Scientists just got some ancient clues about future sea level rise — and it’s bad news

When the researchers first arrived at their field camp at Prudhoe Dome, atop the Greenland ice sheet, they felt they had been swallowed by a monster.

The mountain of ice in northwest Greenland was more than 50 miles wide and 1,600 feet tall. The temperature at its summit was well below 0 degrees Fahrenheit. The scientists’ experiment there — an unprecedented effort to extract bedrock from deep beneath the ice sheet — was routinely disrupted by howling winds and blizzards so dense they blocked the sun. It was hard to imagine that this formidable, frozen expanse could ever disappear.

But the rocks they uncovered on that 2023 expedition contain chemical signatures showing that Prudhoe Dome completely melted within the past 10,000 years — and it might soon suffer the same fate amid modern climate change.

The results published Monday in the journal Nature Geoscience hold a warning for a warming planet, lead author Caleb Walcott-George said. The study suggests that large portions of Greenland were ice-free in Earth’s recent past, when global temperatures weren’t much higher than they are now. If the same melting occurred today, it would raise average sea levels anywhere from 7.5 inches to 2.4 feet.

Present-day melting may not precisely emulate what happened in the past, the researchers acknowledged. The cause of modern climate change — pollution primarily from burning fossil fuels — is distinct from the slight wobbles in Earth’s orbit that triggered warming thousands of years ago.

But researchers said their findings could be used to improve the computer models used to simulate how the ice sheet responds to warming.

“Understanding how the ice sheet evolved in the past … allows us to make better predictions about our future,” said Walcott-George, a glacial geologist at the University of Kentucky.

The Arctic is the fastest warming place on the planet, and Greenland contributes more to rising oceans than any other ice mass on Earth. If the entire ice sheet melted, it would boost global sea levels by 24 feet.

But scientists aren’t certain exactly what will happen as global temperatures continue to rise, said Jason Briner, a geologist at the University at Buffalo and co-author of the new study. Changes in the ice sheet’s elevation and reflectiveness could trigger feedback loops that slow the pace of melting or accelerate it. Some research suggests that Greenland might be approaching a tipping point toward irreversible decline.

Such a catastrophe has happened before, Briner said. In a 2016 analysis of samples from beneath the thickest part of the ice sheet, he helped show that the bedrock was exposed to sunlight sometime in the past 1.1 million years.

The findings raised two urgent questions: What was Earth’s temperature when the melting occurred? And how fast did the ice disappear?

The expedition to Prudhoe Dome — part of a multimillion-dollar project called GreenDrill — aimed to solve those mysteries. By drilling for rocks beneath different parts of the ice sheet and testing those samples to see when they were last exposed to sunlight, the researchers could provide a clearer picture of what happened when Greenland melted away.

Joerg Schaefer, a climate geochemist at Columbia University’s Lamont-Doherty Earth Observatory who led the project with Briner, compared it to a high-stakes medical exam.

But the hostile conditions in far-northern Greenland nearly thwarted the experiment. Blizzards delayed the planes that hauled the drilling equipment onto the ice. A violent windstorm trapped the researchers inside their tents for days. Then the ice around the drill bit cracked, slowing the team’s progress to a crawl.

Finally, on the last day before the scientists needed to start packing up their equipment, they pierced through 1,600 feet of ice and reached the sediments and rocks below.

“It felt like a really big triumph,” Walcott-George recalled. It was only the third time anyone had extracted material from beneath the deepest parts of the ice sheet; scientists have fewer rocks from beneath Greenland than from the surface of the moon.

But he wouldn’t know exactly what he’d uncovered until several months later, when Walcott-George finally examined the sample at a laboratory in Texas.

To figure out how long the material had been covered by ice, he used a technique called luminescence dating.

When sand grains are buried, their crystal structures can trap electrons created by radioactive activity in the surrounding rocks and soil. The longer the sediments remain in the darkness, the more electrons accumulate. But as soon as the crystals are exposed to light, they release the stored electrons in a sudden burst.

If scientists can count those freed electrons and calculate how long it would have taken them to accumulate, they can determine when the sediments last saw the sun.

As soon as he saw the results of his analysis, Walcott-George said he thought to himself, “Oh boy.”

The GreenDrill team had hoped it would find that Prudhoe Dome hadn’t melted since the last interglacial — a geological period more than 100,000 years ago, when global temperatures were slightly warmer than they are today.

But the luminescence measurements suggested that the sediments had been buried for only about 7,100 years. This meant the ice atop Prudhoe Dome had disappeared amid conditions similar to the current climate, when the Arctic was about 3 to 5 degrees warmer than it was in the 19th century.

The discovery bolsters a growing body of evidence suggesting that Greenland’s ice sheet is extremely susceptible to temperature swings, Briner said.

The research helps illustrate the toll of Arctic amplification, which causes the polar regions to warm up faster than the rest of the globe. The phenomenon is driven by the loss of sea ice, which exposes the dark ocean surface to the sun’s radiation and causes it to absorb more heat. Studies show that Arctic temperatures have increased by 3 degrees Celsius just in the past five decades — reaching levels of warmth similar to when Prudhoe Dome last disappeared.

“It’s giving us a sense of X climate change equals X ice volume change,” Briner said. “I think it’s a really important data point from that perspective.”