By Tim Radford / Climate News NetworkThis Creative Commons-licensed piece first appeared at Climate News Network.

LONDON — Human beings have not just started to leave a unique geological stratum that will announce their existence long after the species has been extinguished. They may have altered a climate cycle that has been stable for millions of years and even cancelled – or certainly postponed – the next Ice Age.

Andrey Ganopolski and colleagues from the Potsdam Institute for Climate Impact Research in Germany report in Nature that they took a look at the conditions that determined the geologically recent cycle of Ice Ages.

The advance and retreat of vast glaciers over geological time are the consequence of a mix of factors involving sea, mountains, atmosphere, vegetation and the distribution of continents around the globe.

But ultimately what determines the rhythm of these events is what climate scientists call insolation: how much sun the Earth actually gets in a summer. The Earth’s orbit is not a perfect circle but an ellipse, and the shape of the ellipse and the angle of the Earth’s tilt on its axis change subtly and imperceptibly over cycles lasting tens of thousands of years, which in turn alters the amount of sunshine striking the northern hemisphere.

And if the atmospheric levels of carbon dioxide are not high when the total insolation is near its lowest point, great thick sheets of ice begin to advance over Europe, Asia and North America.

This is enough to explain the last eight Ice Ages. The sequence is punctuated by “interglacials” that tend to last roughly 10,000 years before the ice advances once more.

But at the end of the last Ice Age something happened: humans had discovered fire, and then used it to invent agriculture and metal foundries, and then began to alter the carbon balance in the atmosphere, even before the discovery of fossil fuels.

“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.”

Geologists have a name for this present interglacial epoch. They call it the Holocene. “Even without man-made climate change we would expect the beginning of a new ice age no earlier than in 50.000 years from now – which makes the Holocene as the present geological epoch an unusually long period in between ice ages,” Dr Ganopolski said.

“However, our study also shows that relatively moderate additional anthropogenic CO2 emissions from burning oil, coal and gas are already sufficient to postpone the next ice age for another 50.000 years.

“The bottom line is that 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.”

Geologists have already proposed that the Holocene be renamed the Anthropocene, from the Greek anthropos for mankind. The interest in the Ice Age mechanisms is not new: in the 1970s, climate scientists began to ask whether a glaciated world could come back.

Within a decade, it became clear that atmospheric carbon dioxide levels would be too high to permit another immediate Big Freeze, and would go on rising.

What the Potsdam scientists have done, using climate simulations, is to pinpoint the intricate balance of insolation and atmospheric chemistry that controls the beginning and the end of an Ice Age.

Continuing rise

And once humans had begun to exhume all the ancient sunshine of the Carboniferous Period that ended 300 million years ago, and return it to the 20th century atmosphere as greenhouse gases, the combustion products of coal, oil and methane, the cycle was interrupted.

It is likely to stay warm for a period far longer than all recorded human history so far. Even if humans drastically reduce the combustion of fossil fuels this century, enough will enter the atmosphere to keep carbon dioxide levels high, and global temperatures and sea levels will go on rising.

“Like no other force on the planet, ice ages have shaped the global environment and thereby determined the development of human civilisation. For instance, we owe our fertile soil to the last ice age that also carved out today’s landscapes, leaving glaciers and rivers behind, forming fjords, moraines and lakes.

“However, today it is humankind with its emissions from burning fossil fuels that determines the future development of the planet,” said Hans Joachim Schellnhuber, director of the institute and one of the authors.

“This illustrates very clearly that we have long entered a new era, and that in the Anthropocene humanity itself has become a geological force. In fact, an epoch could be ushered in which might be dubbed the Deglacial.”

Andrew Watson, of the University of Exeter, UK, said the study confirmed what he and others had suspected for some time. “Humans now effectively control the climate of the planet.

If only we were wise enough to be able to use that power responsibly, this might be a good thing, as a planet that avoided major ice ages would probably be better for most of the species living on it. Unfortunately, I don’t think we’ve reached that level of wisdom yet.”

Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.

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