Remember this movie from 2004 called The Day After Tomorrow? It was a great action movie starring
Dennis Quaid and a very young Jake Gyllenhaal which in the best traditions of Hollywood
blockbusters provided two hours of fabulous entertainment but bore very little resemblance
to real-world events or possibilities. It portrayed the Atlantic Ocean circulatory system,
known as AMOC, collapsing within days, resulting in an instant ice age across most of the northern
hemisphere. The film's overly exaggerated dramatic depiction earned the less than complimentary
nickname of 'the Towering Inferno of the climate change era', but the basic premise of the
storyline was based, albeit very loosely, on real scientific observations of changes that have been
happening in our oceans since the middle of the last century. Changes that look like they may be
slowing down the global currents that distribute heat and energy around the entire planet. So,
how close to the mark did Hollywood get? Is this circulatory system at any real risk of grinding
to a halt? And if it does, will the northern hemisphere really be plunged into a new ice age?
Hello and welcome to Just Have a Think. The gulfstream here is an extraordinary force of
nature. As it passes the southern tip of Florida its waters are very warm indeed, averaging more
than 24 degrees Celsius. They're also very wide, at about a hundred kilometres or so,
and very deep - up to a thousand metres, or more than 3,000 feet. And all that water is traveling
at something like six miles an hour. That movement transports about four billion cubic feet of water
per second, which for us metric types is about 113 million cubic metres. Now to most people I imagine
both those sound like very large and completely abstract numbers. So let's try this - a hundred
and thirteen million cubic meters of water per second is more than the amount of water carried by
all the rivers of the world combined! So you know, it's pretty epic! But as mind-bogglingly immense
as that is, the Gulf Stream forms only part of a wider system called the Atlantic Meridional
Overturning Circulation, or AMOC, and it was the abruptly collapsing AMOC that formed the basis of
the plot in The Day After Tomorrow. 'Meridional' means southerly, and 'Overturning' refers to
different bodies of water rising and sinking above and below each other depending mainly on their
temperature and salinity. Cold dense water travels southwards in a very deep ocean layer all the way
from the north Atlantic right down to Antarctica and across to the Indian and Pacific Oceans until
the phenomenon known as upwelling brings the deep water back into the warm top layer of the oceans,
where it travels northwards again to re-join the Gulf Stream. The result is a completely
interlinked and interdependent system called the Thermohaline Circulation or Global Ocean Conveyor
Belt. We looked at the whole system in detail last year and you can click up there somewhere to
jump back to that program. Essentially though, the process ensures the world's oceans are continually
mixed and heat and energy are distributed around the earth. And just to give you an idea of the
staggering scale of the system, a full circuit can take up to a thousand years to complete! Anyway,
back up here at North Carolina, the Gulf Stream takes a right turn and starts crossing the
Atlantic. As it moves northeast it loses heat as a result of evaporation of the very warm surface
water caused by the ocean winds. That heat energy transferred into the atmosphere is enough to make
a difference to the local climate. Temperatures in my part of the world are on average about 10
degrees Celsius warmer than areas at the same latitudes over North America, and many studies
have attributed that differential in large part to the bonus heat our atmosphere receives over here
from the Gulf Stream. But evaporation only removes water from the ocean flow. It leaves behind the
salt. And salt molecules get trapped in the gaps between the water molecules. All those filled
up gaps create a fuller or denser structure. So now we've got dense, cold, salty water combining
with much fresher waters towards the Arctic Circle. And as those two bodies of water mix,
the denser water inevitably sinks, as Bill Nye is so ably demonstrating for us here. Those sinking
waters get caught up in the very deep currents of the AMOC and off they go on their millennial trek
around the globe. All that clever stuff was mostly worked out by this guy. He's called Henry Stomell,
and in 1961 while he was at Harvard University he published this research paper outlining the
principles that have since become the foundation of quite a lot of modern oceanography. In April
2018 a new research paper was published by Levka Cesar and Stefan Rahmstorf from the
Potsdam Institute for Climate Impact Research in Germany. They used a very high definition computer
modelling program called CM2.6 to analyse sea surface temperatures around the entire
circulatory system. This simulation of the model outlines the city of New York to give us an idea
of the size of each square or pixel on the models grid. That high level of definition makes this one
of the most sensitive and accurate modelling programs available. The research team set up
two simulations in the CM 2.6 modeller. Firstly, a control simulation showing what happens over 80
years if atmospheric CO2 concentrations were fixed at the level they were at in 1860, and a second
simulation where CO2 levels were increased by 1% each year for 70 years until they doubled and then
kept at that level for the next 10 years. So the areas turning red indicate a surface temperature
increase above the control model, and areas turning blue indicate a decrease below the
control. The CO2 doubling simulation predicted a very pronounced cooling of the area of ocean just
below Greenland and a noticeable warming in the Gulf Stream close to the eastern coastline of the
United States, strongly indicating a big slowdown in heat being transferred across the Atlantic.
