Ancient warning of a rising sea
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This is a fossilized coral — one of dozens scattered across the Indian Ocean archipelago of the Seychelles. It is a piece of history, hidden in plain sight.
It tells of a time 125,000 years ago, when a modest increase in Earth’s temperatures caused ice sheets to melt and global sea levels to surge.
The height of the coral — which can grow only underwater — marks where the ocean once reached, and where it could someday rise again.
As human-caused climate change remakes the world’s coastlines, these fossil relics from Earth’s past are helping scientists understand how future sea level rise could unfold.
By studying the long-dead coral reefs, researchers have revealed not only how high sea levels can reach, but where the deluge will hit hardest. As temperatures surge and ice sheets melt, the fossils show, the oceans won’t rise evenly around the planet. Instead, the loss of polar ice will trigger profound changes in Earth’s gravity and shape — which, in turn, will create dramatic disparities in where ocean water flows.
But the most worrying lesson from this research may lie in the differences between the time of the corals and today. Researchers warn that the warming brought about by human-generated greenhouse gas emissions is unlike anything that has previously happened in the Earth’s history. That means modern sea level rise may not exactly mirror what happened last time — it might be worse.
‘All this could be covered again’
In all the world, there are no islands like the Seychelles.
Most oceanic islands are volcanic, formed by magma that wells up as tectonic plates drift over hot spots in the Earth’s mantle. Others are coral atolls — low-lying, ring-shaped landforms created when sea level drops below the surface of a reef.
But the Seychelles are fragments of an ancient continent dropped in the middle of the sea. Hundreds of millions of years ago, the granite peaks that make up much of the archipelago — including the main inhabited islands of Mahé, Praslin and La Digue — were sandwiched between India and Madagascar in a giant landmass known as Gondwana. When the supercontinent broke apart, and India drifted northeast to its current position, the Seychelles were scattered behind like breadcrumbs.
Isolated by the wide blue expanse of the Indian Ocean, the Seychelles became a haven for plants and animals found nowhere else: 500-pound tortoises, opulent rare birds, a palm tree that produces the heaviest seeds on Earth. When early European colonists first encountered the islands, the shimmering coastlines and lushly forested mountains reminded them of the biblical Garden of Eden. Today, some 300,000 foreign tourists — three times the Seychelles’ population — flock to the country’s beaches every year.
To Andrea Dutton, a geologist at the University of Wisconsin at Madison, the Seychelles’ geologic singularity made it the perfect place to conduct research on ancient sea levels. The granite landmasses do not sink in the manner of atolls and volcanic islands, which subside as they drift away from the mantle plume that created them. The islands were also far enough from former ice sheets that they were less affected by changes in the Earth’s shape caused by ice pressing down on the crust.
“The Seychelles was really a serendipitous find in many ways,” Dutton said. The islands’ long-term stability made them ideal for comparing sea levels from ancient times to those seen today. If researchers found evidence of a reef on land that is now above water, they could be relatively certain it wasn’t because the land had moved — it was because the oceans had changed.
Unearthing the future
Dutton arrived on La Digue, the country’s third-most-populated island, in the summer of 2009. With only a rough geologic map and local lore to guide them, she and her colleagues began to scour the island seeking traces of chunks — the fossilized form of a coral’s calcium carbonate skeleton. But many of the sites that residents already knew of were too weathered and broken for the kinds of chemical analysis Dutton’s team hoped to do. They would have to find lesser-known fossils, ones that were hidden enough to remain almost pristine.
For days, the researchers scrambled over rocks and burrowed into crevices, dodging spiderwebs and drawing quizzical looks from beachgoers. The stifling humidity left them drenched in sweat. The sharp quartz crystals of the granite bit into their palms.
Finally, at the edge of a beach called Anse Source D’Argent, Dutton’s colleague Dan Zwartz came upon a column of coral wedged between two boulders the size of trucks. The rest of the team clambered over to the fossil, running their hands along the jagged surface. Perched beside the outcrop, they could look across the tops of palm trees and see the ocean gleaming more than 25 feet below.
