Ancient moss under a Canadian ice cap shows humans derailed the next ice age in the 1960s

Buried under a Delaware-sized ice cap in the Canadian Arctic, tiny mosses have been waiting two millennia to tell their story. Now, newly exposed by melting ice and dated by scientists, they carry an uncomfortable message: the next ice age had already begun — and human-caused warming cancelled it, probably sometime around 1960.

That is the central finding of a study published in the journal Science by Nicolás Young, a geochemist at Columbia University's Lamont-Doherty Earth Observatory, and colleagues. To piece together the complete history of the Barnes Ice Cap's advances and retreats, the team drew on two independent dating methods. The ice cap sits on Baffin Island in Canada's Nunavut territory and is the last surviving fragment of the Laurentide Ice Sheet, the colossal glacier that once buried most of Canada under two miles of ice and held enough water to raise global sea levels by roughly 80 meters.

It was just getting started, everything was normal, the next glaciation was about to happen — and yet it stopped. It's a very powerful example of how we have jolted the system.— Nicolás Young, geochemist, Lamont-Doherty Earth Observatory, Columbia University

The story the ice cap tells begins with a retreat. Using beryllium-10 — an isotope that accumulates in rock surfaces exposed to open sky — the researchers calculated that the Laurentide had shrunk to roughly its present footprint by about 3,600 years ago, and likely reached its minimum size around 3,000 years ago. Then something changed.

About 2,000 years ago, the ice started growing again. The evidence for that reversal comes from the mosses. After the Laurentide retreated, tundra plants colonised the newly bare ground. When the ice crept back, it smothered them in place — killing them instantly but preserving them intact beneath the cold, non-sliding ice. Because Barnes is frozen directly to its bedrock rather than sliding over it, it acts less like a bulldozer and more like a glass case, entombing whatever lay beneath it. By radiocarbon-dating the dead plants, the team could pinpoint the moment the ice arrived at each spot — what glaciologists call a kill date.

There were all these plants growing, just like today. And included in that are mosses, these little mosses that are rooted in the ground. So they're there, they're living their life, and eventually the Laurentide Ice Sheet starts to re-expand. And when that ice margin covers the moss, it literally kills it.— Nicolás Young, geochemist, Lamont-Doherty Earth Observatory, Columbia University

About 70 percent of the kill dates cluster around 1800 CE — the peak of the Little Ice Age, when Northern Hemisphere glaciers were at their most extensive in recent millennia. Young interprets that not as a separate event but as the culmination of an advance that had been running continuously for two thousand years. Nearby smaller ice caps on the same plateau reinforce the timeline: nearly 200 kill dates collected from beneath them in earlier studies show not one is older than 2,000 years, suggesting the entire region began re-expanding at roughly the same moment.

The mechanism behind that 2,000-year-old turn is a tug-of-war between two planetary forces. Solar energy reaching high northern latitudes peaked around 10,000 to 11,000 years ago and has been declining ever since, nudging the climate toward glaciation. Over the same period, between 8,000 and 2,000 years ago, atmospheric carbon dioxide climbed by roughly 20 parts per million, providing a warming counterweight. According to the study, the declining solar input eventually overwhelmed that modest CO₂ rise, tipping the balance toward ice growth.

Then came industrialisation. Historic aerial photographs document a reversal in the ice cap's fortunes: the Barnes Ice Cap's edges have been pulling back at an average rate of 4 meters per year since the 1980s. By the 1960s, summer temperatures had risen enough that the ice cap was losing more mass each summer than it gained each winter. Two thousand years of net expansion were undone. Models now project the Barnes Ice Cap will melt completely within the next few centuries — erasing the last physical remnant of the Laurentide Ice Sheet.

Modern retreat of this glacier has reversed 2,000 years of net expansion — that's a pretty decent chunk of time.— Nicolás Young, paleoclimatologist, Lamont-Doherty Earth Observatory, Columbia University

The findings carry an unexpected historical footnote. In the early 1970s, scientists meeting at Brown University concluded that a new glaciation was so imminent they sent a warning letter to President Richard Nixon. Media coverage of that meeting, combined with a cooler stretch of weather in the 1970s, has since been cited by climate change deniers as supposed proof that scientists once predicted global cooling — and therefore cannot be trusted on warming. Young says the new evidence actually vindicates those scientists: a glaciation was beginning, they just lacked the physical proof. It would have taken roughly 30,000 years for the Laurentide to regain its continent-spanning mass, but the process had started.

I think this study actually helps show that scientists were right about a pending glaciation. They couldn't quite prove it, but now we sort of have the evidence.— Nicolás Young, paleoclimatologist, Lamont-Doherty Earth Observatory, Columbia University

The study also challenges the conventional scientific framing of when a glacial period begins. Previous model-based research has suggested the next glaciation would not start for thousands or even tens of thousands of years. Young and his colleagues argue that the correct marker is the first renewed growth of an ice sheet — not the eventual formation of continent-spanning glaciers. By that definition, the next glacial period had already begun 2,000 years ago, and has now been interrupted.

We have a decent understanding of how this enormous ice sheet grew and disappeared over thousands of years from proxy records, but catching the start of this process with direct observations is new. Our approach helped us see the first steps of ice-sheet growth preserved in the landscape. It gave us a rare opportunity to understand how re-glaciation begins and how the Laurentide Ice Sheet re-forms.— Joerg Schaefer, climate-geochemist, Lamont-Doherty Earth Observatory, Columbia University

Researchers not involved in the study called the dataset compelling but noted open questions. Meredith Kelly, a professor of earth sciences at Dartmouth, praised the data as "awesome" and said the study shows a "clear pattern of what the ice cap was doing," though she told Wired she would like comparisons with sites elsewhere in the world to more fully understand the global mechanisms of re-glaciation. Andrew Wickert, an associate professor of environmental science at the University of Minnesota Twin Cities, said the implications were significant regardless.

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.— Andrew Wickert, associate professor of environmental science, University of Minnesota Twin Cities

Young expects the ice itself to eventually settle any remaining uncertainty about the 2,000-year start date. Since 1960, the Barnes margin has pulled back between 200 and 800 meters, uncovering mosses that were killed from around 1000 CE onward. The oldest, most decisive samples — the ones that would directly date the initial re-advance — are still locked under ice. Young says that with further retreat over the coming decades, the opportunity may arise: "If we went back there in 20 years or something, as the ice margin has retreated a little more, we actually might be able to sample those 2,000-year-old plants." The retreat that ended the ice age, in other words, may yet hand scientists the clearest proof that it happened.

Why it matters — The study provides the first direct physical evidence — not just model projections — that a natural glacial cycle was already underway when human carbon emissions reversed it, reframing both the timeline of ice ages and the measurable scale of humanity's impact on Earth's deepest climate rhythms.

Sources differ on When the Laurentide reached its minimum size: About 3,600 years ago (arstechnica.com) vs About 3,000 years ago (news.climate.columbia.edu)

Reported by arstechnica.com, wired.com, news.climate.columbia.edu, newscientist.com