How clams could help solve one of the biggest climate change mysteries

There was nothing outwardly impressive about the clam sitting on the deck of the research vessel Bjarni Sæmundsson. The dull, gray creature, dredged from the muddy bottom of the North Atlantic, was no different from the millions of mollusks caught and cooked each year for chowder.

But this clam was destined for something greater than a soup pot.

The humble bivalve - nicknamed Hafrún, an Icelandic word meaning “ocean mystery” - was the longest-living individual animal ever found by scientists. For 507 years it bathed in the shifting currents off the coast of Iceland, watching the surrounding water become more or less salty, enduring the rise and fall of ocean temperatures. And as the years passed, it recorded those observations in the molecular makeup of its shell, tracking the trajectory of a changing planet from a spot no human could reach.

Once it had been viewed beneath a microscope, Hafrún the clam would turn into a historian, giving researchers new insight into the mysteries of the deep sea. It would serve as a benchmark, allowing experts to make sense of changes they see in the ocean today. And it would become an oracle - helping scientists predict whether human-caused warming has pushed the Atlantic’s sensitive circulation system toward a tipping point that could devastate the modern world.

Experts have long known that the Atlantic Meridional Overturning Circulation, or AMOC - the system of ocean currents that transports heat and salt between the Southern and Northern Hemispheres - can suddenly and irreversibly shut down as a result of rising temperatures. A growing number of computer simulations, including two preliminary analyses published this summer, have suggested a collapse could occur as soon as 2050.

Yet many researchers warn that those findings can’t be confirmed without real-world data - and scientists haven’t been monitoring the AMOC long enough to detect trends in its behavior or determine exactly when it might change.

Enter the clams.

The ocean history these organisms record provides “a natural laboratory to really understand the system,” said David Reynolds, a marine climate scientist at the University of Exeter. “We can answer questions we couldn’t ask before.”

Earth’s circulatory system

Scientists often refer to the AMOC as a conveyor belt of the ocean, carrying water across hemispheres and from the surface to the ocean depths. But that metaphor belies the sheer size and complexity of the current, which is more akin to a human’s circulatory system. It is a core component of a healthy planet - as vital as blood, as powerful as a beating heart.

The cycle starts in the South Atlantic, where the surface of the ocean is warmed by the tropical sun. As the water drifts across the equator and starts moving northward, it begins to evaporate - concentrating salt in the current that’s left behind.

That warm, salty water is propelled through the Caribbean, past the East Coast and across the Atlantic toward Europe, where it runs into the frigid waters of the Arctic. Laden with salt and starting to cool, it becomes denser and sinks toward the bottom of the ocean. This deep water then travels south along the seafloor, getting warmer, fresher and less dense until it finally hits Antarctica and is pulled back to the surface.

The world as we now know it is a product of this vast overturning. The AMOC moves carbon deep into the ocean and transfers heat at a rate of one quadrillion watts per second - 50 times the rate of energy use by humankind. It shapes the band of clouds that encircles the Earth at the equator, delivering rain to Africa and the Amazon, and brings balmy temperatures to Northern Europe, explaining why Scotland is much milder than Alaska and Newfoundland despite sharing the same latitude.

Differences in temperature and salinity are the engine of the AMOC. As long as North Atlantic water is salty enough - and therefore heavy enough - to sink as it cools, the system is self-reinforcing.

Yet over the past century, humans have warmed the planet by more than a degree Celsius (1.8 degrees Fahrenheit) and caused the Greenland Ice Sheet to melt at a pace now exceeding 270 billion tons of ice per year. That influx of freshwater interrupts the salty northbound current, slowing its descent toward the seafloor.

If meltwater continues to flood the North Atlantic, many experts fear the AMOC may cross a tipping point at which it can no longer sustain itself, abruptly and irreversibly shutting down.

A study published this February in the journal Science Advances used the amount of freshwater moving around the South Atlantic to suggest the AMOC had gotten much weaker. Some of the same researchers suggested in preliminary analyses this summer that the system is likely to collapse around the middle of the century if the world remains on its current warming trajectory, and that it could tip even if humanity manages to limit warming to just over 2 degrees Celsius (3.6 degrees Fahrenheit).

Meanwhile, real-world observations have revealed subtle signs that the system isn’t functioning as it should. Even as the rest of the planet warms as a result of greenhouse gas emissions, a patch of water below Greenland has gotten colder - a hint that the ocean conveyor belt isn’t carrying as much warm water as it once did.

In October, dozens of top climate scientists issued an open letter calling on Nordic leaders to “take seriously” the risk of an AMOC collapse in the coming decades. Even a partial shutdown could lead to dangerously harsh winters in Northern Europe, as well as sea level rise on the east coast of the United States and dramatic shifts in rainfall around the equator.

