He's wearing well isn't he, old Chris Packham? He'll be 58 you know on the fourth of May. May
the fourth be with you Chris! Anyway, I don't if this goes out around the world
but it's called Blue Planet Live. If you can find it, I recommend you watch it because it's
basically a week's worth of programs taking a health check of our oceans. I recorded this
little snippet from Sunday's program because Packham made a really good point "you know
I've been sat here and I've been looking out of this lagoon into the Pacific Ocean I've been
thinking to myself - we give all of these oceans different names; Atlantic, Pacific,
Indian, but there are no fences between them. They are all joined up. And as a consequence of
that all the ecosystems that we've been looking at and enjoyed by all these fantastic animals
are connected to one another. They're dependent upon one another. And ultimately our health and
our future is dependent on them as well." That reminded me that I haven't really yet got to
grips with how all that works. So brace yourself friends because this week I'm going to apply my
considerable ignorance and incompetent to the task of understanding the Thermohaline Circulation!
Hello and welcome to just have a think. A couple of programs ago I touched on the possibility of
a September ice-free Arctic in the coming years - something that's become known as
a blue ocean event or BOE, and I made a note to take a close look of what would
be the consequences if that happened. But in order to do that, I first really need to get
my mind around all the complicated interactions of driving forces and currents in our oceans,
and it turns out that that is a science fest of technical terms, abbreviations and acronyms. So
here goes... 'Thermo' is from the Greek word thermos, which means heat. Haline comes from
the Greek word for saltiness. Now we've obviously got English words for these things but I guess the
scientists figured that the 'hottie-salty' circulation was probably not appropriate,
so thermohaline it is. In fairness it's actually sometimes also referred to as the 'ocean conveyor
belt', 'the great ocean conveyor' or the 'global conveyor belt'. So how does it work? Well let's
start with the surface currents. According to Wikipedia the movement of surface currents pushed
by the wind is fairly intuitive. For example, the wind easily produces ripples on the surface
of a pond thus the deep ocean devoid of wind was assumed to be perfectly static by early
oceanographers. Idiots!! Nowadays of course our scientists have all sorts of fancy instruments
and contraptions which show quite clearly that there are significant currents in the depth that
can move as quickly as 1 metre per second. The main driver for these currents is differences
in water density which are driven by temperature and salinity. Warm water expands, so the water at
the top of the ocean which is warmed by the Sun is much less dense than the cold water at the
bottom of the ocean. Salt makes water more dense because the salt actually fills up the gap between
the water molecules. So you get a higher density for the same volume of water. Here's America's
favourite pretend wacky scientist Bill Nye showing us how salty water sinks below fresh water...
The more salt in the water, the heavier it is. So it sinks...see! Nice one Bill. So up here in the
northern latitudes at the North Atlantic warm water from the Gulf Stream arrives,
driven largely by the wind. When it reaches here it gets cooled by the generally cold temperatures
of the ambient air and also by the cold winds that blow around in the region. As that wind whistles
over the surface it causes a lot of the water to evaporate, for reasons that are outside the scope
of this video. But that evaporation only takes the water molecules, not the salt in the water.
So the seawater that's left behind gets more salty and more dense and that makes it sink. Scientists
call this the 'North Atlantic deep water'. Which seems reasonable! I should say here that other
seas also feed the North Atlantic deep water - for example the Labrador Sea off the coast of
Northeastern Canada, where the cold water sinks to depths of 3,000 metres at a rate of 10 centimetres
a second. Anyway, when all this cold dense water hits the bottom it spreads out. It can't really
go up into the Pacific from here because the Bering Strait's too narrow, so it takes the
course of least resistance which is southwards, very slowly into the depths of the Atlantic,
flowing down around South Africa, where all sorts of exciting things happen. the deep water travels
right down to Antarctica where it gets caught up in the most powerful currents on the planet - the
Antarctic Circumpolar Current, moving about 140 to 180 million cubic metres of water per second
in a rotation around the icy continent. Down here in Antarctica a really important process occurs
known as 'Overturning'. Oceans have layers. The top layer is about a hundred metres deep and gets
enough sunlight for organisms like phytoplankton to do photosynthesis. And phytoplankton are of
course the first stage of the ocean food chain, and absolutely essential to all ocean life. As
phytoplankton die they sink and collect on the ocean floor. In Antarctica the freezing
cold surface water sinks and as it sinks it forces the nutrient-rich deep water to rise. Overturning
in Antarctica moves about 35 to 45 million cubic meters of water every second from the ocean bottom
to the surface in a sort of gigantic tumbling rotation. So the Antarctic circumpolar current,
combined with this nutrient bonanza from the overturning process, makes the waters around
Antarctica an ideal habitat for marine mammals. Many types of whales migrate there every year
to feed on phytoplankton and other tiny sea creatures churned up by the overturning waters,
so it's a vital part of the ocean ecosystem. Anyway, back to our circulation journey. Parts
of the circumpolar current flow northward and move towards the Pacific and Indian
Oceans where a gradual warming occurs as the two bodies of water mix. This results
in another effect called 'upwelling', which once again causes the rich deep sea nutrients to be
brought to the top layer of the ocean. This top layer warm water then flows back around
the southern tip of Africa and into the Atlantic to continue the circuit up into the Arctic once
again. And here's my favourite statistic of the whole thing - it takes about five hundred years
for the conveyor belt to turn over the oceans waters and make one complete circuit around the
planet. So I think we can say that science has established that this system is an absolutely
crucial supplier of nutrients to the ocean food chain and a redistributor of heat around the
planet. Now at this stage some of you are probably wondering why I haven't made any mention of AMOC,
which is the Atlantic Meridional Overturning Current - that's the one that featured in the
2004 film 'The Day After Tomorrow'. Thermohaline Circulation and AMOC are terms that are often
used interchangeably in papers and articles, but as far as I can make out - and help me out
here folks if I've got this one wrong - strictly speaking thermohaline circulation is driven purely
by the differences in temperature and salinity that we've just looked at whereas AMOC refers
to the entire overturning circulation of our oceans driven both by the deep water currents
and by the top surface currents predominately driven by the wind. So they're pretty close,
but they're not exactly the same. And then on top of both those things we've also got
the not inconsiderable effects of the Gulf Stream, which is connected to the overall system but again
strictly speaking not part of the Thermohaline Circulation. The Gulf Stream starts in the Gulf of
Mexico and follows the eastern coastlines of the United States before crossing the Atlantic Ocean.
