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.
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| The Earth seen from space |
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.
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.
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."
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