- 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)
