Climate Denial Crock of the Week

High quality video above conveys daunting new observations of southern ocean dynamics that could affect critical global circulation.

We’ve talked a lot, and I’ve produced several videos, documenting slowdown in the Atlantic Meriodonal Overturning Current (AMOC), due to processes in Greenland and the Arctic. Here is perhaps even more concerning evidence of related processes occurring in the Antarctic.

Australian Broadcasting Company (ABC):

A “dramatic” change to ocean circulation could unfold in the Southern Ocean over the next three decades with wide-reaching effects on weather and fisheries, according to researchers.

The landmark study, published in Nature on Thursday, examined waters at the deepest layers of the ocean that play a crucial role in circulating heat and nutrients around the globe.

Deputy director of the Australian Research Council’s Centre for Excellence in Antarctic Science, Matthew England, who coordinated the study, said the results were both significant and “concerning”, likening their projecting to the premise of The Day After Tomorrow.

The fictional film, which was based on the real-life slowdown of the Atlantic Meridional Overturning Circulation current, saw polar melting disrupt the North Atlantic current, setting off a chain of events that influenced weather around the globe.

“In our simulation, [the slowing circulation] in the Antarctic outpaces the North Atlantic by two to one,” Dr England said.

“We know so much about the Atlantic overturning and it’s been such an established part of science, so much so that a film has been made about it.

“And here we have an overturning circulation that’s just as important to humanity, where we still don’t understand why things are changing, what the drivers are, and what the future is.”

The findings of the study all have to do with the production of incredibly dense water, formed around Antarctica, known as Antarctic Bottom Water.

The water is essentially the by-product of sea ice formation around Antarctica, which leaves behind very salty and cold water, which can sink to the deepest layers of the ocean.

It is a key part of the conveyer-belt-like system of underwater currents known as the “overturning circulation”, which cycle heat, carbon, and nutrients around the globe.

But oceanographer Adele Morrison, who was one of the authors of the paper, said simulations had shown the overturning circulation would slow down considerably based on a high-emissions scenario.

“By 2050 we’re looking at a 40 per cent reduction of the abyssal overturning circulation,” Dr Morrison said.

Dr Morrison said the slowdown was driven almost exclusively by the melting of ice sheets and shelves.

“So this puts extra fresh water into the ocean around Antarctica,” she said.

“And it’s this freshwater that reduces the density and lightens the waters around Antarctica.

“Therefore you don’t get as much descending of those dense waters into the abyss, and you get a reduction in the overturning circulation.”

Melting of ice around Antarctica is a direct consequence of climate change.

Dr Morrison said slowing the overturning process could have dire consequences for marine life.

She said this flow-on effect would play out over a much longer time frame.

“We’re studying the dense water formed around Antarctica and its descent into the abyssal ocean,” she said.

“But most of the impacts are then felt when the waters rise back up.

“And so the timescales are a bit slower, more sort of century-type timescales.”

Guardian:

The research looked at what would happen in the deep ocean around Antarctica if the fresher water melting from ice sheets was added to climate modelling.

The modelling for the study assumed that global greenhouse gas emissions remained on their current path, but England said lower emissions could lessen the amount of ice melting which – in turn – could slow the decline.

The slowdown in the deep ocean current relates to the amount of water that sinks to the bottom and then flows north.

Dr Qian Li, formerly of the University of New South Wales and now at the Massachusetts Institute of Technology, was the lead author of the research, coordinated by England.

The study did not attempt to explain or quantify the knock-on effects, but the authors wrote the slowdown would “profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon and nutrients, with impacts felt throughout the global ocean for centuries to come”.

In a briefing, the authors said the deep ocean current influenced the climate around the world, with the potential to radically shift rainfall.

England said the slowdown of the deep ocean current caused those deep waters to heat up.

But as that deep water becomes isolated, it could then cause the upper ocean around the continent to get hotter, kicking off a feedback loop where more melting causes accelerated slowing of the current, which then causes more heating and more ice sheet melt.

The deep water that warms the fastest in the study, England said, was in the same areas – particularly in west Antarctica – where ice sheets were already vulnerable and melting.

“We don’t want to set off a self-reinforcing mechanism in those places,” he said, adding that the slowdown effectively stagnates the deep ocean, starving it of oxygen.

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Below, my video interview with J.P. Steffenson in Greenland discusses potential impacts of change in the global overturning current.