According to the study, the next Ice Age is due between 10,000 and 100,000 years from now - but global warming may step in to prevent it.
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PARIS: Scheduled shifts in Earth's orbit are due to plunge the planet into an Ice Age perhaps 10,000 years from now, but the event may be averted by man-made greenhouse gases, say scientists.
They caution, though, in a study reporting the find in the British journal Nature, that this news is not an argument in favour of global warming, which is driving imminent and potentially far-reaching damage to the climate system.
Big freeze
Earth has experienced long periods of extreme cold over the billions of years of its history. The big freezes are interspersed with "interglacial" periods of relative warmth, of the kind we have experienced since the end of the last Ice Age, around 11,000 years ago.
These climate swings have natural causes, believed to be rooted particularly in changes in Earth's orbit and axis that, while minute, have a powerful effect on how much solar heat falls on the planet.
The two researchers behind the study built a high-powered computer model to take a closer look at these intriguing phases of cooling and warmth.
In addition to the planetary shifts, they also factored in levels of carbon dioxide (CO2), found in tiny bubbles in ice cores, that provide an indicator of temperature spanning hundreds of thousands of years.
The model revealed dramatic swings in climate, including changes when Earth flipped from one state to the other over a relatively short time, said one of the authors, geoscientist Thomas Crowley of the University of Edinburgh in Scotland.
Abrupt bifurcations
These shifts, called "bifurcations," appear to happen in abrupt series, which is counter-intuitive to the idea that the planet cools or warms gradually.
"You had a big change about a million years ago; then a second change around 650,000 years ago, when you had bigger glaciations; then 450,000 years ago, when you started to get more repeated glaciations," said Crowley. "What's also interesting is that the inter-glaciations also became warmer."
According to the model, authored by Crowley and physicist William Hyde of Toronto University in Canada, the next "bifurcation" would normally be due between 10,000 and 100,000 years from now.
The chill would likely induce a long, stable period of glaciation in the mid-latitudes, smothering Europe, Asia and North America to about 45 to 50º latitude with a thick sheet of ice.
However, there is now so much CO2 in the air, as a result of fossil-fuel burning and deforestation, that this adds a heat-trapping greenhouse effect that will offset the cooling impacts of orbital shift, said Crowley.
Critical state
"Even the level that we have there now is more than sufficient to reach that critical state seen in the model," he said. "If we cut back [on CO2] some, that would probably still be enough."
In September, a scientific research consortium called the Global Carbon Project (GCP) said that atmospheric concentrations of CO2 reached 383 parts per million (ppm) in 2007, or 37 per cent above pre-industrial levels.
Present concentrations are "the highest during the last 650,000 years and probably during the last 20 million years," the report said.
"No justification"
Crowley cautioned those who would seize on the new study to say "'carbon dioxide is now good, it prevents us from walking the plank into this deep glaciation'."
"We don't want to give people that impression," he said. "You can't use this argument to justify [man-made] global warming."
Last year, the U.N.'s Nobel-winning Intergovernmental Panel on Climate Change (IPCC) said that greenhouse-gas emissions were already inflicting visible changes to the climate system, especially on ice and snow.
Left unchecked, climate change could inflict widespread drought and flooding by the end of the century, translating into hunger, homelessness and other stresses for millions of people.