“We were so excited,” Dutton said. “They were the highest corals we had seen anywhere.”
Because corals can grow only underwater, the fossils proved that oceans here were once at least as high as the top of the outcrop. The limestone column was like the notches parents make on kitchen walls to track their children’s height: a marker of what the world once was, a reminder of how much it had changed.
Eventually, Dutton’s findings would be published in a scientific journal and cited by the United Nations Intergovernmental Panel on Climate Change. They would add to a growing realization among researchers that sea levels in Earth’s recent past reached heights that would be devastating today.
But even in the moment, as she gazed out at the ocean, it was clear to Dutton that her discovery was cause for alarm. Most of La Digue — a quaint community of colorful bungalows and unpaved streets — was well below the level of the fossils.
“Oh, this is for real,” Dutton recalled thinking, as she watched wave after wave roll toward her. “All this could be covered again.”
A ‘detective of Earth’s past’
Contrary to depictions in grade-school textbooks, the Earth is not a perfectly round sphere, spinning through space on a precisely predictable path. The planet is bulging and irregular — jagged along mountain ranges and wide in the middle. It teeters like a top as it rotates on its axis. The shape of the Earth’s annual journey around the sun is in flux, as Jupiter and Saturn play gravitational tug-of-war with our orbit.
All this commotion has a profound effect on climate, altering how the sun’s energy falls across the surface of the Earth. If conditions are right and the Northern Hemisphere, which is home to most of the Earth’s land area, receives a little less solar energy, it can cool the entire globe, plunging the world into an ice age.
But if the Northern Hemisphere gets a bit of extra sunlight, it can trigger a cascade of processes that cause the ice sheets to melt. Temperatures rise, and the world enters a warm period known as an interglacial.
For the past several thousand years, humanity has enjoyed a particularly stable and mild interglacial. Relatively constant temperatures and greenhouse gas concentrations enabled the development of agriculture, the construction of cities, the invention of writing and plumbing and antibiotics.
Then people started burning fossil fuels and pumping billions of tons of extra carbon into the air, warming the planet at least 1.2 degrees Celsius (2.2 degrees Fahrenheit). The Earth’s average temperature has come closer to that of the Last Interglacial — the warm era before this one, between 129,000 and 116,000 years ago, when Dutton’s corals grew.
The extreme height of the Seychelles fossils indicated what a repeat of Last Interglacial temperatures could do to the Earth’s ice sheets — and by extension, to global sea levels. But it wasn’t enough for Dutton to know how high the oceans might get. She also wanted to understand how fast they could rise. As temperatures spiked, would the poles tend to melt gradually, like ice cubes on a warm day? Or could parts of the ice sheets suddenly collapse, causing a huge surge in sea levels over a short span of time?
In the first case, the Seychelles would have many decades to shift homes and infrastructure away from the coasts. In the alternative scenario, the country could be inundated at any moment — requiring difficult decisions about what places to protect and what to relinquish to the sea.
“If we could say, hey, the last time this happened, it happened in 500 years or a thousand years or whatever … I think that would be a huge contribution in terms of coastal planning,” Dutton said.
So, four years after her initial visit, Dutton returned to the Seychelles, this time with a larger team, more advanced equipment and a new mission. She didn’t just want to know how high the corals got — she wanted to understand the timing of how the reefs grew. She needed to scour each outcrop for tiny details and overlooked clues that could reveal how the corals lived and died.
This was the part of her work that Dutton loved most. It made her feel like a “detective of Earth’s past,” she often said — as though she and her colleagues were investigating a crime millennia after the fact.
They would need all their wits and powers of observation — plus some advanced chemical analysis — to piece together what little evidence remained.
The most scientifically valuable outcrops — like this fossil near Grand Anse Beach — were tucked among piles of granite, which protected them from weathering by wind and rain.
Because the fossil was so well-preserved, Dutton could distinguish subtle variations in shape and texture between the lowest and uppermost parts of the outcrop. Perhaps, she thought, each layer represented a different period of growth, and the variations reflected changes in the reef over time.
