Image copyright Ittiz Image caption Earth has been through a cycle of ice ages and warm periods over the past 2.5 million years

The next ice age may have been delayed by over 50,000 years because of the greenhouse gases put in the atmosphere by humans, scientists in Germany say.

They analysed the trigger conditions for a glaciation, like the one that gripped Earth over 12,000 years ago.

The shape of the planet's orbit around the Sun would be conducive now, they find, but the amount of carbon dioxide currently in the air is far too high.

Earth is set for a prolonged warm phase, they tell the journal Nature.

"In theory, the next ice age could be even further into the future, but there is no real practical importance in discussing whether it starts in 50,000 or 100,000 years from now," Andrey Ganopolski from the Potsdam Institute for Climate Impact Research said.

"The important thing is that it is an illustration that we have a geological power now. We can change the natural sequence of events for tens of thousands of years," he told BBC News.

Earth has been through a cycle of ice ages and warm periods over the past 2.5 million years, referred to as the Quaternary Period.

This has seen ice sheets come and go. At its maximum extent, the last glaciation witnessed a big freeze spread over much of North America, northern Europe, Russia and Asia.

In the south, a vast expanse of what are now Chile and Argentina were also iced up.

Planet rock

A fundamental parameter determining what dips Earth into an ice age is the changing nature of its orbit around the Sun.

The passage around the star is not a perfect circle and over time our planet's axis of rotation also rocks back and forth.

These movements alter the amount of solar radiation falling on the Earth's surface, and if a critical threshold is reached in mid latitudes in the Northern Hemisphere then a glaciation can be initiated.

Dr Ganopolski colleagues confirm this in their modelling but show also the role played by the concentration of greenhouse gases in the atmosphere.

And one of their findings is that Earth probably missed the inception by only a narrow margin a few hundred years ago, just before the industrial revolution took hold.

"We are now in a period when our (northern) summer is furthest from the Sun," the Potsdam researcher explained.

"Under normal circumstances, the interglacial would be terminated, and a new ice age would start. So, in principle, we are in the perfect conditions from an astronomical point of view. If we had a CO2 concentration of 240 parts per million (200 years ago) then an ice age could start, but luckily we had a concentration that was higher, 280ppm." Today, industrial society has taken that concentration to over 400ppm.

Fast metabolism

The team says that an interglacial climate would probably have been sustained anyway for at least 20,000 years, and, very probably, for 50,000 years, even if CO2 had stayed at its eighteenth century level.

But the almost 500 gigatonnes of carbon that has been released since the Industrial Revolution means we will likely miss the next best astronomical entry point into a glaciation, and with a further 500 gigatonnes of emissions the "probability of glacial inception during the next 100,000 years is notably reduced", the scientists say in their Nature paper.

Add a further 500 Gt C on top of that and the next ice age is virtually guaranteed to be delayed beyond the next 100,000 years.

Commenting on the study, Prof Eric Wolff from the University of Cambridge, UK, said: "There have been previous papers suggesting that the next ice age is many tens of thousands of years away, and that the combination of seasonal solar energy at the latitude where an ice sheet would form, plus CO2, is what determines the onset of an ice age. But this paper goes much further towards quantifying where the limits are.

"It represents a nice confirmation that there is a relatively simple way of estimating the combination of insolation and CO2 to start an ice age," he told the Science Media Centre.

And Prof Chris Rapley, from University College London, added: "This is an interesting result that provides further evidence that we have entered a new geological [Epoch] - 'The Anthropocene' - in which human actions are affecting the very metabolism of the planet."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos