Earth’s oceans are connected by a vast, global conveyor belt of moving water.
A planetary circulatory system that transports heat, nutrients, and even animals around the
globe.
And just like your circulatory system keeps you alive, Earth’s network of moving ocean
water keeps the planet healthy.
It powers a huge part of Earth’s weather and climate.
It shapes whole ecosystems.
Ocean currents have even influenced the history of human civilization…
And the engine of this whole global ocean current?
It’s powered by just salt, some sunlight, and a little wind.
Pretty cool!
But you know what’s not pretty cool?
CLIMATE CHANGE.
And as we continue to belch out gigatons of prehistoric carbon into the atmosphere like
a big sweaty, hot blanket, Earth’s ocean current circulatory system might be about
to break.
Here’s how it could happen, and how to stop it.
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In the 1800s, a message in a bottle was thrown from a ship in the Indian Ocean, with just
a piece of paper inside.
It washed up here - in Australia - where a beach-goer stumbled upon it.
But the bottle wasn’t found in the 1800s
It was found in 2018 - 132 years after it was tossed into the ocean.
The note inside wasn’t from a shipwrecked sailor or some long-lost love letter.
This bottle was one of thousands of similar bottles released by the German Naval observatory
between 1864 and 1932, as a kind of ocean tracking device in a huuuge experiment to
figure out ocean currents.
That’s a lot of commotion.
Get it?
Co-motion.
Because they’re currents?
You might have heard that oceans cover two thirds of the Earth’s surface.
I mean, there’s a reason we call it the blue planet after all.
But that doesn’t really do it justice.
The oceans weigh a few thousand million million million pounds.
That’s hundreds of times heavier than the entire atmosphere.
Yeah, take that AIR.
And moving all that water around are ocean currents.
The oceans aren’t really separate things.
They’re tied together by a huge ocean current - the ocean conveyor.
It carries truly ridiculous amounts of water - some sections have a current of around 10
Sverdrups.
What’s a Sverdrup, you ask?
Not a new lounge chair from Ikea.
It’s the special unit that ocean scientists use to talk about the vast quantities of water
going round in the oceans.
One Sverdrup is a current of 1 million cubic meters passing by per second.
If you’re having trouble picturing that ridiculously big number, one sverdrup is roughly
the current of all the Earth’s rivers.
Combined.
As my favorite ocean current-riding turtle would say: “Whoa” So 10 Sverdrups in some
parts of the ocean’s deep conveyor current… is kind of a lot.
But the circulation is also huge in terms of time - it can take one bit of water a thousand
years to make one loop around the swirling global path.
So what makes this huge conveyer run?
The first thing you need to know about how ocean currents work is… there are lots of
ways that ocean currents work..
Near the surface, ocean currents are controlled mostly by wind.
Just like blowing across the top of water pulls the water along with it.
Surface currents are influenced by tides, the rotation of the Earth, even the shape
of land.
But more than 9/10ths of the oceans water is pulled by a deeper force: density.
Hot water is less dense than cold water.
Because its molecules are actually a bit farther apart, you can’t fit as much water in the
same volume.
So a volume of hot water weighs a little bit less than the same volume of cold water
And if you mix them, cold water sinks below the warmer water.
The blue water is much much colder than the red water.
Just like cold ocean water, you can watch that layer of cold water sink beneath the red warm water.
If we speed this up you can even see the force of the rising and falling water densities creating a current in this tank.
This is one of the major driving forces of Earth’s deep ocean circulation: colder waters
sinking and warmer waters rising up.
Of course, the ocean isn’t just pure water.
It’s salty water…
This is just normal table salt, but that's also what makes the ocean salty.
And this is a tiny little ocean.
In salt water, the dissolved salt ions fill the unused space
between water molecules, which means there is more mass in a given unit of volume of
salt water compared to fresh water.
So saltier water is denser than fresher water.
Okay, that looks like the ocean.
We pour that into just normal fresh water, you can see that the saltier
water sunk below the fresh water because it's more dense.
These two factors together, temperature and saltiness, are crucial to how the ocean conveyor
circulates.
What’s called thermohaline circulation: thermo- referring to temperature and -haline
to saltiness.
In high latitudes, cold water on the surface of the ocean gets saltier as water evaporates
thanks to wind.
And when sea ice forms, it pulls pure water out of the ocean, and leaves salt behind,
making the ocean water saltier.
Just like we saw in our experiments, saltier colder water is now denser, and thus heavier,
so it sinks deep into the ocean and moves along the depths until it can rise to the
surface.
Heat from the sun eventually warms the cold water at the surface, where evaporation makes
the water saltier.
This warm salty water is carried northwards by large powerful wind-driven ocean currents
like the Gulf Stream, up the U.S. east coast, then into the North Atlantic region where
it releases heat into the atmosphere and warms Western Europe.
This water becomes very cold and dense again, sinks to the deep ocean, and the cycle continues.
The formation of cold, salty water in the north Atlantic acts like a pump, driving nearly
half of Earth’s deep ocean water circulatory system.
