Savannah Geary (they/them): Deep beneath the waves of the Atlantic Ocean, a huge current called the Atlantic Meridional Overturning Circulation, or AMOC, pulses across the planet.

This current circulates water, heat and nutrients between the northern and southern hemispheres, and whether or not you know it, you probably benefit from it. But while researchers understand that rising temperatures on Earth will likely affect how the AMOC flows, it's proving difficult to predict exactly how or when it will all go down.

And it turns out the solution to this massive, global problem might be hiding in a pretty unassuming place. That is, an unremarkable looking clam.

(0:36) [ INTRO MUSIC ]

(0:40) Savannah: From the surface, the ocean might seem like it’s not doing all that much, but there’s a lot happening under the waves. Massive underwater currents twist and weave down there, moving water and nutrients all around the world.

The AMOC isn’t the only oceanic current system, but it's gigantic. It's vital to many marine ecosystems in the Atlantic, and it’s a big reason the Earth's climate works the way it does. It works like an industrial-sized conveyor belt, constantly moving water between Earth's poles.

This movement helps with temperature exchange, circulating carbon and salts, and transporting tons of other nutrients needed to sustain life. As water warmed in the south Atlantic is pushed northwards, it cools and evaporates. This evaporation leaves salts  and other minerals behind, which makes the water more dense.

The changes in water density that come with latitudinal changes in temperature and salt are what keeps the AMOC chugging along. Salt water moves up through the Caribbean and along the eastern seaboard to the Arctic. As it travels, the warm, salty water cools and sinks to the depths of the North Atlantic.

Then it begins its return journey south, becoming less salty on the way. As the waters warm and desalinate, they become less dense, and rise to the surface. By the time that water reaches the South Atlantic, it's ready to start the whole cycle all over again.

This transfer of nutrients and temperature exchange across the ocean plays a major role in ecosystems all along the way. Trust me, it’s all a lot more complex than it sounds. Even though researchers have  been studying it for decades, some elements of how the AMOC does what it does remain mysterious.

But one thing we do know is that as the AMOC snakes its way through the Atlantic, it's not just influencing life under the sea. It's also a huge reason Earth's climate and weather systems work the way they do. This massive current plays a crucial role in transporting heat to different parts of the world, sending rain clouds to the Amazon and to Africa, warm air to northern Europe, and creating all sorts of weather patterns that make climate conditions in and around the Atlantic what they are.

However, as the oceans become warmer due to climate change, the AMOC is likely to change as a result. Problem is, researchers don't  know enough about the AMOC to be able to model what that’ll look like, or even when it’ll happen. One big question the researchers have is about melting ice.

All the sea ice up in the Arctic might be floating in the ocean, but the ice itself is fresh water. And when it melts, the result is less salty seawater.. Rising temperatures prevent  new ice from forming as well.

And while that sounds like  it’d keep the salt levels lower since that fresh water isn’t turning into ice, in some cases, it does the opposite. This happens because of something called brine rejection. See, salt molecules don't easily fit in between the frozen water crystals, so when water freezes, it spits the salt out, making the surrounding liquid water even saltier and denser.

That brine that oozes out of the ice as it grows makes the nearby water good and salty, so if you get less ice formation, the salt levels in the area get way out of whack. The phrase “in that area” is doing a lot of lifting here. The conditions throughout  the AMOC are really variable, so changes in one place don’t mean that they’re happening everywhere, which makes the whole thing even more complicated to untangle.

The Arctic ice situation means that the salty waters that currently plunge kilometers under the surface of the Atlantic between Greenland and Iceland might not be so salty going forward, meaning they won’t flow as  far down as they do now. What that means for the overall movement of the AMOC is…. unclear. Part of what makes the AMOC so hard to study is that it’s unbelievably slow -- it takes around a thousand years for a drop of water to complete the entire circuit through the northern and southern hemispheres.

So the water that's finishing up its trip through the AMOC today started its journey when the Byzantine Empire was still a thing. And while the process takes a while, there's evidence that the AMOC is slowing down even more as a result of warming oceans and lower salinity in the northern latitudes. Because ocean salinity is one of the AMOC's big drivers, there's some urgency to understand the effect freshwater is having on the system.

Lowered salinity could be weakening the current, and there could be a salinity level so low that the AMOC just… Stops. And this is what has climate scientists so panicked. Because a broken AMOC would result in major shifts in global weather patterns.

You find the word "catastrophic" used pretty liberally in the literature about the  potential collapse of the AMOC. But nobody really knows how likely this collapse is. Even though researchers have been tracking lots of ocean variables for more than sixty years, they’ve only been directly monitoring the AMOC since… 2004.

That's just a tiny fraction of the time it takes for the AMOC to complete a full cycle, so it just isn’t enough information to make judgement calls with. Plus without any historical data on cycle variations in the AMOC, it’s hard to know which changes are warning signs and which aren’t really cause for concern. And here's where the clams come in.

Hafrun, the longest-lived animal ever known, is a clam, and more specifically, an ocean quahog. Or at least, it was. Hafrun was dug up off the coast of Iceland in 2006, and died as scientists were figuring out how old it was.

Turns out it was 507 years old, which means it’s the oldest known animal to have ever lived. This thing was alive for the fall of the Aztec empire and the life of William Shakespeare, and it was done in by a couple researchers with a shovel. But I promise, Hafrun didn’t die in vain.

These researchers determined just how old Hafrun was by counting the thin rings on its shell, because clam shells grow like rings on a tree. Thanks to a field of research called sclerochronology, scientists can look at an animal’s hard tissues and figure out from the growth rings what was going on each year it lived. It works on anything from bones to corals and yes, shells.

Wide bands in the clam's shell indicate productive, warm years, while thin ones show colder, leaner times. Not only that, carbon and oxygen isotopes trapped in the layers preserve a record of other climatic variables like sea temperature during the year, even in something growing underwater. And that brings us to why Hafrun is so special, aside from being such an elder.

Since Hafrun lived on the edge of the AMOC, on what’s called the North Icelandic Shelf, each ring of its shell provides a year-by-year record of changes in its environment through that time. By studying Hafrun's shell and hundreds of other ocean quahogs, scientists can actually put together a continuous record of climate conditions on the North Icelandic Shelf that stretches back 1,300 years. The clam shells hold a record of all the disturbances to the AMOC in that time -- or at least the ones that affected bivalves.

For instance, they found that slowdowns in clam growth at around 1330 CE were consistent with changes to a bit of the AMOC called the sub-polar gyre, a current that dumps warmer water into the Icelandic shelf. The growth rings were super thin around this time, meaning the clams weren’t doing so hot. And researchers think they struggled to thrive because they were deprived of warm water, and the nutrients that come with it.

This dip in temperature and clam food also coincides with an event known as the “Little Ice Age,” which was a miniature climatic upheaval that caused big problems for humans all over the world. Because the clams had their finger -- or I guess, foot? -- on the pulse of AMOC, they experienced the Little Ice Age before anybody else in the world sensed a temperature dip. Not only was the AMOC a harbinger of a future climatic shift, the clams were the first to register the effects.

The slowdown in clam growth happened years earlier than the land-based temperature records marking the 14th century cool down. Tying the clamshell record to the AMOC has helped researchers get a much better picture of what changes in the current might do to the rest of the world, and what warning signs to look for ahead of them. Since we'll have a better understanding of historical tipping points, we can respond to threats sooner -- thanks to a bunch of mollusks.

Who knew that clams were good for more than just chowder?

(8:54) [ outro]