Burning fossil fuels may have stopped the Earth from entering another ice age, a new study suggests. The research, published Thursday in the journal Science, uses samples of ancient vegetation and rock records to analyze the growth—and sudden retreat—of an ice cap that is the last vestige of an ice sheet that once covered much of North America.

The findings shows the Barnes Ice Cap, which covers parts of Canada’s Baffin Island, started growing a few thousand years ago before rapidly reversing course beginning in the 1960s as carbon pollution ramped up.

“A question that’s in the popular consciousness is, have we caused enough global warming to stop the next ice age from happening?” says Andrew Wickert, an associate professor of environmental science at the University of Minnesota Twin Cities, who was not involved in the research. The new research shows that “even as we were just starting to ramp up emissions, we’ve already done enough damage to cause the major North American ice sheet to already start retreating. That’s significant.”

The planet regularly goes through phases where ice extends down from the high latitudes—what we colloquially call "ice ages"—due to natural shifts in the Earth's rotation and orbit, known as Milankovitch Cycles. Those are followed by interglacial periods where ice retreats toward the poles.

The last ice age ended 12,000 years ago. At its peak, what's known as the Laurentide Ice Sheet covered all of Canada and extended as far south as New York and the Midwest. It retreated to form the Barnes Ice Cap, which covers an area the size of Delaware on Baffin Island.

Unlike glaciers, which can move, the Barnes Ice Cap is frozen to the bedrock, preserving the vegetal record of what grew around it under the ice. That quirk allowed the scientists leading the new study to glean information about how much it has grown—and shrank.

To determine when it first reached its modern size, they used an exposure dating technique to calculate roughly how long rocks several meters from the glacier’s edge had been exposed to the sky. Using measurements of Beryllium-10, an element that forms on the surface of exposed rock, the authors calculated that the ice sheet first reached the size it is today around 3,600 years ago.

Next, they wanted to see how much the ice cap had regrown from that point. To do this, they turned to the humble denizens of the tundra in front of the glacier: mosses and lichen.

“After the ice cap retreated, mosses eventually colonized this newly exposed land,” says Nicolás Young, a paleoclimatologist at the Lamont-Doherty Earth Observatory at Columbia University and lead author of the new paper. “These mosses are living there, and things are normal—then, eventually, the ice cap starts to re-expand. The ice covers the moss, and kills it.”The traces of the dead moss are preserved in newly reexposed land, allowing Young and his team to piece together a timeline of how the ice has moved across the landscape.

Thanks to these records, Young and his coauthors estimate that the glacier started growing again around 2,000 years ago. This may have signaled the start of another glaciation period.

But starting in the 1960s, the ice cap has done something that wouldn’t correspond with another ice age: It started getting smaller again. The team used historic aerial photos that showed that the ice has reversed course, retreating significantly back from its new margins. The Barnes Ice Cap’s edges have retreated an average of 4 meters per year since the 1980s. That coincides with a period of rapidly increasing carbon emissions, which has caused ice to go into retreat worldwide. The ice cap offers a particularly stark example, though, given the growth prior to the 1960s.

“Modern retreat of this glacier has reversed 2,000 years of net expansion—that’s a pretty decent chunk of time,” Young says.

Meredith Kelly, a professor of earth sciences at Dartmouth, says the data gathered in the study is “awesome” and that the study shows a “clear pattern of what the ice cap was doing.”However, Kelly says that she would like to see a comparison of some of the data to other sites around the world to more fully grasp how, exactly, the Earth was moving into its glaciation period before the effects of human-caused warming kicked in. While we have a broad understanding of what kicks off periods where ice can grow, the exact mechanisms are still something that require more research.

The evidence presented in this paper, Young says, could also validate a decades-old hypothesis that has long been held up by climate deniers as “evidence” climate change is a hoax. In the early 1970s, scientists gathered for a meeting at Brown University to discuss hypotheses of when the next ice age might happen. (Young says that two of his coauthors on this paper were at that meeting.) The scientists concluded that a glaciation period was so imminent that they sent a warning letter to President Richard Nixon.

“I think this study actually helps show that scientists were right about a pending glaciation,” Young says. “Media coverage of the meeting, as well as a colder period of weather in the 1970s, have long been pitched by climate deniers as “proof” that global warming isn’t actually happening. It very much is, though, and the new research actually adds to the evidence.