Deep in our oceans lurk a series of mysterious, invisible global ocean currents,
that possess the power to alter life on Earth as we know it.
They are intrinsically linked,
working together like giant conveyor belts
to pump oxygen, life, heat, salt, and nutrients
seamlessly and efficiently around our blue planet.
They are the unsung heroes in regulating global climate,
the systems that so often take a backseat in the climate conversation.
That is, until something goes wrong.
Perhaps the most influential of these ocean current systems is the AMOC -
The Atlantic Meridional Overturning Circulation -
a keystone of Earth's climate system
and a system scientists have warned isn’t just slowing down,
but could actually collapse as early as 2025.
While this has happened before, it hasn’t happened for nearly 10,000 years.
And there is one big difference this time: We’re here.
How can something so life altering have sneaked under the radar without us knowing?
If the worst were to happen, what would happen to life on earth?
I’m James Stewart and you’re watching Astrum Earth.
Join me in this video as we find out what’s going on below the surface.
Ok, so that’s pretty intense isn’t it? This is the year we’re in - 2025
but considering the AMOC collapse is what inspired the 2004 film ‘The Day After Tomorrow’
it doesn’t feel like that is just around the corner does it?
There doesn’t seem to be much coverage or even urgency on this subject.
Indeed a lot of people won’t have ever heard of the AMOC.
Perhaps that’s one of the big problems with a force you can’t see or measure
because the truth is, there have been indications
that the AMOC has been slowing down for the last 60 or 70 years as our planet has warmed,
and some scientists have gone a step further,
suggesting the AMOC has declined by at least 15% since 1950
and is in its weakest state in more than a millennium.
The big clue? A big cold blob that’s appeared over the Northern Atlantic.
Bizarrely, this region is the only place
in the world that has cooled in the past 20 years or so,
while everywhere else on the planet has warmed.
Any guesses on why this might be?
Yep, it is the AMOC slowing down.
One of the AMOC's main jobs is to shift heat around;
it’s one of our planet’s largest heat transport systems,
it is moving the equivalent of 50 times the amount of energy humanity uses,
the same amount of energy that flows through one million power stations all at once.
So when it slows down, this region gets colder.
Since 2021, the general thinking in the Intergovernmental Panel on Climate Change (IPCC)
was that the probability of crossing the tipping for a collapse point this century was less than
10% with ‘medium’ confidence. But as recently as October 2024,
44 climate scientists from 15 countries sent an open letter to the Nordic Council of Ministers
suggesting the risk of the AMOC collapsing has so far been greatly underestimated
and is higher than previously thought.
A new study, published in Nature Communications used sea surface temperature data stretching back
to 1870 as a way of assessing the change in strength of AMOC currents over time,
ultimately estimating it could collapse between 2025 and 2095…
so which paper is correct? I decided it was about time we found the answers.
So let’s dive into the AMOC;
what it is, how it works, previous ice ages, how it’s evolved,
what happens if it collapses,
and perhaps most importantly - when or if it will collapse.
I suppose a good place to start when you’re trying to work out why something isn’t working,
is to understand what it is in the first place.
What does this mysterious invisible beast that could bring down life as we know it actually do?
It's easy to forget that we are all connected by one enormous interconnected body of water
and the AMOC helps make sure our oceans remain as one, like a sort of giant mixing bowl.
It moves water, heat, nutrients, dissolved gases and microscopic
life around Earth through ocean currents. The AMOC is a brilliantly complicated system
of those ocean currents and eddies, which are circular currents of water.
It forms a glorious, giant vertical loop that spans the entire length of the Atlantic Ocean,
and it would look like a huge piece of spaghetti draped across the map.
And it’s those currents which are pretty interesting here,
because there are a few different types of them. Tidal currents occur close to shore and
are influenced by the sun and moon. Surface currents, which are influenced by the wind,
and much slower currents that occur from the surface to the seafloor. They're driven by changes
in the saltiness and ocean temperature. Those last ones are the ones we’re most interested in here.