The researchers used handheld drills to extract several small, cylindrical cores from the outcrop. When they returned to their labs, they analyzed the samples for two particular elements — radioactive uranium and thorium-230 — which would help them understand when the corals grew.
Uranium, which is present in seawater, gets incorporated into coral skeletons as reefs grow. But over thousands of years after a coral dies, the uranium decays into a “daughter” element: thorium-230. Comparing the ratio of those elements, combined with other kinds of analysis, revealed that this coral had lived about 126,000 years ago.
Details within the outcrop helped determine what corals composed the ancient reef. These flower-shaped holes are characteristic of corals from the Goniastrea genus — usually found within three to six feet of the sea surface. Now Dutton knew not only the age of the fossil but the probable height of the ocean when it was growing.
Dutton and her colleagues performed the same careful analysis for each part of the outcrop. Atop the Goniastrea layer was a blanket of calcium carbonate secreted by a species known as Millepora exaesa, which tends to grow atop dead reefs. Just above the gap was another thin layer of coral whose uranium-thorium ratio suggested that it was roughly 123,000 to 124,00 years old.
The team detected similar patterns at seven other outcrops around the Seychelles. At each site they examined, the reefs seemed to grow a few feet, die, grow back even higher, die again and then grow back once more — all in a span of just a few thousand years.
That pattern suggested that the waters around the Seychelles rose, fell and then spiked back up several times — with each spike raising sea levels even higher than they were before. If plotted on a graph, the changing height of the ocean would look less like a smooth diagonal line than a broken staircase for giants, climbing upward in short, dramatic surges.
The dating methods Dutton’s team used allowed them to pinpoint the timing of these pulses to within a 500-year span. But details in the fossil reef’s structure suggested that some of the spikes in sea level may have happened even faster than that, within the 100- to 200-year life span of a single coral.
If oceans rose like that again, Dutton said, it would be devastating for small island nations like the Seychelles.
“Because that means it’s not just going to be gradual,” she said. “It could be like, whoop, there goes a whole half a meter all at once.”
The vanishing coast
For centuries, life in the Seychelles has centered on the meeting of land and sea. The low-lying coastal plateaus that curve around the bases of the steep granite islands offer the only flat ground in the country. Much of the capital city, Victoria, has been built on reclaimed land along the shore. The airport, power stations, hospitals and about 90 percent of the population can be found within a few yards of sea level. The country’s two biggest industries — tourism and fishing — depend upon pristine waters and immaculate beaches.
“The coast is our number one asset,” said sustainable-development consultant Didier Dogley, who previously served as the Seychelles’ tourism minister and environment chief. “Very early on, we understood that the only way to survive on this rock is to protect the nature that’s around us.”
But the warming, surging ocean threatens that crucial coastline — and with it, the Seychelles’ entire existence.
Fig. 2.
Areas expected to be at risk of high-tide flooding in 2100
Beau Vallon
5,598 people
Victoria
29,640 people
MAHÉ ISLAND
Airport
Grande Anse
4,136 people
Anse Boileau
5,378 people
Indian
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SEYCHELLES
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Satellite image by Planet Labs PBC
Beau Vallon
5,598 people
Victoria
29,640 people
MAHÉ ISLAND
Airport
Grande Anse
4,136 people
Anse Boileau
5,378 people
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Satellite image by Planet Labs PBC
Beau Vallon
5,598 people
Victoria
29,640 people
MAHÉ ISLAND
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4,136 people
Anse Boileau
5,378 people
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Satellite image by Planet Labs PBC
SEYCHELLES
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Beau Vallon
5,598 people
Victoria
29,640 people
MAHÉ ISLAND
Airport
Grande Anse
4,136 people
Anse Boileau
5,378 people
Indian Ocean
Satellite image by Planet Labs PBC
SEYCHELLES
Beau Vallon
5,598 people
Victoria
29,640 people
MAHÉ ISLAND
Airport
Grande Anse
4,136 people
Anse Boileau
5,378 people
Indian Ocean
Satellite image by Planet Labs PBC
Observations from the nation’s single tide gauge show that local sea levels are rising by about six millimeters per year — twice the global average rate. Repeated bleaching events have destroyed about 90 percent of the coral reefs that ring the islands, diminishing their ability to absorb the force of incoming waves. Rising water temperatures have also intensified the cyclones that form in the Indian Ocean, creating powerful storm surges that inundate the Seychelles’ shores.