It flows at a speed of about 6 miles an hour at the surface and as it makes its way north it
passes very close to Europe. There appears to be a reasonable scientific consensus that this warm
water flow past the shores of Europe is a major contributing factor in the continent remaining on
average about 5 degrees Celsius warmer than areas of Canada that are at exactly the same latitude.
Recent articles such as this one in The Guardian from April 2018 reported evidence suggesting the
Gulf Stream is at its weakest in 1,600 years, and that the consequence of this may be a significant
cooling effect on Europe's climate. The report told us the warm Atlantic current linked to severe
and abrupt changes in the climate in the past is now at its weakest in at least 1,600 years,
new research shows. The findings, based on multiple lines of scientific evidence,
throw into question previous predictions that a catastrophic collapse of the Gulf Stream would
take centuries to occur. And it goes on to say such a collapse would see Western Europe suffer
far more extreme winters, sea levels rise fast on the eastern seaboard of the US, and would disrupt
vital tropical rains. The new research shows the current is now 15 percent weaker than around 400
AD - an exceptionally large deviation, and that human-caused global warming is responsible for
at least a significant part of the weakening. While the threat to the eastern seaboard and
the tropical rains may be very real possibilities, some climate scientists are starting to find that
the very thing that causes the Gulf Stream to slow down - human induced climate change - might also,
ironically, be minimizing it, at least across in Europe. This report from the BBC published
in July 2018 looks at a paper by Professor Kakit Tung of the University of Washington. The report
tells us while the waters in the North Atlantic will definitely cool as a result of changes in
the flow, the experts say it's likely that the UK will see continued impacts of climate change over
the next 20 years according to this study. The air temperatures globally will be warming and there's
no barrier for that so there won't be much cooling in the UK. You'll probably still see the normal
global warming, said professor Tong. Tongs theory is that the speed of the AMOC circulation may be
cyclical in nature, maybe over decadal timescales. He says when the current is faster more of the
warm salty tropical water travels to the North Atlantic. Over years this causes more glaciers
to melt and eventually the fresh water makes the surface water lighter and less likely to sink,
slowing the current. And that gives us a sort of self-regulating cycle. Tang tells us when the AMOC
was at a minimum, between 1975 and 1998, more heat entered the atmosphere and global temperatures
gradually went up. When the current started to accelerate from the mid 1990s to the early 2000s
and sink more heat, it coincided with a so-called slowdown in the pace of global warming. Now the
authors of the paper say that the slowdown in AMOC since 2004 means less heat going into the sea
and more heat going into the atmosphere, which is just about the worst coincidental timing we could
possibly have, given the amount of global warming we're causing through our human CO2 emissions!
But if Tang's theory is correct, then the cycle will shift and eventually the AMOC will start
accelerating again. And surely that'll mean that more heat from the atmosphere is sequestered down
into the depths of the ocean. And that will bring some relief in the global temperatures, right? But
what if something happens to stop it speeding up again? Just as a reminder of how complex and
intricately interconnected our global life-support system really is, let me leave you with the
abstract of yet another scientific research paper - this one a senior thesis presented
to the Faculty of the Department of Geology and Geophysics at Yale University by Holden Leslie-
Bole, Alexey Fedorov and Mary-Louise Timmermans on the second of May 2018. As anthropogenically
forced climate change intensifies, the loss of Arctic sea ice will become an important factor
affecting global weather and climate patterns. In particular it's been shown recently that Arctic
sea ice decline can contribute to the slowdown of the Atlantic Meridional Overturning Circulation
which modulates climates not just in Atlantic but globally. And that's why, as I mentioned earlier,
in an upcoming program I'm going to be having a look at the threat of a Blue Ocean Event up
in the Arctic. That's it for this week though. I'm gonna go and have a lie down to let my brain cool
off a little bit. Please do give us a like and a share if you found the program useful, and if you
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cost you a thing to do that. You just need to click that link there. As always, thanks
very much for watching. Have a great week, and remember to just have a think. See you next week