This process is called Thermohaline circulation (thermo meaning temperature,
and haline meaning saltiness). The AMOC is part of this Thermohaline
circulation conveyor belt, circulating water from north to south and back again
in a long cycle within the Atlantic Ocean. It drives warm water northwards along the
ocean’s surface and cold, deep waters back southwards. In doing this, it delivers heat
and nutrients to colder latitudes and transfers carbon to the ocean depths.
Things ‘begin’ off the east Coast of North America where the Gulf Stream, part of the AMOC,
carries warm water from the Florida Straits up to the Grand Banks off Newfoundland. From there,
the North Atlantic current then travels across the ocean toward Europe and the
Norwegian sea. When this warmer water reaches the subpolar regions, near Greenland or Arctica,
not only does it cool down as it loses heat to the atmosphere, but it also becomes more dense.
The cooling forms sea ice, and as the ice forms salt is left behind in the ocean water.
That large amount of salt makes the water much denser, causing it to sink down in
the ocean to a depth of 2-3,000 metres where it’s then carried back southwards.
That’s where the ‘overturning’ in ‘AMOC’ comes from - this sensation
creates a vertical “overturning” circulation throughout the ocean basin as the water is
eventually pulled back up towards the surface and warms up, thus completing the cycle… much
like your central heating system works at home. Every second, it moves an incredible 17 million
cubic meters of water northwards, equivalent to around 7,000 Olympic-sized swimming pools.
This in turn moves 1.2 peta Watts (PW) of heat,
100 times bigger than the total global energy produced on Earth from all power
sources and all of that is critical in regulating the climate around the world.
As impressive as all of that is, and it really is, it all happens rather slowly,
which is quite surprising for such an awesome system. In fact, the AMOC’s entire circulation
cycle is slow – it takes an estimated 1,000 years for a parcel (any given cubic meter) of water to
complete its journey along the belt and it really can’t afford to get any slower - but sadly,
that’s where the problem lies; because it does seem to be slowing down… but why?
Before we press on, let’s do a quick terminology tidy up with the AMOC and the Gulf Stream, as the
two are often confused. As we just touched on, the Gulf Stream is a part of the AMOC system,
not the system itself. It's like an internal organ the AMOC couldn’t function without.
The Gulf Stream originates at the tip of Florida. It's a warm and swift Atlantic Ocean current that
follows the eastern coastline of the US and Canada before crossing the Atlantic Ocean
towards Europe. Its job is to ensure that the climate of Western Europe is much warmer than
it would be otherwise. For example, where I am today is about the same distance from the
equator as the cold regions of Canada, yet here in England we enjoy a much warmer climate. If
it weren’t for the warm water of the Gulf Stream, England would have a much colder climate - perhaps
not full ‘Day After Tomorrow’ vibes but the UK would be at least 3-4 degrees Celsius
cooler and in mainland Europe things would be colder still, by least 10 degrees Celsius.
It’s like a really fast highway - part of the journey taken by the warm water through the
AMOC as it travels from the South Atlantic to the far North. When the highway ends,
the warm water takes different, smaller route, continuing its journey northwards.
As it moves through the Atlantic, it loses heat through evaporation. This leaves behind cooler,
saltier water which eventually hits the fresher waters of the North Atlantic.
The Gulf Stream is caused by a large system of circular currents and powerful winds,
called an oceanic gyre. There are five oceanic gyres on Earth - the North Atlantic Gyre,
South Atlantic Gyre, the North Pacific Gyre, the South Pacific Gyre and Indian Ocean Gyre. Given
its location on Earth, the Gulf Stream is part of the North Atlantic Gyre.
Now we’ve cleared up our currents, let’s focus back on the AMOC and more
specifically the impact climate change is having (and has had) on it. As we touched
on at the start, everything on Earth is connected, and the same is true here.
