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

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 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 today’s 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. 

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 CSIRO oceanographer Dr Steve Rintoul, who helped produce the projections and has spent his career studying how the Southern Ocean around Antarctica affects earth’s systems.

‘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’s lying 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 University of NSW Professor Matthew England, who coordinated the new study.

Dr Rintoul says the new modelling is vastly more sophisticated than any previous work focused on Antarctica, and that means scientists can have a far greater degree of confidence in its projections.

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

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

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

‘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 projections have been published in the journal Nature.