Scientists have warned that the catastrophic collapse of a critical Atlantic Ocean current system is on the cards if human-caused climate change continues.
While the collapse would have the most dramatic impact in Europe, potentially causing massive and unprecedented temperature drops, the Southern Hemisphere and New Zealand would also face huge challenges as a result.
A 2021 study found that the system was weaker than at any other time in the past 1600 years and a new study has confirmed that the Earth is on a bad path.
The Atlantic Meridional Overturning Circulation (AMOC)
The AMOC works like a giant global conveyor belt, taking warm water from the tropics toward the far North Atlantic, where the water cools, becomes saltier and sinks deep into the ocean, before spreading southward.
The strength of the AMOC is immense, moving about 18,000,000 m3/s of water. For comparison, the Amazon River moves 200,000 m3/s.
Scientists believe it shut down over 12,000 years ago following rapid glacier melt.
A paper published in February, 'Physics-based early warning signal shows that AMOC is on a tipping course', warns it could soon tip into a shutdown again as freshwater floods the ocean due to ice melting from climate change.
The paper warns that were the AMOC to collapse, there would be "a very strong and rapid cooling of the European climate with temperature trends of more than 3C per decade".
That's compared to current trends of about 0.2C per decade.
"No realistic adaptation measures can deal with such rapid temperature changes," the paper said.
The study adds to the growing body of evidence that the AMOC may be approaching a tipping point and that it could even be close.
The study
Professor Christina Hulbe of Otago University's School of Surveying is a glaciologist who specialises in ice shelves and has spent time in Antarctica.
She told Newshub "these models give us results that we should have high confidence in".
Prof Hulbe explained the paper's methodology takes a system in a steady state and then nudges it to see where the tipping point is.
In this case, they used a complex model and very gradually increased the amount of freshwater in the North Atlantic to see when the AMOC would change state and stop working as it does now.
"When you hit that tipping point, all of a sudden that response gets giant," she said.
"It's really fast. There's something like 3C per decade of cooling in Europe - that's mind-blowing."
Dr Erik Behrens, an ocean modeller at NIWA, compared the tipping point to steadily adding drops of water to a bucket of water until "just a tiny bit makes it overflow".
Impact on New Zealand
While the AMOC primarily transfers water across the Northern Hemisphere, its shutdown would also have massive implications for the Southern Hemisphere.
Prof Hulbe explained: "We've got water that's getting cold enough and salty enough that it sinks in the north. It then starts pulling back south. As it comes down, it gets entrained in the current that goes all around Antarctica."
This is called the Antarctic Circumpolar Current.
She explained, "there's a net heat flow from the south to the north, all the way through that upper part of that circulation, and if we shut that down, the heat stays here with us".
This would accelerate all the impacts of climate change already being felt in the Southern Hemisphere.
Prof Hulbe said the impacts on New Zealand would be far-reaching, and impacts could be felt even if it doesn't shut down and just slows.
There would be more marine heatwaves, changes in ocean productivity that would hurt marine fisheries, and further damage to coral systems.
New Zealand's agricultural sector would also be impacted by changes in rain distribution and how much snow would hit the country.
The study suggests the AMOC's collapse could also cause sea levels to surge by around 1 metre.
Dr Behrens said the AMOC shutting down would "not be immediately drastic for New Zealand," but he confirmed that "we are not completely out of the woods", adding it was "hard to tell at the moment".
Regardless of the immediate climate impact on New Zealand, Prof Hulbe said the massive climate impacts felt by Europe would undoubtedly have flow-on effects for New Zealand.
"We're a big, connected, world so a big climate event in Europe would also affect New Zealand.
"There's just no adapting to 3C per decade that looks anything like we're used to," she said.
Dr Behrens also warned that the collapse of ice shelfs on the Southern Pole could lead to the shutdown of ocean circulation systems which would mean "oceans which just look more like a lake because you don't have any sinking water replenishing the ocean".
The outlook
The study does not give specific timeframes for when the potential collapse could happen, saying that more research is needed.
"But we can at least say that we are heading in the direction of the tipping point under climate change," said René van Westen, a co-author of the paper.
Dr Behrens said he understands how the public can get bored with hearing that each year is the new hottest on record, but approaching a tipping point for the AMOC is alarming because of the rate at which it would accelerate climate change.
Dr Behrens said there are "some mechanisms out there which are, simply speaking, turning the lights on or off," but made clear it will not be as catastrophic as movies like The Day After Tomorrow.
Dr Behrens explained that these tipping points are known because of observations of what has happened during warming and cooling periods in the Earth's past, but he made clear that the rate of change we are seeing now is definitely caused by humans.
"The rate of change experienced in past years is just completely off the charts. It has not been seen and cannot be explained with natural variability".
While the planet's future can often feel hopeless, Prof Hulbe made clear that humanity is "not on a one-way course".
"The moment we achieve net zero, many of these things stop," she said.
"We have it within our power to avoid this."
She argued that, as a small and relatively well-off country, New Zealand emits more than its fair share of greenhouse gasses.
"Our neighbours in the Pacific need us to do the right thing," she said.