Remember the big clue we talked about earlier? The big cold blob
that’s the only place on Earth in the last 20 years to have cooled?
Well that’s not just a surface cooling,
that extends down to a depth of 2,000 metres and is the smoking gun to a slowing AMOC.
You see, as well as that area getting colder, its salinity is also declining;
it’s at its lowest levels since measurements began
120 years ago. Because of this, the water is an awful lot less dense.
And that's really significant because it’s becoming much harder for that water to sink
down and join up with the deep, colder current we talked about before. One of
the main reasons Scientists think this could be is there's lots more fresh water melting off
ice into our oceans due to a significant increase in greenhouse gas emissions.
At least two studies analyzing state-of-the-art climate models and observations have shown “that
the recent North Atlantic warming hole is of anthropogenic origin” and is caused by
reduced northward oceanic heat transport related to greenhouse gas emissions.
One of the main reasons we’re able to attribute some blame towards humans is by looking back to
the past. To understand conditions before regular temperature measurements began,
we must turn to proxy data: the traces of past climate change left behind
in slowly accumulating archives such as ice sheets or seafloor sediments.
Proxies like the ratio of oxygen isotopes found in the microscopic skeletons that
make up much of deep seafloor sediment provide a record of past surface water temperatures.
The size of sediment grains on the ocean floor reveal current speeds above it,
and allow us to reconstruct past sea surface temperatures and other parameters.
What they suggest is a long-term weakening of the AMOC since the early or even mid-twentieth
century. Climate models have long predicted its decline in response to global warming,
and the physics behind these predictions is understood. In addition, the paleoclimatic
data also strongly points to human activities as the cause, in that AMOC weakening coincides
with the period of unprecedented modern global warming. In short, it’s very likely that humans
have significantly increased the conditions in which the AMOC is prone to being unstable.
We can observe several changes here. There's a rise in greenhouse gas emissions and arctic
amplification. There’s also an increasingly enhanced water cycle, intensified by more energy
from the sun, creating more extreme weather, like floods and droughts. There’s more evaporation in
the subtropics and more precipitation in the high latitudes. These things are all happening before
we’ve even mentioned melting sea ice. Here comes the big one - the Greenland ice sheet is melting
at a rate of around 270 billion tons per year and this is what makes the water less and less salty.
The freshwater dilutes the salty water and this is salt advection feedback (a positive feedback
mechanism that affects the strength of the AMOC) ... as the major tipping point that ultimately
leads to a slow down, or collapse of the AMOC.
In 1961, US oceanographer Henry Stommel recognized
how the Atlantic waters’ salinity leads to an AMOC tipping point. In other words,
the AMOC flows because the Northern Atlantic is salty, and it’s salty because the AMOC flows.
It's chicken and egg, or in more technical terms, a self-sustaining feedback effect,
which works the other way around as well. If the Northern Atlantic becomes less salty
because of an inflow of freshwater, the water becomes less dense and the
AMOC slows down. It brings less salt to the region, which slows down the AMOC further.
One of the most interesting things in this crazy cycle of ocean systems and
currents is that the AMOC has a bit of a track record of being less than stable.
In other words, it’s collapsed before…
and its collapse has historically been linked to some pretty extreme climate events.
I sometimes forget we’re still in an ice age (albeit an interglacial one),
but the climate during the last glacial period was also far from stable. To give us a clue into
past AMOC behaviour we can examine some dramatic climate changes that have happened in the recent
past. “Recent,” that is, from a paleoclimate perspective - the last 100,000 years or so.
One thing we know from studying the paleoclimate is that some of the most abrupt,
and striking temperature changes, which are known as Dansgaard-Oeschger events (or
D/O events) - they're periods of abrupt warming followed by a period of slow cooling that occurred
during the last ice age - they were likely caused by instability in the AMOC. This graph shows
temperature reconstructions from ocean sediments and Greenland ice over the last 60,000 years.
