TRANSCRIPT
The Atlantic meridional overturning circulation or AMOC is a system of currents, known as the 'conveyor belt of the ocean.
Warm, surface water flows north - bringing heat to Europe in a current known as the Gulf Stream.
As it nears Greenland, the water cools and becomes saltier, allowing it to sink deeper as it's carried back south.
But as the planet gets warmer, AMOC is getting weaker.
Too much fresh water from ice melt in Greenland can dilute the saltiness of the water flow, preventing it from sinking.
There has long been speculation that overtime the system could shutdown.
Professor Matthew England is a researcher on large-scale ocean circulation and its influence on regional and global climate at the University of New South Wales.
"So the AMOC is this Atlantic overturning circulation that takes a lot of heat from the tropics in the Atlantic, moves that heat up to Northern Europe, and it releases that heat to the atmosphere, keeping that part of the world much more mild in its climate, It's an overturning circulation that's been there and steady for thousands and thousands of years. And so if it were to slow down or collapse, it would be a major disruption to our climate system."
Now, we have fresh insight into what such a collapse could look like.
Using computer modelling, scientists in the Netherlands simulated the flow of fresh water until AMOC reached a so-called tipping point .
Rene Van Westen - Postdoctoral researcher at Utrecht University, co-conducted the study.
"So when you're adding more and more fresh water into the Atlantic Ocean, the circulation strength slowly decreases until a critical point and then you see an acceleration in the strength decline, and that is what we call the tipping point, so it seems like you're falling off a cliff."
While the researchers cannot provide a timeline of when this threshold will be reached, they were able to estimate how quickly the impacts would unfold.
"With our study, we cannot say when this will happen. But maybe if we cross this tipping point in the future, these changes will happen within 100 years time... and that is maybe on a human timescale relatively short, but on a human timescale, it's really abrupt."
So what are the changes?
The study predicts temperatures in Europe could drop by 3 degrees each decade, and sea levels there could rise by a metre.
The southern hemisphere, including Australia, would become warmer and more prone to flooding.
And the wet and dry seasons in the Amazon would flip severely disrupting the rainforest's ecosystem.
Still, there remains uncertainty about when or even if this ‘tipping point’ will happen.
Josh Willis, a climate scientist at NASA says in the simulation, a significant amount of freshwater was added - much more than what is currently entering the ocean due to ice melt.
"We need to be prepared for this kind of thing to happen, but it doesn't look like it's imminent in the next decade or two decades, where we would see a complete shutdown of the overturning and re-arrangement of the entire climate in the northern hemisphere. That seems unlikely right now, but we are tracking this very carefully and it's something we have to keep our eyes on."
The issue is a key focus of the Ocean Sciences meeting in US city of New Orleans this week.
Professor Matthew England, who is at the global conference says overturning circulation in the Southern Hemisphere is also a concern.
"There's a lot of talks on this overturning circulation. I think about two days worth of talks all up. There's a lot of work going on to studying its behaviour, lots of observations are being made, lots of model simulations. So there's a lot of interest in this overturning circulation along with other topics like the El Nino and La Nina system. Also the circulation around the Antarctic margin because we also have an overturning circulation off the Antarctic margin that we estimate has been slowing down and will continue to slow down. So we're seeing big, big changes to the ocean circulation globally."
Reaching a tipping point this century is slim, but the potential consequences, scientists say, should command action.
