At first, Andrew Christ was ecstatic. In soil taken from the bottom of the Greenland ice sheet, he’d discovered the remains of ancient plants. Only one other team of researchers had ever found greenery beneath the mile-high ice mass.

But then Christ determined how long it had been since that soil had seen sunlight: Less than a million years. Just the blink of an eye in geologic terms.

And it dawned on him. If plants once grew at multiple spots on the surface of Greenland, that meant the ice that now covers the island had entirely melted. And if the whole Greenland ice sheet had melted once in the not-so-distant past, that meant it could go again.

“Oh my god,” he thought.

The findings, published Monday in the Proceedings of the National Academy of Sciences, indicate that the biggest reservoir of ice in the Northern Hemisphere can collapse due to relatively small increases in temperature over a long period of time. That makes it even more vulnerable to human-caused warming, which is causing the Earth to warm faster now than at any other period in its history.

“We know the Greenland ice sheet has this threshold,” Christ said — and humanity is pushing it.

Greenhouse gas emissions from human activities have already raised global average temperatures more than 1 degree Celsius (1.8 degrees Fahrenheit) since 1880. Greenland is losing ice at its fastest rate since humans invented agriculture, causing about 14 millimeters of sea level rise in the past half-century.

If the island’s entire ice sheet were to melt now, global sea levels would rise by more than 20 feet.

Scientists observe a lake of glacial meltwater drain beneath Greenland's ice sheet in July 2018. This research could help better understand global sea rise. (Sam Doyle and Tom Chudley)

“We don’t want to see what that looks like,” said Christ, a geologist at the University of Vermont. “It underscores the urgency of needing to change the way things are going right now.”

The story of this soil sample is almost as dramatic as the data it contains. It comes from the bottom of an ice core taken during “Project Iceworm,” a failed Cold War effort to hide nuclear missiles beneath Greenland’s ice.

Camp Century, in the far northwest of Greenland, was to be a base for the U.S. military project. Housing, dining and medical facilities, all powered by a nuclear reactor, were dug into the ice.

In the “battle between man and nature … man has brought his greatest scientific achievement — power from the atom — to the very top of the world,” broadcaster Walter Cronkite declared during a visit in 1960. “But can he live here? Can he stop the crushing force of the ageless ice?”

To disguise the true purpose of the venture, the United States solicited scientists to conduct research at the site. Among the experiments was a first-of-its-kind project to obtain an ice core that spanned the entire depth of the ice sheet.

“That ice core revolutionized our understanding of Earth’s past climate,” Christ said. By measuring the types of oxygen contained within each layer of ice, researchers could get a rough estimate of how warm it was when the water froze. Analyses of ice cores from Greenland and elsewhere have allowed scientists to reconstruct a record of global temperatures going back tens of thousands of years.

But the roughly 12 inches of soil from the bottom of the Camp Century ice core was never studied.

Meanwhile, the folly of Project Iceworm became apparent when the ice began to shift. Tunnels collapsed. Equipment got crushed. The nuclear reactor was swiftly dismantled, and the camp abandoned. Any scientific materials collected were sent off to laboratories and rarely thought of again.

More than a half-century later, glaciologist Jørgen P. Steffensen was conducting an inventory at the University of Copenhagen’s ice core repository when he stumbled upon what looked to be cookie jars full of sand, clay and soil. Curious, he shipped them to a few colleagues for analysis.

The samples arrived at the University of Vermont in coolers stuffed with freezer packs. Christ, then a doctoral student, was tasked with sifting through the material, pulling out intriguing fragments and placing them on microscope slides.

He leaned in to look, and then his eyes went wide. “It’s twigs and leaves and moss,” he said, “freeze dried for hundreds of thousands of years. They literally look like they could have been alive yesterday.”

Christ summoned his adviser, Paul Bierman, and an undergraduate assistant in the lab. They all began jumping up and down, laughing and shouting, while a high school tour group looked on in bemusement.

“This is the most exciting day of my scientific life and you’re here for it,” Christ recalls telling the teenagers.

The fossils were passed along to plant experts for further analysis, and Christ set about trying to determine when they might have grown. He used a technique called cosmogenic nuclide dating, which estimates the amount of time rocks have been buried by analyzing particles created when materials are exposed to radiation from space.

Early analysis suggests that the plants are no older than a million years, which suggests they must have grown during the epoch of repeated ice ages known as the Pleistocene. During that time, greenhouse gas concentrations in the atmosphere were far lower than current levels, and the Earth was rarely as hot as it is now.

“If we had found a much older age, it would have been impressive, but it might not have been as scary,” Christ said. “Because what we found means the ice sheet melted away and raised sea level within a climate system kind of like ours. That, as a climate scientist, has more gravity.”

Further study is needed to narrow the time frame during which this melting might have occurred, Christ said. It likely happened during a period of relative warmth known as an “interglacial.” The ice sheet would have dwindled slowly, like an ice cube moved from the freezer to the fridge.

The current melting in Greenland is more akin to an ice cube moved from the freezer to an oven. And humans keep turning the oven temperature up.

The entire Greenland ice sheet is unlikely to collapse in the immediate future.

“But if we warm up the ice sheet more than 1.5 degrees Celsius” — a number United Nations scientists have identified as the threshold for catastrophic climate change — “the risk of complete removal gets higher and higher,” said Isabella Velicogna, an ice expert at the University of California at Irvine who was not involved in the new study.

The fact that this has happened due to natural causes in the past is not a reassurance, she added. Geologic records show the toll of past climatic changes on animals and ecosystems, and offer a warning about what humanity could face.

Unless the world drastically reduces its greenhouse gas emissions, scientists project Greenland could lose as much as 35,900 billion tons of ice over the rest of the century. The subsequent sea level rise could exceed three feet, dramatically worsening hurricanes and floods.

Christ tried to imagine what must have been going through the minds of the men who obtained these samples back in the 1960s. The notion of human-caused global warming barely existed. People were much more concerned about the looming threat of nuclear war.

And yet, “even by that point in time in history, we had already pushed Earth’s climate system into unknown territory,” Christ said.

The findings offer a kind of poetic justice, he continued: More than 60 years ago, Americans went to Greenland believing they could conquer the ice. But they came away with a harsh lesson in humanity’s fragility.