The green line refers to sediment data and the blue line refers to ice core data. What’s
interesting here is as the blue and green lines drop down rapidly throughout the time period,
they coincide with Heinrich-events, marked red, and D/O events which are numbered.
This demonstrates an unstable AMOC in all likelihood led to these dramatic spikes.
During the Younger Dryas period, which was a period of extreme climate change,
the AMOC slowed abruptly around 12,500 years ago. This caused a period of near-glacial temperatures
in the northern hemisphere. This was caused by the melting of the Laurentide ice sheet,
which resulted in several mass iceberg surges into the North Atlantic Ocean. They're known
as Heinrich events. At certain times, these ice sheets released large amounts
of freshwater into the North Atlantic. We can see recorded evidence of these
events in North Atlantic marine sediments as layers with a large amount of coarse-grained
sediments derived from land. These layers, which are continuous across large areas of
the North Atlantic, are evidence for both an increase in icebergs discharged from the
Laurentide ice sheet in North America and a southward extension of cold, polar waters.
Scientists have hypothesized that these freshwater dumps reduced ocean salinity enough to slow
deepwater formation and therefore the AMOC. Since the thermohaline circulation plays an important
role in transporting heat northward, a slowdown would cause the North Atlantic to cool. Later,
as the addition of freshwater decreased, ocean salinity and deepwater formation
increased and climate conditions recovered. Okay so hopefully by now you’ve got the
general gist of this pretty large, pretty real problem, so we turn to the big question of WHEN
is it going to collapse? Note the use of when here - the thing that struck me the most when
recording this video is that there are so many different papers written on the AMOC and they
all have different answers to that question of when it might collapse, and the one thing they
ALL seem to agree on is that if we continue on our current trajectory, it’s no longer a question of
IF but indeed when this collapse occurs. As I stated at the start of the video,
until a few years ago the general thinking in the Intergovernmental Panel on Climate Change (IPCC)
was that the probability of crossing the tipping point this century was less than
10% with medium confidence. The latest, sixth IPCC report found that, even for
a low emissions scenario, the AMOC will weaken between 4% and 46% by the year 2100, depending
on the model. In the high emissions scenario, the reduction ranges between 17% and 55%. But
there are now conflicting reports suggesting that a collapse could happen as early as 2025…so what’s
going on? Who’s right, and who’s wrong? Great question and I wish it was as
straightforward as that! Lots of it comes down to how the AMOC is modelled, and the factors and
physics taken into account to do that. A big hole in all of these models is that beyond paleoclimate
data, we haven’t really been measuring this stuff for long enough to give us accurate data. The IPCC
report used a model called CMIP6, consisting of the “runs” from around 100 distinct climate models
being produced across 49 different modelling groups. Critics have noted that the AMOC model
they used for this system was a ‘too stable’ AMOC - when they input the AMOC to the model,
they had to tune it (as you would a car) and the suggestion is that it was a little too well tuned,
so didn’t represent a realistic scenario estimates for an AMOC collapse, hence churning out a rather
conservative estimate. For many critics, it goes beyond being just conservative however,
and becomes a significant underestimate. On the opposite side of the spectrum we’ve
got papers saying it’s going to collapse between 2025-2095 with 95% confidence. How
did they get those numbers? These scientists used a different type of modelling called reanalysis,
looking to move past the limitations of our short observational record of the overturning, relying
instead on sea surface temperature (SST) of the North Atlantic subpolar gyre, although
SST as a measure of AMOC collapse is also widely debated in the scientific community.
The actual observations of AMOC since 2004 have long since discredited the evidence that
the authors of this paper are using; the 5 data points they showed were collected
several years apart by ship surveys, and it is well known and well established
that they give a highly misleading impression of AMOC decline. To be fair,
the authors acknowledge in the discussion that there is large uncertainty in their conclusions.