And then they compared that CO2 simulation with actual real world observation of changes in sea
surface temperatures between 1870 and 2016. And it looked like this. So what's going on? Well,
most people already know that the increased levels of atmospheric CO2 are causing the warming of our
planet, but the science bods tell us that that warming is happening at least twice as rapidly
up in the Arctic, and that means accelerated melting of the Greenland ice sheet and a rapidly
decreasing volume of sea ice sitting on the Arctic Ocean. All that melting is releasing huge
quantities of pure water into the salty ocean which has the effect of freshening the water.
Fresh water is less dense than salty water so less of it sinks and that slows down the engine of the
great global thermohaline circulation. The Potsdam team showed a decline in the AMOC of about 15%
since the mid 20th century in fact recent years it looks like the AMOC has reached its lowest
point since records began in 1880. Those findings tallied with other research in this paper, also
published in April 2018, that found the AMOC was the weakest it's been for around 1,600 years. The
special report published by the Intergovernmental Panel on Climate Change or IPCC in October 2018
reviewed all the available scientific evidence and concluded that the AMOC was indeed very likely
to weaken more over the course of this century. The report stated "such a weakening would have a
cooling effect on the climate around the North Atlantic region as the northward heat supply
is slowed down. This effect is included in the climate projections but the direct warming effect
from rising concentrations of greenhouse gases is stronger. So the net result is still warming over
land regions." That cold blob under Greenland is indeed causing cooler North Atlantic weather but
meteorologists have found that it also tends to cause an air pressure distribution that channels
warm air northwards over Europe, and that's making European summertime heat waves more likely
and increasing levels of storminess across the continent. Even further south, on the warm-water
return leg of the global circulation, the slowing conveyor belt means greater heat retention
along the western side of Africa, bringing the likelihood of worsening drought across the Sahel
region. Across the other side of the pond the slowdown of the Gulf Stream means above average
sea level rises on the eastern seaboard, and the extra energy from the ocean surface is getting
scooped up into the local atmosphere causing more powerful hurricanes across the southern states. As
long as we continue to pump greenhouse gases into our atmosphere all those climatic consequences
will most likely continue to worsen as the AMOC continues to weaken. Eventually if our emission
levels don't change, the system could slow down so much that its ability to maintain momentum will
grind to a halt and will reach a catastrophic tipping point. Thankfully all the existing
scientific evidence suggests we're not approaching that point anytime soon, but that doesn't leave
any room for complacency. Arctic sea ice volumes have been reducing very dramatically in recent
years. The lowest extent on record occurred in 2012 - a year that was unusually warm as a result
of a very strong El Nino event. Most of the years since then have hovered around record low levels,
and then we come to 2020. Here's what the plot looked like at the start of the year. The red
dotted line is the 2012 record low year and the dark grey line is the 1991 to 2010 median. Our
2020 blue line starts off fairly predictably, but right around the end of February it takes
a noticeable dip downwards, well below the dotted red line of the record 2012 year. That drop does
of course coincide with the start of the global shutdown and a massive drop in atmospheric levels
of reflective sulphate and nitrate particles. We looked at the debate around global dimming
a couple of weeks ago and some commentators have pointed to this dip in the graph line as evidence
of global brightening causing abrupt global warming. It remains to be seen whether that
downward movement is a blip or a true reflection of significant increases in arctic temperatures,
but if that trend line continues all the way to the annual sea ice minimum in September then we
may well be heading for a new record low that would most likely exert a noticeable influence
on AMOC slowdown. This article, published in February 2020 at Carbon Brief by Dr. Richard
Wood and Dr. Laura Jackson from the UK Met Office Hadley Centre, includes this simple graph taken
from Henry's Stommel's original 1961 research paper. The units of measurement, by the way, are
apparently called 'Sverdrups', because, you know, why not? One Sverdrup denotes 1 million cubic
metres of water transported per second. Stommel had identified that if enough fresh water from
ice melt got into the circulation then eventually a tipping point will be reached which would create
a very rapid collapse of the AMOC. Doctors Wood and Jackson point out that under scenarios of
continued high greenhouse gas concentrations a number of models project an effective AMOC shut
down by 2300, and when the whole system shuts down the models do indeed suggest a widespread cooling
across the northern hemisphere that could outweigh the effects of global warming. Not that anyone
should take any comfort from that either. Other consequences are likely to include major shifts in
rainfall patterns causing huge storms over Europe, and a sea-level rise of about half a metre around
the North Atlantic Basin, in many cases actually worsening the already terrible effects of our
already rapidly changing climate. The entire world would experience major socio-economic consequences
with catastrophic impacts on agriculture, wildlife, transport, energy demand and coastal
infrastructure, resulting in the likely loss of hundreds of millions of lives. Unsurprisingly,
a great deal of time and money is being spent researching ways to develop early warning
monitoring systems at various points around the AMOC circulation. Scientists hope that by tracking
medium-term variations in salinity levels in subtropical and subpolar waters they might be able
to forewarn an AMOC collapse decades in advance. Of course, the easiest and most blindingly obvious
solution to this future calamity is to rapidly reduce our use of fossil fuels in transport &
industry, embrace renewable energy, and regenerate our land and forests back to the sustainable
levels that existed in pre-industrial times. I wonder if we will? No doubt you've got your views
on the subject, so get scribbling down there below in the comments section, and I'll be keen to read
your feedback. That's it for this week though. A massive 'thank you' as always, to the channel
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