Humans halted the next ice age in the 1960s — and that's not good news

For two thousand years, a patch of ice on a remote Canadian island was doing exactly what Earth's ice has done for millions of years: slowly, steadily growing. Then, around 1960, it stopped — and began to shrink. A new study published in the journal Science argues that this reversal marks the moment humans accidentally canceled the planet's next ice age, and that future emissions could keep the next one from arriving for hundreds of thousands of years.

The ice in question is the Barnes Ice Cap, a Delaware-sized remnant of the ancient Laurentide Ice Sheet sitting on Baffin Island in Canada's Nunavut territory. Stretching from the Arctic Circle all the way to Illinois at its height around 20,000 years ago, the Laurentide Ice Sheet reached thicknesses of up to two miles in places. Today, the Barnes Ice Cap is all that remains of it.

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

Young and colleagues at Columbia University and the University of Colorado Boulder reconstructed the ice cap's history using two techniques. Because the Barnes Ice Cap is frozen to its bedrock and does not slide, it has preserved the plants it buried as it grew. The team gathered long-dead mosses near the ice cap's edge and used radiocarbon dating to determine when they were last alive, effectively tracking the ice margin as it advanced and killed successive generations of vegetation. They also measured levels of beryllium-10 — a radioactive isotope that accumulates on rock surfaces exposed to the sky — to calculate how long nearby rocks had been ice-free, pinpointing when the ice first reached its modern extent at roughly 3,600 years ago.

Together, the records told a clear story: the ice sheet reached its smallest size about 3,000 years ago, then began re-expanding around 2,000 years ago as temperatures cooled slightly. The researchers argue that regrowth was the opening chapter of Earth's next glacial period. Historic aerial photographs then showed that starting in the 1960s, the ice cap reversed course entirely. Since the 1980s the Barnes Ice Cap's edges have been pulling back at an average rate of 4 meters per year — undoing in a matter of decades what took 2,000 years to accumulate.

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 timing of the reversal — coinciding with the mid-20th-century acceleration in greenhouse gas emissions — is central to the study's argument. Young said climate pollution has 'essentially overpowered' the natural mechanisms that 'want to make that region cool.' According to climate models, the Barnes Ice Cap could vanish entirely within 300 years. If it does, the study's authors warn, it could alter the dynamics of future glaciations, since the Laurentide Ice Sheet has historically served as the anchor for continent-spanning ice growth.

A perspective piece accompanying the study in Science, written by glaciologist Shaun Marcott of the University of Wisconsin-Madison, puts the long-term stakes plainly. Marcott writes that human emissions could delay the next ice age by hundreds of thousands of years, and that whether glaciation ever returns 'is left to choices made by humanity and tied to the future carbon emissions.' He notes that moving away from fossil fuels might give natural climate cycles a chance to rebalance, though the outcome remains uncertain.

Humans are now geologic agents capable of thwarting millions of years of natural glacial cycles and offsetting Earth's climate for hundreds of thousands of years to come. The absence of future glacial cycles will be profound in the context of the geologic history of the past 2 million years and will have ramifications that will extend to other biological and physical systems that coevolved with the ice ages.— Shaun Marcott, glaciologist, University of Wisconsin-Madison

Young was emphatic that none of this should be read as good news. Glacial periods unfold over thousands of years, and it would have taken millennia before a growing ice cap affected everyday human life. The real significance, he said, is what the finding reveals about the scale of human interference in Earth's systems — and the direction of travel. Young put it directly: 'We've probably put off any chance of any other future glaciation for a very, very long time.'

The research also casts new light on a long-running dispute about scientific credibility. In the early 1970s, scientists — including two who are co-authors on this new paper — met at Brown University to discuss the timing of the next ice age and sent a warning letter to President Richard Nixon. Media coverage of that meeting, and a cooler weather spell in the decade, have since been cited by climate skeptics as evidence that scientists once predicted global cooling and therefore cannot be trusted on global warming. Young argues the new study vindicates those 1970s scientists: a glaciation really was underway. It was simply overtaken by something faster.

I think this study actually helps show that scientists were right about a pending glaciation.— Nicolás Young, paleoclimatologist, Lamont-Doherty Earth Observatory, Columbia University

Not all researchers are convinced the evidence goes as far as the authors claim. Dartmouth College earth sciences professor Meredith Kelly, who had no role in the study, praised the data as 'awesome' and acknowledged it reveals a clear picture of the ice cap's behavior, while adding that she would want to see the findings set against glacier records from other regions before the underlying mechanisms can be fully understood. Andrey Ganopolski, a physicist at the Potsdam Institute for Climate Impact Research, goes further, arguing that other modeling studies — including his own — conclude the next ice age would not have been expected for another 50,000 years even without global warming, and that the growth of the Barnes Ice Cap alone is 'insufficient to support the conclusion that a new glacial cycle had already begun and was subsequently aborted.' Young and his co-authors counter that the first regrowth of the Laurentide Ice Sheet is itself the start of an ice age, and that waiting for enormous ice sheets to form before declaring one has begun sets the bar too high.

Andrew Wickert, an associate professor of environmental science at the University of Minnesota Twin Cities who was not involved in the research, said the finding carries weight regardless of where one draws the definitional line. He told Wired: '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.' He added: 'That's significant.'

Why it matters — The study provides the first direct observational evidence that human emissions have interrupted a natural planetary cycle that has governed Earth's climate for millions of years — with a companion analysis warning the disruption could last hundreds of thousands of years.

Sources differ on Whether the Barnes Ice Cap's regrowth constitutes the start of a new glacial cycle: Yes — the first regrowth of the Laurentide Ice Sheet counts as the start of an ice age; we need not wait for enormous ice sheets to form. (cnn.com (Young et al., study authors)) vs No — the growth of the Barnes Ice Cap alone is insufficient to conclude a new glacial cycle had begun; other modeling suggests the next ice age was 50,000 years away regardless. (cnn.com (Andrey Ganopolski, Potsdam Institute)) vs The data is compelling but comparisons with glaciers in other regions are needed to fully understand the mechanisms. (wired.com (Meredith Kelly, Dartmouth))

Reported by cnn.com, wired.com, gizmodo.com, iflscience.com, science.org