Almost every winter, torrential rains compounded by increasingly high tides lead to flooding in La Digue’s main village. Streets turn to unnavigable pools of mud. The community’s septic systems become overwhelmed, spilling sewage into the open.
Even when the sun shines, the encroaching ocean is expanding marshes and contaminating ground water, says Melton Ernesta, a 64-year-old bar manager and community leader. As a board member for the L’Union Estate Park and demonstration farm, he has watched crops wither in soil that has become contaminated by salt.
And at Anse Source D’Argent — the scenic beach where Dutton found her first fossils — acres of soft white sand have been washed away by the steadily rising tide. The water seeps under walls that are supposed to contain it and scrapes earth from beneath the roots of the shading takamaka trees.
Just a few feet away, a chunk of ancient coral looms over the eroded beach path. Like many La Digue residents, Ernesta grew up hearing about the history the fossils represented and the warnings for the future they held.
“It’s very emotional for me to know that where we are standing now, everything was underwater,” Ernesta says. “Maybe now, the planet is taking it back.”
Finding fingerprints
Alone amid a vast ocean, nearly 1,000 miles from the nearest land, the Seychelles can seem separate from the rest of the globe — quite literally an island on its own.
But nearly every Seychellois will tell you the opposite is true. Most of their food must be imported from elsewhere. Their cars, appliances and fuel all come from overseas. Though the nation has banned plastic bags, polyethylene flotsam from far-flung places routinely washes up on their shores.
The coronavirus pandemic was a stark reminder of the country’s vulnerability to world events, Ernesta says. With less than a third of its usual tourism revenue, GDP fell by more than 10 percent, according to a U.N. Development Program report. Unemployment doubled. Prices spiked.
Nothing makes Ernesta feel his country’s interdependence more than climate change. The Seychelles accounts for less than a tenth of a percentage point of all the greenhouse gases ever released into the atmosphere. Indeed, with half of its land area and 30 percent of its oceans protected, the country’s forests and seagrass meadows currently pull more carbon out of the air than its people emit.
Yet the nation is listed sixth in the Notre Dame Global Adaptation Initiative’s ranking of countries most exposed to climate impacts. U.N. scientists project that the sea level around the Seychelles will rise by another 2 to 2.5 feet by the end of the century — enough to inundate 1 in 8 buildings and 42 percent of major roads on the main island of Mahé, according to the country’s coastal management plan. Scientists have identified almost 150 endangered species on the islands that could be driven to extinction by climate change.
A recent report from the International Monetary Fund found that coping with global warming will cost the Seychelles about 30 percent of its gross domestic product. The country is wealthier per capita than any other African nation, thanks largely to an economy fueled by carbon-intensive international travel. But its small size still leaves it short on the resources and expertise needed to protect against rising sea levels, surging ocean temperatures, intensifying rainfall and escalating heat.
“We are suffering, but we are not the ones damaging,” Ernesta says.
Seychellois say their survival depends on larger nations with more economic clout drastically reducing their own carbon pollution to avoid further warming. So far, the opposite is happening: Global greenhouse gas emissions set a new record this year.
“It’s like, when the elephants fight, the grass gets crushed,” Ernesta adds. “We are the grass.”
There is another force linking the Seychelles to distant parts of the planet — one that scientists are just beginning to understand.
As researchers like Dutton accumulated evidence of ancient sea levels all over the globe, they started to notice surprising discrepancies in their data. Fossils on one continent seemed to suggest sea levels that were inches, even feet higher than fossils of the same age from another part of the world. In some cases, sea levels in far northern locales seemed to fall even as waters were rising everywhere else.