Yet ongoing uncertainty about the AMOC’s future makes it hard to communicate the threat, scientists say.

Computer simulations may suggest a collapse is possible, said Maya Ben-Yami, a climate scientist at the Potsdam Institute for Climate Impact Research, but they don’t perfectly simulate the complexity of the real world. She found that the models used to predict AMOC’s behavior are too full of uncertainties to pinpoint exactly when the system could collapse. And direct measurements of the current’s strength, gathered by a network of floating sensors, only go back two decades - not nearly long enough to detect significant changes in a system that takes 1,000 years to complete a single cycle.

“If you want to do some sort of extrapolation into the future, you need something that goes back much further in the past,” Ben-Yami said.

The trees of the sea

Sclerochronologists - scientists who tell time through shells and bones - can’t take credit for discovering that clams are astute record keepers. That honor goes to Aristotle, who in fourth century B.C. observed that lines on the animals’ shells represented a year’s worth of growth.

But it wasn’t until the last few decades that researchers realized they could use those lines to learn about the history of the ocean, much the way they use tree rings to understand past temperature and weather on land.

A wide line in a shell generally indicates a year of warmer waters and abundant food, Reynolds explained. The types of carbon and oxygen atoms found in the lines can point to whether the clams’ habitat was dominated by deep Arctic water or warm surface currents from the south.

Few species are more valuable for this research than Arctica islandica - the group of bivalves that includes Hafrún as well as many of the clams eaten by humans. In the frigid waters of the North Atlantic, where creatures grow old by growing slow, there are Arctica islandica that were alive to see the invention of thermometer, the founding of New York and the discovery that the sun sits at the center of the solar system.

No other organism has been tracking the ocean in as precise detail, for as long - not even humans.

“That’s how they got the nickname, ‘the trees of the sea,’” Reynolds said.

Nearly two decades ago, as a young researcher, Reynolds was hired to help sort through thousands of living and dead clams that had been collected by the RV Bjarni Sæmundsson. In a laboratory at Bangor University in Wales, he painstakingly sliced shells open and peered at them under a microscope, looking for patterns and seeking out overlaps in their rings. Measuring down to a fraction of a millimeter, he drilled microscopic samples from each annual line and determined their chemical compositions.

By matching up the shells from clams of different ages, Reynolds and his colleagues were able to create a timeline that extended 1,357 years, all the way to the middle of the 7th century.

The chronology, published in 2016, remains the longest annual record from anywhere in the ocean, Reynolds said. Other kinds of evidence, such as layers of seafloor sediment, extend further back in time. But they can’t be pinpointed to specific years, making them less useful for cause-and-effect analyses.

“You have to have absolutely dated material to diagnose what’s happening and what’s changing first,” said Iowa State University geologist Alan Wanamaker, who also worked on the clam timeline. “For the first time we could say, ‘Did this cause this to happen? Or did it respond because that happened?’”

Crucially, the clam data came from a spot-on the border between the AMOC and Arctic waters, where ocean conditions are heavily dependent on the strength of currents flowing from the south. And the chronology encompassed a dramatic climatic shift: the transition from the relatively warm ocean conditions of the Medieval period, to a much cooler 500-year stretch known as the Little Ice Age.

That got the attention of Beatriz Arellano-Nava, a fellow researcher at the University of Exeter who specializes in climate tipping points. She was on the hunt for subtle changes in the AMOC’s behavior that might indicate it was on the precipice of “critical slowing down,” she said.

A tipping element in Earth’s climate, experts often say, is like a person leaning back in a four-legged chair. As long as it isn’t pushed too far, the system can quickly recover from disturbances and return to its original, stable state. But as it tilts toward its tipping point, the system becomes wobbly. Its variations get bigger and last longer. This lack of resilience is thought to be an early warning signal of a tipping element nearing collapse.

Perhaps, Arellano-Nava thought, the clams might have captured this kind of signal in the years leading up to the Little Ice Age, which spanned from roughly 1300 to 1800. And if scientists could identify the warning signs that preceded the Atlantic’s sudden cold snap more than 700 years ago, it might help them understand what is happening to the AMOC today.

Warning signs

In the year 649, paper money was a hot new invention, Visigoths controlled Spain and Portugal, and a young clam was beginning its life near the Icelandic island of Grimsey.

The bivalve was born at an opportune time, just before a natural increase in solar radiation and decline in volcanic activity that contributed to higher temperatures in the Northern Atlantic. As the clam aged, it enjoyed longer growing seasons and increasingly abundant food - allowing it to add ever-thicker annual lines to its shell.

