A couple of years ago I made a video based on a scientific paper from a
team at Potsdam University in Germany led by the climatologist and oceanographer Stefan Ramstorf.
That research found the Gulf Stream and Atlantic Overturning Meridional Circulation or AMOC had
slowed down by about 15% since the middle of the last century. A collapsing AMOC was of course
the main feature of the Hollywood blockbuster The Day After Tomorrow, which in true Hollywood
style exaggerated the consequences out of all proportion and had the whole of North America
under several feet of ice within about 48 hours. That impossibly rapid pace of change is thankfully
not going to happen, but it is perfectly true to say that the AMOCs circulation has collapsed
before. In fact it's collapsed several times over the course of Earth's long history and records
show that when it happens it does have a very significant impact on global weather patterns.
In an attempt to better understand why our global ocean circulation appears to
be slowing down again now, and whether it might grind to a complete halt in the near future,
scientists have been searching for evidence of what caused previous collapses of the system
to establish whether those same phenomena are happening today. And now one group has
published their findings in a new scientific paper. So what's going on? Is it us with our
climate warming greenhouse gas emissions or is it a completely natural process that would have
happened anyway? And if the AMOC system is collapsing, how quickly will it happen? And
what will be the consequences? So many questions and so little time. So let's dive straight in!
Hello and welcome to Just Have a Think. So what is the AMOC then anyway? Well, Atlantic means... well
it means it's in the Atlantic. Meridional is just a fancy word for southern or southerly, so that
tells us a bit about its direction. Overturning refers to different layers of ocean water rising
and sinking below each other depending on their relative temperature and salinity, and Circulation
is hopefully pretty self-explanatory. Over in the Gulf of Mexico the water is really
very warm indeed. About 24 degrees Celsius or 75 degrees Fahrenheit, and it's moving at a decent
pace too. About 6 miles or 10 kilometres an hour, shifting more water every second than
all of the rivers in the world combined. That warm water comes across the Atlantic towards
Europe and as it does so some of its heat is lost to the atmosphere through evaporation. The result
of that is that temperatures in my part of the world are, on average, about 10 degrees Celsius
warmer than areas at the same latitude across the pond. But evaporation only liberates water,
it doesn't take any of the salt. That means the concentration of salt in the remaining current of
water increases along the way. Eventually the warm salty water hits the cold fresh water below the
Arctic Circle and because it's denser it sinks and gets caught up in the deep southerly currents that
carry it all the way south, around Antarctica, across into the Indian and Pacific Oceans,
and eventually back up to the Gulf again, as part of the global Thermohaline Circulation. It's
a process that ensures the waters and essential nutrients in our oceans are continually mixed and
that heat energy is distributed around the planet. It's a torturous journey though. A whole circuit
can take up to a thousand years to complete. AMOC is a big driver of the overall system,
so if it slows down or grinds to a halt then the consequences aren't local, they're global.
Climatologists and oceanographers like Stefan Ramstorf tell us that climate change is indeed
having an effect on the AMOC system. As we've discovered in previous videos here on the channel,
human-induced greenhouse gas emissions and Arctic amplification mean that the Arctic is warming at
a much faster rate than the global average. That warming is melting ice from the Arctic Ocean and
from the Greenland ice sheet, vastly increasing the amount of fresh water in the surrounding seas.
As the water freshens and becomes less salty it gets less dense and therefore less likely
to sink and join up with the deep cold currents. And that slows down the AMOC system. This paper,
published in 2018, found that right now AMOC is the weakest it's been for about 1600 years.
A more recent study, published in July 2022 by teams at the University of Bremen in Germany and
the University of Sao Paulo in Brazil, analysed sediment course from water almost 2,000 metres
deep at a site to the east of Newfoundland in the subpolar western North Atlantic. Those
sediment cores hold hidden information going back thousands of years. Layers and layers of plankton
build up over those millennia to provide a chronological database of ocean conditions,
a bit like the rings of a tree can reveal what the weather was like in each growth season.
In this case the scientists studied the ratio of magnesium to calcium in a species of plankton
with the spectacular name of Neoglobaquadrina Pachyderma Sinistral, or N. Pachyderma Sin.
going back 250 years at a time over 27,000 years. And through the clever jiggery-pokery that modern
day science allows, they used this data as a proxy for historical subsea surface temperatures down
to depths of about 150 metres. But N. Pachyderma Sin. also contains a stable oxygen isotope called
Delta O18, which can be used as a proxy for ocean salinity over the same period. So now the team had
a comprehensive record of ocean temperature and salinity going back far enough to span periods of
previous AMOC collapses. The data demonstrated that warming of the waters at high latitudes
of the North Atlantic closely corresponded to times of a weakened AMOC and preceded a massive
release of an armada of icebergs from Canada and the U.S into the Atlantic which caused enormous
freshening of the surrounding waters, resulting in the rapid and total collapse of the AMOC system.
