has a dramatic impact on the overturning circulation that regulates Earth's climate.

They looked at 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.