“Humanity narrowly escaped a glacial inception in the middle of the Holocene, which was almost suppressing the formation of civilization,” says Hans Joachim Schellnhuber, one of the paper’s three authors and founding director of the Potsdam Institute for Climate Impact Research (the other authors are also affiliated with the institute).

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Moreover, the study says, massive human greenhouse gas emissions since that time have likely “postponed” what might otherwise be another ice age “by at least 100,000 years.”

The new research is based on the idea that there are two key factors that shape whether the Earth goes into an ice age (or glacial period) or not. There’s one that humans can influence, as well as one they really can’t.

The factor out of our control is the Earth’s Milankovitch cycles, which describe the erratic way in which the planet orbits the sun and spins on its axis over vast time periods. The Earth’s orbit grows slowly more and less elliptical, even as the angle of the planet’s axial tilt, and the wobble of the poles as the planet spins (much like what you see with a spinning top), also change slightly over thousands of years.

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All of this can affect the delivery of sunlight over different parts of the Earth and the nature of the seasons — for instance, causing summers to be colder — and thus, whether it’s possible to build up huge ice masses on land. Critically, how much sun the Earth’s northern hemisphere high latitudes receive in summer shapes whether ice can build up there over long periods, the new study says.

But there’s also a second factor that’s in our control — how much carbon dioxide is in the atmosphere. We are able to turn this knob by how many forests we cut down and how many fossil fuels we burn, both processes that transfer carbon from the land (or beneath it) into the atmosphere.

Atmospheric carbon dioxide traps heat and so causes an overall warming effect, and this will happen no matter where the planet is in its various orbital cycles. And if there’s enough of it, it can counteract the tendency of these cycles to make and then unmake ice ages.

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“We wanted to understand what is really triggering glacial inception, and what we found is an amazingly simple function, which is the ratio between insolation around 65 north latitude and the CO2 content of the atmosphere,” Schellnhuber said. “And this is more or less summarizing the two key factors in the development of glacial cycles over the last at least 800,000 years.”

Specifically, using analysis of past planetary glaciations and a computer model of the Earth that is able to predict their occurrence, the researchers found that carbon dioxide concentrations were only slightly too high to push us into glaciation a few thousand years ago. Instead, we enjoyed the relatively friendly (for humans) interglacial climate of the Holocene.

“The Earth system would already be well on the way towards a new glacial state if the pre-industrial CO2 level had been merely 40 [parts per million] lower than it was during the late Holocene,” the authors write. Indeed, they note that about 800,000 years ago, orbital alignments were similar but carbon dioxide concentrations were around 240 parts per million, and glaciation did indeed occur.

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It’s important to note that at present, the Earth is not fully deglaciated — we still have major ice sheets atop Greenland and especially Antarctica. However, in a period of glacial maximum, ice sheets would also cover northern Europe and much of North America, placing vastly more total ice atop land and thus leading to sea levels radically lower than they are at present.

A key question raised by the research then becomes why carbon dioxide concentrations were higher during the Holocene — the current geological epoch that began 11,700 years ago, unless you agree with some scientists that we’ve now entered the “Anthropocene” — and whether this is natural. Here, the authors walk up to, but do not fully embrace, the idea that humans substantially altered the climate long before the industrial revolution, and specifically, that they upped atmospheric carbon dioxide concentrations from around 240 parts per million to 280 parts per million through deforestation, agriculture and other means.

This idea is credited to University of Virginia climate scientist William Ruddiman, who was not involved with the current paper. Asked to comment on it, Ruddiman agreed with the work overall but questioned why the authors didn’t more fully embrace his thesis (which they call “debatable”), and the idea that pre-industrial humans headed off a coming ice age through their alterations of the planet’s land surface, even without mass burning of fossil fuels (yet).

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“While there is little doubt that industrial-era anthropogenic emissions are now forestalling any possibility of a new ice age, the evidence shown here suggests that this major human intervention started millennia ago,” Ruddiman said in a statement to The Post.

If Ruddiman’s hypothesis is right, then combined with the new research, it would suggest that burgeoning human civilization basically headed off an ice age before that ice age could complicate matters for human civilization, making its establishment and growth considerably more difficult, especially in some key northern hemisphere regions.

And just as the current study’s analysis allows for an explanation of why we did not recently begin an ice age, so it also allows for an assessment of when another is likely to occur. After all, we’ve since gone far beyond any pre-industrial alterations of Earth’s climate and driven atmospheric carbon dioxide levels up to 400 parts per million, with peak concentrations still not yet achieved.

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If we had maintained planetary carbon dioxide levels at about 280 parts per million — the pre-industrial level that apparently averted ice age onset — then the research suggests that we may then have enjoyed a relatively stable climatic optimum, conditions that were friendly for civilization for about 50,000 years, without tipping into glaciation or experiencing extreme warming.

Instead, and depending on how much more carbon dioxide we emit, the research says, the planet may not see large ice sheets build for 100,000 years. (It takes a very long time for atmospheric carbon dioxide to be naturally removed again from the atmosphere.)

Indeed, with all the atmospheric CO2, we’re currently headed in the opposite direction: We’re now seeing major changes in Greenland and Antarctica, though it’s not clear yet just how much of these remaining ice sheets we could lose.

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“The next two glacial inceptions will be suppressed by the current cumulative emissions, plus the emissions we will unavoidably have over the next 40 to 50 years, even if we keep global warming below 1.5 to 2 degrees,” Schellnhuber said.

“The ice ages are called off, if you like, by human interference,” he said.

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