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.
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