Scientists say greenhouse gas emissions have canceled the next ice age

At a time of intense planetary warming, it's odd to even contemplate a counterfactual world in which we might instead be in or heading into a glacial period, sometimes more 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 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 (the other authors are also affiliated with the institute).

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.

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.