PARIS (AFP) – Scheduled shifts in Earth's orbit should plunge the planet into an enduring Ice Age thousands of years from now but the event will probably be averted because of man-made greenhouse gases, scientists said Wednesday.
They cautioned, though, 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.
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.
Two researchers 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.
They found dramatic swings in climate, including changes when Earth flipped from one state to the other in a relatively short time, said one of the authors, geoscientist Thomas Crowley of the University of Edinburgh, Scotland.
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," Thomas told AFP.
"What's also interesting is that the inter-glaciations also became warmer."
According to the model, published in the British journal Nature by Crowley and physicist William Hyde of Toronto University, Canada, the next "bifurcation" would normally be due between 10,000 and 100,000 years from now.
The chill would induce a long, stable period of glaciation in the mid-latitudes, smothering Europe, Asia and North America to about 45-50 degrees 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.
"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 percent 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.
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 UN'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
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