Earth was sliding into a new ice age. Fossil fuel emissions reversed it.

For roughly 2,000 years, a remnant of the last great North American ice sheet was quietly doing what ice sheets do at the start of a glacial cycle: growing. Then, in the 1960s, it stopped — and began to shrink. The culprit, a new study argues, was us.

The Barnes Ice Cap, a glacier the size of Delaware frozen to the bedrock of Canada's Baffin Island, has retreated an average of 4 meters per year since the 1980s. That reversal, the authors say, coincides precisely with the period of rapidly accelerating carbon emissions — and it erased 2,000 years of net ice growth in a matter of decades.

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 at Columbia University

Remnant of the vast Laurentide Ice Sheet, the Barnes Ice Cap has shrunk dramatically since that frozen expanse blanketed all of Canada and reached as far south as New York and the Midwest before the last ice age drew to a close 12,000 years ago. Because the Barnes cap is frozen to bedrock rather than flowing like a typical glacier, it has preserved a biological record beneath it — dead mosses and lichen killed and entombed when the ice advanced, then re-exposed as it retreated. Young and his team used those organic traces to reconstruct a precise timeline of the ice's movements.

To pin down when the ice cap first reached its current size, the researchers used a technique called exposure dating, measuring concentrations of Beryllium-10 — an element that accumulates on rock surfaces exposed to the sky — on rocks near the glacier's edge. That calculation placed the ice cap's modern footprint at roughly 3,600 years old. From there, the moss record showed it began expanding again around 2,000 years ago, a pattern consistent with the natural orbital rhythms, known as Milankovitch cycles, that have driven every ice age for hundreds of thousands of years.

Historic aerial photographs then documented what happened next: starting in the 1960s, the ice reversed course entirely. The growth that had been underway for two millennia went into retreat just as atmospheric carbon dioxide from fossil fuel burning was ramping up sharply.

A question that's in the popular consciousness is, have we caused enough global warming to stop the next ice age from happening? 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.— Andrew Wickert, associate professor of environmental science, University of Minnesota Twin Cities

The Barnes Ice Cap finding adds physical, on-the-ground evidence to a picture that climate modelers had already sketched. Research published in Nature in 2016, under the leadership of Andrey Ganopolski at Germany's Potsdam Institute for Climate Impact Research, applied detailed climate models to all eight glacial periods of the past 800,000 years in order to pinpoint the exact conditions under which glaciation begins: a particular pairing of diminished summer solar radiation at 65 degrees north latitude with low levels of atmospheric CO2. The research found that CO2 levels — which stood at 280 parts per million at the start of the industrial revolution and have since risen to 400 ppm — have already pushed the atmosphere well past the threshold needed to prevent the next ice age.

The bottom line is we are basically skipping a whole glacial cycle, which is unprecedented. It is mind-boggling that humankind is able to interfere with a mechanism that shaped the world as we know it.— Andrey Ganopolski, Potsdam Institute for Climate Impact Research

According to the 2016 modeling, keeping carbon emissions within bounds consistent with 2°C of warming — the internationally agreed target — would still leave enough CO2 in the atmosphere to head off ice ages that might otherwise have commenced 50,000 or 90,000 years from now. The reason is the extraordinary persistence of CO2: according to Michel Crucifix at the Université catholique de Louvain, the mean half-life of CO2 in the atmosphere is on the order of 35,000 years, meaning anthropogenic emissions will still be present in 50,000 to 100,000 years' time.

Such long-term consequences may seem surprising, given that the emissions will occur over a few centuries at most. In fact, the mean half-life of CO2 in the atmosphere is of the order of 35,000 years. Consequently, anthropogenic CO2 will still be in the atmosphere in 50,000 years' time, and even 100,000 years, which is enough to prevent any glaciation.— Michel Crucifix, Université catholique de Louvain

The new Barnes Ice Cap study also resolves a long-running piece of climate misinformation. In the early 1970s, scientists meeting at Brown University concluded that a glaciation period was sufficiently imminent that they sent a warning letter to President Richard Nixon. Media coverage of that meeting, combined with a cooler weather period in the 1970s, has since been cited by climate skeptics as supposed proof that scientists once predicted global cooling — and therefore that warming predictions cannot be trusted. Young, whose coauthors include scientists who attended the original Brown University meeting, says the new evidence shows those scientists were correct: a glaciation was indeed beginning. Human emissions stopped it.

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

For some researchers, the scale of the intervention is as striking as the intervention itself. Prof Tim Lenton at the University of Exeter noted that preventing the next ice age required only 500 gigatonnes of carbon in the atmosphere — a threshold humanity has already crossed. Prof Andrew Watson, also at Exeter, drew the starkest conclusion, saying that "humans now effectively control the climate of the planet," before adding: "If only we were wise enough to be able to use that power responsibly. Unfortunately, I don't think we've reached that level of wisdom yet."

Both the Ganopolski modeling study and the Barnes Ice Cap research are cited by scientists as evidence that Earth has entered the Anthropocene — a proposed new geological epoch defined by humanity's capacity to alter planetary-scale processes. Other evidence cited for the Anthropocene designation includes plastic pollution and the mass extinction of wildlife.

Why it matters — The findings show that human carbon emissions have already overridden a natural planetary cycle that has shaped every landscape and civilization on Earth — and that the CO2 already in the atmosphere will continue doing so for tens of thousands of years.

Reported by wired.com, theguardian.com