Nations are scrambling to declare ocean current collapse a security risk amid warnings the fallout could dwarf anything the world has seen.
El Nino has grabbed headlines for decades.
Now its cousins, AMOC and AAIW, are demanding attention.
And not in a good way.
Both are vast oceanic networks of currents.
Like El Nino, both are being supercharged by rapidly warming weather.
Like El Nino, that means well-established weather patterns are lurching in disturbing directions.
The Atlantic Meridional Overturning Circulation (AMOC) carries heat, humidity and nutrients out of the tropics towards the Arctic Circle. Along the way it creates the weather patterns that give Europe its summer.
The Antarctic Intermediate Water (AAIW) is a vast expanse of fresh water fed into the Southern Ocean by Antarctica’s cyclically melting sea ice. This serves to shuffle heat and nutrients between the saltier South Atlantic, Indian and South Pacific oceans.
A slew of studies show clear signs that both are weakening dramatically.
And that has severe implications.
European crop production could collapse as winters get longer and colder. Choked nutrient flows worldwide could devastate already depleted fish stocks.
And the atmospheric jet streams and trade winds that govern global weather could lash out as they struggle to find a new balance.
Some nations are now beginning to act.
Iceland has just declared the shifting currents a national security risk.
The European Union’s decarbonisation chief, Teresa Ribera, recommends Brussels follow suit.
The Netherlands and Finland have initiated their own assessments.
Britain has committed $150 million towards an urgent early-warning system.
And a new joint Australian-Indian study suggests the Southern Hemisphere is also in for a shock.
“It is irresponsible to wait for absolute certainty before preparing,” warns Iceland’s climate minister, Jóhann Páll Jóhannsson.
“Because, by the time the consequences become undeniable, it may already be too late to prevent them from happening.”
Deep impact
“This is not about panic or alarmism,” Mr Jóhannsson adds.
“It’s just responsible governance.”
Icelandic researchers have been warning their country will be “ground zero” once key climate tipping points start tumbling.
Put simply, as the rest of the world burns, an AMOC collapse would subject the volcanic island to a new ice age.
Scientists are racing to understand how great this risk is.
The AMOC acts as a heat pump, carrying warm salty surface seawater up from the tropics and the Gulf of Mexico. As it pushes through the cold North Atlantic, it releases this heat into the atmosphere. This drives the region’s notorious storms. But the humid air also delivers both warmth and rainfall to Europe.
That’s why winter in Canada’s city of Labrador gets frostbitten at -27C, while Britain’s Cambridge – at the same latitude – enjoys a relatively balmy 2C.
As the current cools, the current’s water gets heavier and sinks to the ocean floor. There, it is drawn south again – stirring up fresh nutrients along the way.
Greenland may offer an early warning indicator of an emerging imbalance.
A circular current, or gyre, south of the island helps spin up the broader AMOC network. But rapidly melting glaciers are injecting abnormal amounts of cold, fresh water into the mix.
Fresh water is lighter than salty water. And that’s unbalancing the gyre. Some studies suggest it could shut down as soon as the 2040s.
If it does, it won’t have the same ice age-inducing impact as a total AMOC collapse. But it would lead to colder winters and more frequent droughts.
And further destabilise the AMOC.
Probability estimates for the AMOC shutting down this century vary from 1 per cent to 50 per cent. But most put the odds at about 20 per cent.
But even a weakening would produce dramatic fallout.
Nordic countries will get sharply colder for longer. And projections show this cooling and drying trend extending over much of Western Europe, Greenland and Canada’s Newfoundland.
Surrounding regions, however, will keep getting hotter.
“If one area cools and the rest of the world is still getting warm – the boundaries across that, the thermal gradients will get steeper and steeper and steeper,” says British Antarctic Survey oceanographer Professor Mike Meredith.
“This might bring extreme weather conditions. Those weather events could make anything we’ve seen so far just look like small beer.”
Overlooked, underappreciated
The Antarctic Intermediate Water (AAIW) is not as well studied as AMOC. But researchers are beginning to fear they have underestimated the extent of its influence.