As I mentioned earlier in the video, we already know that the AMOC system is weakening again now,
and we know that the main drivers this time around are an overall warming of the planet's surface and
a much more rapid warming of ocean water at high latitudes causing more intense rainfall,
all of which is being turbocharged by human induced greenhouse gas emissions. The warmer
water is melting and destabilizing glacier sea snouts in Greenland, which is accelerating the
release of enormous volumes of fresh water into the ocean as the melting of the Greenland ice
sheet speeds up. In other words, we humans have rapidly created very similar conditions up in
the subpolar regions that caused a massive release of icebergs thousands of years ago.
Scientists are not projecting an imminent collapse of the AMOC system, but it is potentially a bit
too close for comfort. A 2019 article from the UK Met Office found that while a total AMOC shut down
this century is very unlikely, it does remain a possibility. They also say that if the world
started taking urgent steps to properly mitigate climate change then the weakening could be slowed
or even reversed. But of course we humans are a curious bunch aren't we? And scientists
have been debating the consequences of an AMOC collapse on modern day society for some time now.
In June 2022 another research paper was published that took an in-depth look at those consequences.
It was produced by researchers from the University of New South Wales and it used very sophisticated
model simulations that suggested there would be abrupt changes reaching far beyond the Atlantic
Basin and Northern Hemisphere stretching right around the globe. In what they call the 'AMOC
off' scenario the researchers found that cooling in the North Atlantic would extend southwards,
reaching the northern mid-latitudes within the first 10 years after shutdown and the
tropical Atlantic by year 20, largely as a result of sea surface wind evaporation.
The model also showed cooler sea surface temperatures creeping into the higher
latitudes of the North Pacific by around year 5, becoming well established by year 20. That
North Pacific cooling would eventually cause sea ice to expand in the ocean basin which
would reinforce the cooling as a result of the extra sea ice reflecting the sun's energy
back into space via what's known as the Albedo effect. By about year 30 the model showed that
cooling would start happening in the equatorial Pacific, evolving that region into more permanent
La Nina-like conditions. We took a closer look at the effects of a sustained La Nina quite recently.
Suffice to say they're not great, especially for countries like Australia and Africa.
Then there's the shutdown of a crucial movement of heat from the southern hemisphere to the Northern
Hemisphere by the great Thermohaline Circulation that I mentioned earlier. With that switched off,
most of the ocean heat stays down south with the biggest contrast between Southern and Northern
ocean temperatures occurring after about three decades. To compensate for the lack of oceanic
heat transport, nature turns to the atmosphere in an attempt to balance global temperatures.
There's a strengthening of something called Hadley cells in the northern hemisphere,
which are pockets of warm air that rise at the equator and move towards the poles.
Hadley cells also create the trade winds, tropical rain belts, hurricanes and the Jet Stream, so as
they get stronger all those events become more extreme as well. The study even demonstrated AMOC
connections to atmospheric circulation over the West Antarctic, with indications that the greater
retention of ocean heat in the region would bring about an accelerated melting of the ice sheet,
with all the catastrophic consequences of sea level rise that I'm sure we're all aware of.
Now there's a big caveat here, and that's that the research team concede that these results
were derived from only a single computer model that was set up under pre-industrial conditions.
They suggest future work should examine the impacts of an AMOC shutdown under a range of
scenarios that take into account increasing atmospheric greenhouse gas concentrations.
So, once again we find that the accuracy of climatic predictions are only as good as the
models we base them on, which means there's still a lot of variation in the detail. The
one thing all the models agree on is that an abrupt AMOC collapse would bring about
profound changes to the oceanic and atmospheric systems that all life forms on Earth rely on.
Nature's got all the mechanisms needed to recalibrate back to equilibrium over geological
time scales of course, so dear old planet Earth will quite happily trundle along for another few
billion years or so regardless of any undulations in the cycles of its Control Systems. The threat
of AMOC collapse is not to the planet, it's to all the species that live on the planet,
including we humans. All the indications are that if and when the AMOC does shut down it
will happen very quickly, with consequences kicking in with only a few years, making it
almost impossible for an entire global society to adapt in time to avoid catastrophic consequences.
Alternatively, we could start taking climate science seriously, stop wasting time in pointless
international talking shops, and immediately implement globally agreed actions to eradicate
the combustion of fossil fuels, transform our agricultural practices, and provide support for
developing nations to grow their economies in a sustainable way. So another challenging one
then folks! No doubt there are some strong views out there one way or the other, so if you feel
a burning desire to share your thoughts then the place to do that is in the comments section below.
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very much for watching. Have a great week, and remember to just have a think. See you next week.