The explanation was in the ice. The ice sheets that cover the Earth’s poles are so massive, they exert a gravitational pull on water in oceans. Their weight compresses the continents underneath them and pushes Earth’s crust upward elsewhere — the same way sitting on a couch can cause the cushions to bulge to the sides. These phenomena can cause sea levels to rise near the ice sheets and fall in the mid-latitudes.
Fig. 3.
How ice melting at the poles leads to uneven sea level rise
GREENLAND
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ANTARCTICA
GREENLAND
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When the ice melts, its gravity weakens, and the Earth’s crust is able to bounce back. The ocean falls near the poles, and water surges in the opposite direction.
The result is that each ice sheet produces a distinctive pattern of sea level rise known as a “fingerprint,” which can help scientists understand what places were most affected in the past — or will be most at risk in the future.
The Seychelles, which are located almost on top of the equator, are in the worst possible spot. No matter which ice sheet is melting, the islands will experience 9 to 24 percent more sea level rise than the global average, according to a 2014 study in the journal Quaternary Science Reviews.
This fingerprint research is helping Dutton determine what her Seychelles findings meant for the oceans across the rest of the Last Interglacial world. Meanwhile, in new work presented at recent scientific conferences, Dutton drew on evidence from climate models, deep ocean sediments and polar ice cores to determine which ice sources were most likely responsible for the pulses of sea level rise recorded in the fossil reefs.
Her “working hypothesis,” she said, is that the first burst of melting probably came from Antarctica. Then the Northern Hemisphere started warming up, causing another pulse of sea level rise from the Arctic region. Finally, the northern ice sheets unleashed one last spurt of meltwater, before the planet tilted back into a cooling phase and the Last Interglacial came to an end.
“What’s happening with those pulses of sea level rise is it’s actually the two ice sheets kind of competing against each other — one is getting smaller while the other one is growing,” Dutton said. “And so it’s like this back and forth, and that’s why it goes up and then it goes down a little bit and then it goes up again.”
Alessio Rovere, a coastal geologist at Ca’ Foscari University of Venice, said Dutton’s findings add to a growing body of evidence that the Last Interglacial ice sheets didn’t melt simultaneously, causing sea levels to surge upward in multiple, sudden spurts.
“We can use it as a benchmark for what could happen in a future that is slightly warmer,” Rovere said, helping scientists develop better models of how modern ice sheets will behave.
But there is a key difference between the Last Interglacial and today, Dutton warned. Back then, the poles alternated warming, so melting from one ice sheet was buffered by the other. Now, the whole planet is warming at the same time as a result of human greenhouse gas emissions. Both Greenland and Antarctica are crumbling at once.
And the Seychelles, positioned at just the wrong distance from both ice sheets, will be on the receiving end of that surge.
‘We have to protect’
In recent years, as the revelations of her research have become clearer and the realities of climate change have gotten worse, Dutton has sought any opportunity to talk with the public about her work. She has given presentations to neighborhood groups and coastal planning commissions, appeared on podcasts, testified before the U.S. Senate.
Often, her audiences’ reactions feel painfully shortsighted, she says. People dismiss the lessons of Earth history as too distant to be relevant. They balk at the suggestion of spending money today to protect against the possibility of a disaster tomorrow. They believe that climate change is something that will happen to unknown people, someplace far away.
The geology of the Seychelles defies such myopia. Here, the past is a fossil people can touch. The future is a wave already eating away at the shore.
And every carbon dioxide molecule emitted anywhere on the planet is a threat to Seychellois’ narrow strip of habitable coastline. Each fraction of a degree that the Earth’s temperature rises is a step toward re-creating the world that the fossil corals knew.
With his palm pressed against the Anse Source D’Argent outcrop, Ernesta finds himself thinking about the Seychelles his ancestors encountered when the first people came here 253 years ago. The wide beaches and Edenic forests. The rare plants and unusual creatures.
“I feel that we have to protect,” he says, speaking slowly as he searches for the right words. “We have to protect for generations and generations, [for them] to come and to see our planet. How — how it was. And how it still is today.”