The prime conditions also had a chemical effect on the clam. Warm ocean water tends to contain more of a lighter form of oxygen, known as oxygen-16, while colder waters are disproportionately laden with a heavier atom called oxygen-18. Relatively large amounts of oxygen-16 in the clam’s shell attest to the balmy seas surrounding it.

By the time this clam - the oldest in Reynolds’ chronology - died in the year 988, the Northern Hemisphere was near the peak of a warm period known as the Medieval Climate Optimum. The Vikings had ventured all the way to Greenland and England was temperate enough to grow grapes for wine.

But change was on the horizon. Around 1180, several clams living near Grimsey began to record strange shifts. Stretches of poor growing conditions were more frequent and longer lasting. The ratio of oxygen-16 to oxygen-18 started to fluctuate, hinting at an ocean climate that oscillated between extremes.

“You can tell it is losing stability,” Arellano-Nava said. “The environment is really struggling to maintain equilibrium.”

Other material from the bottom of the ocean, including bits of debris that were originally part of Greenland, suggest that chunks of glacier ice were falling into the ocean around this time. This would have diluted the saltwater of the AMOC, causing it to weaken and creating the fluctuations that got recorded by the clams.

The initial volatility subsided toward the end of the 13th century, but it returned with a vengeance around 1330. First came wild fluctuations in the clam shells’ oxygen ratios. Then indicators of temperature took a nose dive. The growth lines in the shells became thinner, as colder conditions depleted the clams’ food supply.

Above the surface, humans were also struggling to survive. Historical records from Northern Europe document harsh winters and brutal famines, as regional temperatures fell by 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit). Meanwhile, tropical regions saw major shifts in rainfall patterns, disrupting societies from India to the Amazon.

Scientists debate what exactly caused the Little Ice Age: changes in solar radiation, volcanic activity and the reforestation of North America following the decimation of Native American populations have all been blamed. Yet the first unstable episode in the shell record preceded those potential causes, suggesting that the ocean played a critical role, Arellano-Nava said.

Because the changes were concentrated in the North Atlantic, the AMOC couldn’t have completely broken down, she explained. But there did seem to be an abrupt downturn in a crucial component of the system, known as the subpolar gyre, that prevented warm water from flowing to the clams’ seafloor home. Frigid Arctic currents filled the void, helping to trigger a regional cold snap. Meanwhile, solar and volcanic factors likely contributed to and amplified the change.

Tim Lenton, who directs the Global Systems Institute at the University of Exeter and contributed to the Little Ice Age study, called a subpolar gyre downturn “the little brother of an AMOC collapse.”

“The AMOC could step down in strength without completely collapsing,” Lenton said. “And the impacts could hit you quite quickly.”

What’s more, Arellano-Nava added, the clam records show there were warning signs of the coming transition, though no human knew it at the time. The fluctuating growth lines and shifting chemical compositions are indicators of the gyre critically slowing down - the wobbles of someone tilting back in a chair just before they fall.

An uncertain future

To scholars of Earth’s climate, the clam shells and other ancient records are like pieces of a vast puzzle. With each new study, they get a slightly clearer picture of what the world once was.

But the longevity of clams like Hafrún means they’re not solely useful for looking backward.

“Now that we know they are good for recording (the AMOC) critically slowing down, that unlocks the potential of these records to provide early warning signals for the future,” Arellano-Nava said.

She is now studying other clams that lived across the North Atlantic over the past century - and discovered many of the same chemical signals that could be seen in the medieval shells. In unpublished research presented this year at the European Geological Union’s annual conference, she argues that the evidence from the modern clams is an early warning signal that the subpolar gyre is “highly unstable” and approaching a tipping point once again.

That follow-up study has not yet been published in a peer-reviewed journal. But it jibes with other analyses showing instability in the region, as well as a recent paper suggesting that the gyre could collapse much faster than the full AMOC - possibly at a lower level of warming.

That doesn’t necessarily mean the planet is headed for a repeat of the Little Ice Age, Arellano-Nava cautioned. Modern environmental conditions are quite different from those that existed in the 13th century. And it remains to be seen whether the chemical signals that heralded a tipping point 800 years ago have the same implications in today’s much hotter world.

But with the planet now warming faster than ever, Lenton said, there’s plenty of reason to curb greenhouse gas pollution. Otherwise, he fears the AMOC might pass its tipping point before scientists have enough evidence to be certain about its fate.

“I’m not sure [the data] will come in time before nature is giving us the answer,” he said. “Sometimes you know enough to know you have to act.”