- Oceans absorb around 90%
of the heat trapped by rising greenhouse gas emissions.
They warm much slower than the atmosphere
because of the high heat capacity of water.
So on July 3rd, 2023, when we set the record
for the highest global atmospheric temperature
and then broke that record on July 4th, 5th and 6th
I started to wonder what the ocean was doing.
Turns out it was setting heat wave records at the same time.
- The ocean by now has warmed everywhere compared
to the late 19th century
except for one very telling exception.
This cold blob.
- The cold blob is one of the only places
on the planet that has actually gotten colder
since the Industrial Revolution
and not just a little colder.
While our planet's oceans have warmed about
one degree Celsius, this cold patch in the North Atlantic
near Greenland has cooled by nearly the same amount.
And if you find this mysterious cold blob a little creepy,
you should because many scientists see it
as a sign that a vital earth system
could be nearing collapse
with enormous consequences.
- I used to say that the probability
of crossing the tipping point this century
is like single digit percentage.
You don't want this to be even a 5% probability
because of the consequences.
Would you board an aircraft
that has a 5% chance of crashing?
Obviously not.
And there have been a couple of recent studies suggesting
that crossing the tipping point may actually happen
before the middle of this century.
So in the next 20 to 30 years or so.
- This is the AMOC, one of the most important earth systems
for regulating and transporting heat.
In this episode, we're going to find out
how likely it is to collapse
and what the North Atlantic warming hole
or cold blob is all about.
The term cold blob was likely coined by Stefan Rahmstorf,
a German oceanographer who first described this phenomenon
in a paper published in 2015.
He and his colleagues observed a patch
of sea in the North Atlantic, roughly the size of Greenland
that in contrast to the rest of the world, was cooling.
At the time, there were several hypotheses
about what might be causing this unusual cooling
ranging from natural variability and sea temperatures
to the melting of Greenland's ice sheet.
But as more research was conducted, it became clear
that the cold blob was not just
a brief fluctuation or anomaly.
A crucial turning point
in understanding the origins
of the cold blob occurred
when climate models actually predicted its existence.
And what had previously seemed like a paradox,
a cooling region and an otherwise warming ocean now appeared
in models as a warning signal for one
of our planet's most important climate tipping elements.
- The climate models show a weakening of what we call
the Atlantic Meridional Overturning Circulation
or AMOC for short.
It's a thing that breaks down
in the Hollywood disaster movie "The Day After Tomorrow."
- You recall what you said about how polar melting
might disrupt the North Atlantic current.
- [Speaker] Yes.
- Well, I think it's happening.
- Hollywood stretched
what was a relatively new scientific theory
well past its limits back in 2004
but since then, a lot more research has been done.
The AMOC is a giant flow
of water that acts like a global conveyor belt.
Unlike surface currents,
the AMOC is a deep vertical overturning of the ocean
and it's driven
by a process called thermohaline circulation
or changes in the saltiness and temperature of the ocean.
The AMOC transports warm tropical water
into the North Atlantic Ocean where it cools
and forms sea ice.
That process leaves less salty sea ice floating
on the surface and sends saltier, denser water down
towards the ocean floor and drives its circulation back
to the south where it will eventually warm
and complete the circulation.
And what we know is that
this globally important current is slowing.
Here's why.
As Greenland's ice sheets melt, cold, fresh water is dumped
onto the surface of the North Atlantic, diluting the water
thus making it less dense.
Increased precipitation in the far north
and decreased sea ice formation
further dilutes the ocean water.
This less dense water no longer sinks as readily
which impedes the flow of warm water coming from the south.
And today the AMOC is flowing
at its slowest rate in over 1,000 years.
That warm water comes from a current
in this cycle called the Gulf Stream.
And when I heard another PBS Terra show
was going diving there,
I asked if I could come along and swim
in part of the largest energy transfer on planet Earth.
- So Jasmin, where are we and why are we here?
- So right now we're just off the coast of Jupiter, Florida.
We're actually sitting in the Gulf Stream right now
which is a place where a lot of sharks gather.
It's oxygen rich, there's upwelling.
So that deeper, colder ocean water is coming up
bringing a lot of plankton.
And that's really the base of the food chain.
It's a really what we call productive area.
- Okay, we're out here in the water.
So right now we are in the Gulf Stream.
We're closer to the equator.
So that sun's radiation is really strong.
It causes evaporation, it causes the the salt content
in the water to get stronger.
That water then is
transported all the way up to Europe.
And while the Gulf stream is a critical part of the AMOC
it's primarily driven
by surface winds and the earth's rotation.
So it won't disappear if the AMOC collapses
but it will be affected in ways you need to know about.
We'll get to that in a minute.
But first, let's get back to the AMOC.
Climate models are pretty certain
that a warmer world means a weaker AMOC
but there's a big difference
between a gradual linear weakening and a collapse.
And increasingly scientists are worried
that it could collapse altogether.
- If you dilute the surface waters in the north,
the AMOC will get slower.