To bring us back to the big question of WHEN will the AMOC collapse, the paper using SST as a key
measurer concludes that there’s a critical mean of the year 2057; a date supported in another
paper which offers an estimated collapse between between 2037-2064 (10-90% CI) with a mean of 2050.
I’ll let you make your own mind up on a specific date, but one thing seems for
sure - if we keep going the way we are with global warming, something pretty intense
is going to happen in many of our lifetimes. A full AMOC collapse would be a planetary-scale
disaster, there’s no getting away from that but if this thing collapses, what happens to us?
It’s not looking good for Europe, which is where we’ll start. European cities would experience a
3 to 10°C drop in temperatures in a few decades, and in some areas, the effects will likely be even
worse - February months in Bergen, Norway, would become 35°C colder every century for example. It’s
not like you have a new ice age in two weeks, but the Northern Atlantic region and Europe,
in particular, will cool substantially; England, Scotland and Ireland would probably look like
Northern Canada. There will be a BIG increase in winter storms across Europe, and significantly
less precipitation in Western Europe, which would have a huge knock-on effect on agriculture in
places like France, or Great Britain: it would be like trying to grow potatoes in northern Norway.
However, Europe will not be the only region to be affected. The sea level
in the Atlantic Ocean could rise as much as 70 cm, submerging many coastal cities around the
world - there would be densely populated places on Earth where people simply could not live.
Scientists also predict a shift of the tropical rainfall belt to the south,
which is bad because the rains will move away from the rainforests to regions that are not used
to so much rainfall. So this will mean droughts in some regions and floods in others. Rainfall in the
Amazon rainforest would also undergo a drastic change, there’d also be less rain in the Sahel
and a weakening of the summer monsoon in Asia. As the Northern Hemisphere cools, the Southern
Hemisphere will become increasingly warmer. The heat from the Pacific Ocean will not be
transported to the North Atlantic and instead end up staying in the tropics,
causing dramatic temperature increases there. It will also reduce the CO2 uptake of the ocean
because the AMOC sinking in the northern Atlantic takes a lot of CO2 down into
the deep oceans where it is safely locked away from the atmosphere and the last thing we need
right now is more carbon in our atmosphere. There are some people who might suggest actually,
parts of the world cooling might NOT be such a bad thing ... .Considering
all we hear about is the Earth getting warmer, maybe an AMOC collapse might counteract that?
The truth is, there is nowhere we can think of that will be better off, in fact,
it’s the opposite. If it were just a case of averages, then somewhere like Germany
might see a balance just because of where it’s positioned. But weather is not a climate average;
it is seasonal and highly variable. Within the average you can get warm air from the south or
cold polar air outbreaks from the north. These contrasts will be more pronounced if
Scandinavia and Britain cool while Spain and Italy warm for example. This will drive much
greater variability in the weather, which is bad for agriculture, and it will cause more storms;
there’d be major extreme weather events that we have not seen in the past. You get the idea.
Some scientists have suggested an AMOC collapse may actually temporarily delay some of the
issues that the Amazon is currently facing if coupled with the effects of global warming. In
the initial phases of an AMOC slowdown, greater rainfall and lower temperatures in the Amazon
could partially offset any temperature-driven stressors that the rainforest vegetation
experiences, delaying the potential for the Amazon to transition into a savanna-like state.
Still, this would only be for a temporary amount of time and also depends on the relative timing
between an AMOC collapse and global warming. If global warming worsens past a particular
threshold before an AMOC collapse, the rainforest could irreversibly enter a savanna-like state. Due
to these fluctuations in the AMOC, the Amazon could shift its seasons – with the dry season
becoming wet and vice versa – a change that would severely impact the overall ecosystem.
The AMOC is clearly something we need to watch incredibly closely,
but how do we actually do that? How do we keep track of changes in the
AMOC to monitor how quickly or how far the slowing down is progressing?
