Melting Antarctic ice will slow down a major global deep ocean current by 40% by 2050

Melting ice around Antarctica will spark a rapid slowdown of a major global deep ocean current by 2050 that could have devastating impacts on ocean health and the marine food web.

It may also alter the world’s climate for centuries and accelerate sea level rise, a new study suggests.

The scientists warn that if greenhouse gas emissions continue at their existing levels, the current in the deepest parts of the ocean could slow down by 40 per cent in just 30 years.

The knock-on effect would deprive marine life of vital nutrients, change weather patterns and further increase sea levels.

Emissions will need to rapidly fall this decade to avoid these potentially grave consequences, the team of Australian researchers said. 

Warning: Melting ice around Antarctica will spark a slowdown of a major global deep ocean current by 2050 that could have devastating impacts on ocean health and marine life

The scientists warn that if greenhouse gas emissions continue at today’s levels, the current in the deepest parts of the ocean could slow down by 40 per cent in just 30 years. This graphic shows the temperature rises and increased salinity caused by the melting of freshwater ice in Antarctica over the past 20 years, as well as the projections for the next three decades

If they don’t, more sea life could die out, the ocean will struggle to absorb and hold heat, and ice loss will speed up even more.

‘That’s where the urgency comes from,’ said Dr Steve Rintoul, an oceanographer and expert on the Southern Ocean at the Australian government’s Commonwealth Scientific and Industrial Research Organisation.

‘Once we slow down the circulation it’s hard to get it going again. Once we kick this off, we can’t really change our mind. 

‘These changes are irreversible on time scales of many, many centuries.’

The world has very few places that produce water that is cold enough and dense enough to sink to the deepest parts of the ocean, but Antarctica has four such locations.

About 250 trillion tonnes of water sinks to depths below 13,100ft (4,000 metres) near the continent each year, in turn becoming very cold, very salty and oxygen-rich water.

However, the increase in melting Antarctic sea ice is reducing salinity levels, which therefore makes the water less dense and less able to sink.

This is important because it means the nutrient-rich water that sits far below is not displaced in the normal way and spread to other locations in the deep Indian, Pacific and Atlantic Ocean.

Such is the dramatic fallout, that surface ecosystems across the globe are then robbed of that water’s nutrients.

‘If we limit those nutrient upwelling fluxes, and we don’t provide enough nutrients to the basis of the food chain … clearly that has impacts for feeding populations in decades to come, into the 21st century,’ said co-author Professor Matthew England, of the Climate Change Research Centre at the University of New South Wales. 

He added that the whole deep ocean current was heading for collapse on its current trajectory. 

Dr Rintoul, who has spent his career studying how the Southern Ocean around Antarctica affects Earth’s systems, said the new modelling was vastly more sophisticated than any previous work focused on Antarctica.

This, he added, gives scientists even more confidence in their projections. 

Simulations show a slowing of the overturning circulation, which then leads to rapid warming of the deep ocean.

This graphic shows how the ocean near Antarctica has warmed and will continue to do so until 2050 and beyond unless more is done to tackle global warming emissions across the world 

The world has very few places that produce water that is cold enough and dense enough to sink to the deepest parts of the ocean, but Antarctica has four such locations. Unfortunately, however, the melting ice is stopping water from becoming dense enough to reach these levels

‘Direct measurements confirm that warming of the deep ocean is indeed already underway,’ Dr Rintoul added.

The same thing is happening in the northern hemisphere, with the melting of Greenland’s ice cap suppressing the North Atlantic overturning of deep water, according to Professor England.

‘These two water masses (in Antarctica and the North Atlantic) in a way absolutely dominate the ventilation of all of the ocean waters below about 1,500 metres (4,900ft) depth,’ he said.

He added that the North Atlantic system has shut down in the past, and scientists believe it is also headed for a slowdown and potential collapse in the future.

A collapse of both systems would have profound implications for marine ecosystems globally, Professor England said.

The new projections have been published in the journal Nature.