It will transport less salty water from the subtropics
up to the higher northern latitudes,
thereby the density gets even more reduced,
making the AMOC even slower.
And this is a self-reinforcing positive feedback.
If you weaken that AMOC too much, a freshwater cap
will simply develop at the surface.
The sinking motion will stop
and the whole system is going to break down.
- The cold blob could be evidence
that the AMOC has already slowed down
with less heat being transported
from the south and in turn cooling the region.
And the big question is, are we heading for a collapse?
But first, what's the big deal
with this overturning circulation halting?
Well, it turns out
that we can get a pretty good idea of what might happen
by going way back in the climate record.
- We actually have some paleoclimate evidence
of what will happen when the AMOC collapses
because during the last Ice Age,
this has happened several times.
This will massively change
the Northern Hemisphere temperatures.
- The AMOC is currently partly responsible
for relatively temperate climates
in otherwise far north parts of Europe, if it collapsed-
- Would have significant cooling in the northern hemisphere.
More cooling the more we move northward.
So let's say in Germany
it might just be two to three degrees cooling
but then in Northern Norway it may go up to 8
or 10 degrees of cooling.
- The jet stream could also become more unstable
which could cause more extreme weather events
like the polar vortex or heat dome.
- There will be also changes in rainfall patterns.
Most alarming consequence will be the southward chip
of the tropical rain belt
which will affect the monsoon systems
of South America and of Africa.
The current West African monsoon region would basically be
without monsoon.
- Hundreds of millions of people rely
on these monsoons for food production and more.
But the impacts don't stop there.
- We will see massive impacts on the marine ecosystem
which are very difficult to predict
what they exactly will be.
But the whole marine ecosystem,
the fisheries all depend strongly
on the current situation with the AMOC going.
- And another consequence would be seen
along the eastern seaboard of North America.
And this is where the Gulf Stream comes in.
- The sea level on the left hand side
of the Gulf Stream is quite low.
On the right hand side, it's about a meter higher
because of the Coriolis force acting on the Gulf Stream
and pushing the water
to the right away from the American coast.
If you slow down that AMOC and thereby also the Gulf Stream,
the water on the left hand side of the Gulf Stream
will go up leading to a massive extra sea level rise
by up to a meter along North Atlantic shores
when the AMOC collapses.
- A meter of sea level rise would inundate significant parts
of coastal cities like New York, Boston, Halifax.
It wouldn't happen quite like the tsunami
in "The Day After Tomorrow," but that much change
over the course of a few decades is still far faster
than what we can prepare for.
And one last scary consequence, it's estimated
that our oceans absorb about a quarter
of the carbon dioxide that humans release each year.
And the North Atlantic alone may absorb around
1.3 billion tons per year.
About 40% of the global ocean's carbon uptake.
A weaker AMOC could significantly reduce this carbon uptake
thus amplifying global warming.
- It has really reverberations around the world
if this change in ocean circulation happens.
- Okay, so now that we're taking this seriously
what are the chances that this will actually happen?
- I've also myself always considered it
as a low probability,
high impact back in the 90s and 2000s.
But in recent times, there are a number
of studies that really suggest that the risk is much larger
and much closer actually with lower levels
of warming than we have previously thought.
- A new study published in July of 2023 that Dr. Rahmstorf
was not a part of warned that we estimate a collapse
of the AMOC to occur around mid-century
under the current scenario of future emissions.
If that estimate turns out to be true,
many people alive today would see impacts like three feet
of sea level rise in New York City
before the end of the century.
And short of collapse, a weakening
of the AMOC is also something to be concerned about.
- It doesn't have to completely shut down
for us to feel impacts that will be probably quite serious.
And we probably already are seeing some
of those impacts over the last several years.
There's been much above normal temperatures
running along the eastern seaboard
which has had some big impacts on the fisheries there.
- And what's really fascinating
and scary is how all of these changes will interact.
There's still a lot we don't know.
- The combination of natural fluctuations
in the climate system that we've seen many times before
like El Nino, La Nina.
These cycles that have been happening and evolving
over many thousands of years are being disrupted
in a very rapid and difficult to predict way.
This journey that we're on
into uncharted territory is really concerning.
- But in a conversation
about tipping points like the the AMOC,
it's important to remember that at this point, what we do
as humans is very likely to have a bigger impact
on climate change than tipping elements.
Our greenhouse gas emissions are still a big contribution
to our warming world
and that's something that we can control.
And the process of understanding
the uncharted climate territory we're in is not simple.
In 2022, a paper explained the existence of the cold blob
without changing or collapsing ocean currents.
That's why the process
of scientific discovery and debate is so important.
So what impacts are you most concerned about?
Let us know in the comments and see you next time.
We wanna thank Jasmin Graham
for joining us on this episode.
And although I haven't fully gotten over my fear
of swimming with sharks,
it was great to see the amazing work she does firsthand.
We filmed a lot more great stuff with Jasmin
which you can see in her new show, Sharks Unknown.
There's info in the description, check it out
and thanks for watching.
(foreboding music)