Like AMOC, this mass of moving water is central in determining how much, and where, rain falls across southern Australia, Africa, and South America. It also helps shape India’s vital monsoon season.
Now, scientists are seeing falls in salinity and water density that power currents spinning off into the Indian, South Pacific and South Atlantic Oceans.
Here, unlike Greenland, seasonally melting ice is what sustains the pattern.
But Antarctica’s sea ice has all but evaporated. And that’s cut freshwater flows by about a third.
Without this injection of cold, light freshwater, the Southern Ocean’s AAIW is getting saltier and heavier. That means it’s sinking deeper beneath the surface.
This last happened 130,000 years ago. Back then, Antarctica’s climate was closer to Norway’s and Iceland’s than the ice-covered mass it is now.
“That transition took thousands of years, not two decades,” says Bishakhdatta Gayen, a professor at the University of Melbourne and the Indian Institute of Science.
“The Antarctic region is the freshwater engine of oceans around the globe. Melting Antarctic sea ice influences circulation thousands of kilometres away.”
That influence is now in the balance.
“From 2016, Antarctic sea ice shrank to record lows,” Professor Gayen explains.
“This abruptly choked off that critical seasonal surge.”
This circulating current has in the past reduced the effect of atmospheric carbon by capturing it at the surface before injecting it into ocean depths. Any slowdown or halt in this carbon conveyor will therefore accelerate atmospheric warming in the Southern Hemisphere.
It would also push more heat deeper into the ocean. And what effects this would have are as yet unknown.
“It could ultimately influence atmospheric circulation, weather and storm track in the Southern Hemisphere,” warns study co-author Professor Naveira Garabato.
“However, the impact can cascade globally.”
Unknown territory
“We see many climate projections now forecasting that the deep convection in the North Atlantic’s Subpolar Gyre will weaken sharply, or even cease entirely, by around mid‑century,” says Dr Bablu Sinha of Britain’s National Oceanography Centre.
“Our job is to put that kind of result under intense scrutiny – stress‑testing the models with new physics and observations to see whether such a collapse is a real risk, and if so, how and when it might unfold.”
Iceland’s climate minister, Mr Jóhannsson, doesn’t like the odds.
“When it comes to climate tipping points, we won’t be certain until it’s too late to act,” he said.
Just one collapse – be it the AMOC or AAIW – would have dramatic global impact.
If both go down, the outcomes are unimaginable.
But it’s not likely to be an on-off switch.
AMOC is currently estimated to have lost about 15 per cent of its strength in the past decade. That’s the weakest it’s been in over 1000 years. One estimate puts that slowdown at 43 per cent by the end of the century.
But every percentage point brings with it a change in weather.
The worst-case scenario sees Arctic ice gradually extending down Norway to Norfolk in the United Kingdom and covering Finland. Cold snaps of -20C would afflict London, Amsterdam, Brussels and Paris once every decade.
A lesser 20 per cent weakening would make agriculture in countries like Britain, Germany, Canada, Ukraine and Scandinavia increasingly precarious. Some estimates put global cereal price rises at up to 40 per cent.
But only if the global economic order remains calm and stable.
Without it, famine would become likely.
The Alliance to Feed the Earth in Disasters (ALLFED) research group has simulated global crop conditions under a global weather regime without AMOC influence.
It notes that changes vary by region. But Northern Hemisphere production of wheat, maize, and rice is among the hardest hit, with falls of 12 to 19 per cent. Globally, the decline is projected to be about 5.3 per cent.
Meanwhile, the United Nations has abandoned its goal of limiting global warming to 1.5C.
It now says we are on track to generating a 2.8 per cent increase by the end of this century.
Reversing this trend is only possible through massive investment in new technology to suck carbon dioxide and methane out of the atmosphere.
“One of the key roles of government is to identify threats and prepare society for them,” concludes Iceland’s climate minister.
“That is how we approach volcanic eruptions and pandemics, and the same should apply to climate risk.”