There are a lot of big worries when it comes to global warming, but one of the big ones is that it will change the big ocean currents. These currents are important for the role they play in “stirring” the oceans, and delivering oxygen to waters too deep and dark for photosynthesis. The other big role they play is in redistributing heat around the planet. Ireland, for example, is further north than anywhere I’ve lived in the U.S., but its climate is much more stable. It never gets as cold in the winter, and it never gets as hot in the summer, and that’s all thank to the “ocean conveyor belt” current known as the Atlantic Meridional Overturning Circulation, which brings heat north and east from the Gulf of Mexico, and gives western Europe a relatively warm and stable climate. If the current shuts down, Europe suddenly gets a whole lot colder, and some other part of the world gets a whole lot warmer, and the general result is global chaos:

Such an event would have catastrophic consequences around the world, severely disrupting the rains that billions of people depend on for food in India, South America and West Africa; increasing storms and lowering temperatures in Europe; and pushing up the sea level off eastern North America. It would also further endanger the Amazon rainforest and Antarctic ice sheets.

The complexity of the AMOC system and uncertainty over levels of future global heating make it impossible to forecast the date of any collapse for now. It could be within a decade or two, or several centuries away. But the colossal impact it would have means it must never be allowed to happen, the scientists said.

“The signs of destabilisation being visible already is something that I wouldn’t have expected and that I find scary,” said Niklas Boers, from the Potsdam Institute for Climate Impact Research in Germany, who did the research. “It’s something you just can’t [allow to] happen.”

Unfortunately, what we can or cannot “allow” has little bearing on what we do, or rather what is done by the ruling class. As Boers says later in that article, the odds of this catastrophe increase with every bit of CO₂ we add to the atmosphere (and that’s not to mention all the other greenhouse gases we produce). The current has been slowing down, and while there’s not conclusive proof that that’s due to climate change, that certainly seems to be the most likely culprit. These currents are primarily driven by big changes in temperature and salinity at the poles, and shockingly, if you add a whole bunch of fresh water from melted ice into the mix, that’s gonna change conditions.

New research published in Nature reports on an effort to model what’s going on down there, and how increasing meltwater will affect things, and you’ll be shocked to hear that the results aren’t great for us:

Antarctic circulation could slow by more than 40 per cent over the next three decades, with significant implications for oceans and the climate.

Direct measurements taken from the deep ocean have established that warming is already underway.

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

Such a decline would stagnate the bottom of the oceans and affect climate and marine ecosystems for centuries to come.

The results are detailed in a new study coordinated by Scientia Professor Matthew England, Deputy Director of the ARC Centre for Excellence in Antarctic Science (ACEAS) at UNSW Sydney. The work, published today in Nature, includes lead author Dr Qian Li – formerly from UNSW and now at the Massachusetts Institute of Technology (MIT) – as well as co-authors from the Australian National University (ANU) and CSIRO.

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.

“Our 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,” says Prof England.

Modelling the deep ocean

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.

“If the oceans had lungs, this would be one of them,” Prof England says.

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 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.

Impacts of reduced Antarctic overturning

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,” says Prof England.

Co-author Dr Steve Rintoul of CSIRO and the Australian Antarctic Program 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,” says Dr Rintoul.

I wrote about some of those direct measurements back in 2020, and I think this is a good reminder that all of these models are built using real-world data, and repeatedly tested against new data as it becomes available. This is also a good point at which to remind everyone that we will start feeling the effects of a continued slowdown long before, and long after the current can be described as “stopped”. As with so many other aspects of climate change, this isn’t likely to be enough to and civilization by itself, but it will certainly make life harder. Changes to temperature and precipitation will continue to interfere with conventional food production, and it’s hard to say when we’ll be able to see how this is affecting fisheries. The truly catastrophic stuff, like the lower layers of the ocean turning anoxic, are unlikely to happen in our lifetimes, simply due to the sheer size of the oceans – it takes time for things to affect such a large mass.

That said, we have been “affecting” said mass for well over a century, and the scale of that effect has been growing that whole time. It may be that the “less catastrophic” stuff will be more than enough to end our species, so while I feel like we’re on the wrong path, I’ll agree again with Dr Boers above – we cannot allow this to continue.

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