Chapter 1: What is the main topic discussed in this episode?
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Scientists are warning that a crucial system of ocean currents in the Atlantic may be on course to collapsing. Warnings that a key Atlantic ocean current may be approaching collapse. The Gulf Stream is a part of it.
There's been a lot of news about ocean currents recently, and one in particular. The Atlantic Meridional Overturning Circulation, or AMOC, is at risk of collapse with dire global climate consequences. So where do currents like this come from, and how do they work? And what happens when these big water movements stop? I'm Jonathan Webb and this is Lab Notes from ABC Radio National.
Helping me to get to the bottom of ocean currents today is Professor Laurie Monville from the University of New South Wales. Hello, Laurie.
Hello, Jonathan.
Before we focus on the Atlantic, can you give us a bit of a picture of these large ocean movements globally? What creates them and what makes them get faster or slower?
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Chapter 2: What is the Atlantic Meridional Overturning Circulation (AMOC) and why is it important?
And what would make those currents change today?
what we have in the North Atlantic, but also in the Southern Ocean, are formation of deep waters. Okay, so if we go back into the Atlantic, we have the Gulf Stream that bring warm and salty waters from the Caribbean Sea into the North Atlantic. When those warm water travel northward, they lose their heat to the atmosphere. So that's why Western Europe has a relatively mild climate.
So when you have those very dense water, they're probably denser than the underlying water, therefore they sink. And they sink to a depth of about 1,500, 2,000 meters. And then they flow southward in the Atlantic Basin and into the Southern Ocean. And then there they can mix with other water masses and spread into the Indian Ocean and the Pacific Ocean at depths again.
Water all around the ocean, if you wish, are connected, are interconnected. And what happens in the North Atlantic doesn't stay in the North Atlantic. It has impact all around.
And why are scientists paying such close attention to the AMOC at the moment?
We know that over the last 20 years, the AMOC has weakened. Now, the issue we have with the AMOC is that we don't have many direct observations. So the direct observation of this system of current, if you wish, started in 2004. So we only have about 22 years. So I think there's no question as to whether the AMOC has declined over the last 20 years.
But the issue is, is this decline part of the natural climate viability or is it part of a longer term decline? And so the scientific community is quite split on it at the moment. Some think that the AMOC has been declining since the 1960s. And some think what we are observing is just part of the natural climate viability.
What would be making it slow down?
So global warming is the answer.
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Chapter 3: How do ocean currents like the AMOC work?
How does that slow down a huge ocean current?
So waters now become warmer. So if they are warmer, they are less dense. Then there's also the issue of sea ice melting. So if we warm the atmosphere, we're decreasing the extent and volume of Arctic sea ice. And so we've seen this incredible, or horrific, I should say, decrease in summer sea ice in the Arctic extremely rapidly, more rapidly, in fact, than what the climate model can simulate.
So this adds fresh water to the system as well, meaning that the water in the sub-polar North Atlantic becomes warmer and less salty. that means that they are less dense. That means that they can form a bit less of those North Atlantic deep water that are the source of the air mock.
Right. So that difference in density, which was driving, and energy, I guess, which was driving the movement of the current, is reducing because you've got extra water coming in and extra warmth, all making things less dense. Right.
So this is what we call the salt advection feedback, which is quite a mouthful. But basically what it says is that as you start slowing down, forming less deep water, you're bringing less of the salty water to the North Atlantic, which makes the water less dense and you form less. So it's like reinforcing, if you wish.
As a feedback loop.
As a feedback loop, yes.
So there are conflicting findings and perspectives by the sounds on, like, will it collapse completely? How likely do you think it is?
So climate models all agree on the fact that the AMOC will weaken over the coming century. And unfortunately, it seems like we have a commitment to about 2060, meaning whatever the trajectory we choose, we're kind of committed to the AMOC change to about 2060. So this is due to the inertia in the system.
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