Scientists measure the AMOC using scientific instruments deployed in different latitudes
across the North and South Atlantic Ocean. The National Oceanography Centre (NOC) is
the UK lead of two international programmes in the North Atlantic:
Rapid Climate Change (RAPID) and UK Overturning in the Subpolar North Atlantic (OSNAP).
Both programmes deploy sensors attached to wires that are hundreds to thousands of metres deep,
known in oceanography as moorings. These instruments have special sensors
attached to them which measure things like ocean current speed,
temperature, salinity and direction of the water crossing the arrays.
One of the major things to keep an eye on in relation to ‘when’ this collapse
MIGHT BE imminent, the alarm bell if you will, is the flow of water in the Atlantic,
which has been measured with the Rapid project since 2004. The other measurements
help us monitor water mixing during winter in the northern Atlantic and Nordic seas.
If the deep mixing starts to decline a lot,
that could be an early indicator that we are approaching a tipping point.
Whilst there are some signs of this, as we’ve shown,
we don’t have enough data yet to be sure when that might be.
Ok, final question - we know the AMOC collapsed
before and it’s come back. Could it just do that again?
Well, sort of… maybe. Let’s say it did collapse, even if that freshwater input into the oceans (ice
melting) decreases to current levels, the AMOC may remain in a collapsed state. The ability of the
system to not return to the initial state once the forcing is reversed is referred to as hysteresis.
The UK’s met office models show that there may be a temporary resilience period in the AMOC
after a threshold has been crossed, during which the AMOC could still recover if the freshwater
input is reversed rapidly. In one experiment (using modelling) the MET Office conducted,
the AMOC recovers if freshwater inflow ceases after 20 years but NOT if it
ceases after 50 years. This gives a 20-50 year temporary resilience,
but the temporary resilience period would be lower if freshwater was added at a higher rate.
The last time, it took about 1,000 years to recover, though the past is not a direct
analogue because there is also massive CO2 forcing this time – CO2 is already
higher than any time in 15m years and we are still in an ice age after all.
Based on what I’ve read, and what we’ve discussed, it’s hard to get away from the fact that if we
keep going as we are there is likely to be a significant slowing down, or even collapse,
of the AMOC in many of our lifetimes - 2025 we can almost certainly rule out based on the data points
and models used being unreliable. In contrast, the IPCC’s estimate around the end of the century also
seems too optimistic. The decade that does seem to come up more reliably however is the 2050’s.
The answer to stopping this is as obvious as you might have expected, and an answer
you probably already knew. We must significantly slow down the rate at which our planet is warming.
It’s so simple, and so obvious and yet sometimes it can feel like such a huge undertaking.
Often when we think of climate change we think of the things we experience or can
see -, the droughts, the extreme weather, the temperatures, the floods, and melting sea ice,
but rarely do we think about what we can not see, the underlying causes. Forces just like
the AMOC that go under the radar, yet have the ability to severely alter life on earth.
Planet Earth has demonstrated a resilience to extreme events throughout its history, and it
finds a way to bounce back and respond, and it would do so again if or when the AMOC collapses.
Whilst a collapsing AMOC is not ‘new’ news geologically, we’ve never had such an advanced,
populous civilisation living on it, when it’s happened before.
So, although planet Earth itself will survive this event, whether humanity would,
or at the very least billions’ of people’s current way of life would, is a more serious question.
The AMOC demonstrates just how intricate and delicate the systems that govern Earth are,
and it’s as mesmerising and awe-inspiring as it is deadly and dangerous.
Much like so many other things on planet earth,
the AMOC joins the long list of things we urgently need to take care of,
and in my view has to be near, or even at the top of that list.
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We want to show you a side to it that maybe you haven't seen before.
To do this, this channel will leave no stone unturned in terms of the data,
the research and the analysis you’d expect
but it'll combine that evidence based investigation with excellent storytelling
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Thanks for watching, and we’ll see you in the next one.