At a time of intense planetary warming, it’s odd to contemplate a counterfactual world in which we might be in or heading into a glacial period, popularly called an “ice age.”
But new research published Wednesday in the influential journal Nature suggests that we may have had a close scrape with such a period earlier in the current geological epoch known as the Holocene — and that pre-industrial human modifications of the climate through agriculture, fires and deforestation might have just barely staved it off.
“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.
Moreover, the study says, massive human greenhouse gas emissions since that time have probably “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 Earth goes into an ice age. There’s one that humans can influence, as well as one they really can’t.
The factor out of our control is 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. 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.
All of this can affect the delivery of sunlight over different parts of Earth and the nature of the seasons and thus, whether it’s possible to build up huge ice masses on land. 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 into the atmosphere.
Atmospheric carbon dioxide traps heat, causing 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.
Using analysis of past planetary glaciations and a computer model of 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.
“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.