This section of the great ocean conveyor is called the Atlantic meridional overturning
circulation - or AMOC.
The heat released by the AMOC has huge climate effects, bringing moderate rains to Europe
and bringing about a million power plants-worth of heat to some parts of the planet!
Consider the fact that London in the UK is roughly the same latitude as Calgary, Canada,
but Calgary is way colder!
And that’s thanks - in part - to the heating Europe gets from the AMOC.
And researchers have argued that a big part of Europe’s political and economic power
over the last several millennia has come from the moderate climate conditions brought by
the AMOC.
Basically, if northern Europe was as cold as its latitude suggests, then maybe the Romans
don’t Empire as far, the Vikings don’t Viking everywhere, and the English and the
Dutch and the Spanish would have spent more time huddling next to the fireplace instead
of colonizing every country on the map.
All this has climate change saying... that’s a pretty important ocean circulation… it
would be a shame if something happened to it.
So yeah, that’s a pretty important ocean current.
Which is why it would be really bad if climate change broke it.
And that might be happening.
You see, as we heat the world, we melt ice.
And when ice in, say, Greenland melts, it flows into the North Atlantic.
But ice is made of fresh water, not salty sea water.
If melting ice in Greenland dumped enough fresh water into the North Atlantic - right
where salty water normally sinks as part of the AMOC - it could lower the density of that
sea water and slow down the sinking that pumps the global ocean conveyor.
And by “could”, I actually mean “probably would”.
Climate scientists are pretty confident that if human emissions of greenhouse gases follow
worst case scenarios, we’d melt enough ice to see some slow down in this ocean circulation.
And slowing down this huge Earth-sized heat transfer system would come with serious consequences.
It could shift the rain belt in West Africa, bringing drought to millions of people.
It would disrupt fisheries that millions depend on for food and money, and bring higher sea
level rise to the East Coast of the US.
Greenland holds enough ice that, if it melted, one-fifth of the world’s rivers-worth of
freshwater could flow into the north Atlantic.
Computer simulations show that with that much melting the AMOC would power down - basically
turning off the deep Atlantic current.
And like that iPhone 4 you have in the back of your drawer, it would take painfully long
to start up again.
Like centuries.
The odds of the AMOC shutting down within the next hundred years is what climate scientists
call a "low probability, high impact" event.
Which is boring climate scientist speak for: it’s probably not going to happen anytime
soon, but on the small chance that it does, it would be a massive disaster.
Like, SO bad.
It would majorly disrupt weather patterns and could make some parts of Europe a full
10°C (18°F) colder than they otherwise would be.
The impacts would be far reaching, affecting farming, drinking water, extreme weather,
sea levels, you name it.
And… we’re pretty sure that this has happened before.
Climate scientists think an interruption of the AMOC in the past may have triggered a
sudden extremely cold spike in the climate of the Northern Hemisphere… we’re talking
a millennium-long mini-Ice Age, just twelve thousand years ago.
OK that might not sound too recent - but in terms of Earth’s history, that’s a blink
of an eye.
And while that ocean current breakdown was part of natural climate cycles, what’s happening
today is thanks to us, and it’s happening even faster.
This image shows how much different parts of the world have heated up over the last
hundred years or so.
And there’s a reason they call it global warming: pretty much everywhere is heating
up.
Everywhere, that is, except this cold spot in the North Atlantic.
That spot is known as the North Atlantic cold blob.
It’s a sign that cold water isn’t sinking there as fast as it should, and that less
warm water is being pulled north as a result.
It may mean the AMOC pump is already weakening, maybe to the slowest it’s been in over a
thousand years.
Of course, the science still isn’t settled, because with something as big and complex
as the ocean it can be tough to confirm exactly if and how much these massive global currents
are changing.
But what scientists are certain of is that if climate change continues at its current
pace, currents like the AMOC will break.
Shutting down - or even slowing the AMOC would be a huge climate change disaster.
But it’s far from the only harm we’re doing to our oceans - and a lot of those other
impacts are being felt right now.
There’s sea level rise, plastic pollution, ocean acidification, ecosystem destruction.
All of these things… really suck.
And they don’t just suck because they’re harming the ocean.
They suck for humans too.
Billions of people around the world live near the ocean, and rely on it to support their
lives.
So what can we do to protect our oceans and protect ourselves?
We can be more careful about what we take out of the ocean, and what we put in.
But to truly keep our oceans safe, we need to stop climate change, and that means we
have to stop emitting greenhouse gases.
If we can get to zero emissions in the next few decades, that’s a good prescription
for our oceans AND the currents that keep them healthy.
Stay curious.
Our PBS friends across YouTube are celebrating Earth Day — follow me over to Animal IQ
on Terra for an episode all about the incredible minds of elephants (and why we have so much
more to learn about these endangered animals).
Tell them Joe sent you!
One more thing — for those of you in the US, you should check out Greta Thunberg: A
Year to Change the World, a new 3-hour Earth Day broadcast event on your local PBS station.
The special is going to premiere on Thursday, April 22nd at 8/7 central, and will then repeat
starting Wednesday, April 28th at 8/7c.